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daemon: add seccomp filter for slirp4netns.
The container that slirp4netns runs in should already be quite difficult to do
anything malicious in beyond basic denial of service or sending of network
traffic. There is, however, one hole remaining in the case in which there is
an adversary able to run code locally: abstract unix sockets. Because these
are governed by network namespaces, not IPC namespaces, and slirp4netns is in
the root network namespace, any process in the root network namespace can
cooperate with the slirp4netns process to take over its user.
To close this, we use seccomp to block the creation of unix-domain sockets by
slirp4netns. This requires some finesse, since slirp4netns absolutely needs
to be able to create other types of sockets - at minimum AF_INET and AF_INET6
Seccomp has many, many pitfalls. To name a few:
1. Seccomp provides you with an "arch" field, but this does not uniquely
determine the ABI being used; the actual meaning of a system call number
depends on both the number (which is often the result of ORing a related
system call with a flag for an alternate ABI) and the architecture.
2. Seccomp provides no direct way of knowing what the native value for the
arch field should be; the user must do configure/compile-time testing for
every architecture+ABI combination they want to support. Amusingly enough,
the linux-internal header files have this exact information
(SECCOMP_ARCH_NATIVE), but they aren't sharing it.
3. The only system call numbers we naturally have are the native ones in
asm/unistd.h. __NR_socket will always refer to the system call number for
the target system's ABI.
4. Seccomp can only manipulate 32-bit words, but represents every system call
argument as a uint64.
5. New system call numbers with as-yet-unknown semantics can be added to the
kernel at any time.
6. Based on this comment in arch/x86/entry/syscalls/syscall_32.tbl:
# 251 is available for reuse (was briefly sys_set_zone_reclaim)
previously-invalid system call numbers may later be reused for new system
calls.
7. Most architecture+ABI combinations have system call tables with many gaps
in them. arm-eabi, for example, has 35 such gaps (note: this is just the
number of distinct gaps, not the number of system call numbers contained in
those gaps).
8. Seccomp's BPF filters require a fully-acyclic control flow graph.
Any operation on a data structure must therefore first be fully
unrolled before it can be run.
9. Seccomp cannot dereference pointers. Only the raw bits provided to the
system calls can be inspected.
10. Some architecture+ABI combos have multiplexer system calls. For example,
socketcall can perform any socket-related system call. The arguments to
the multiplexed system call are passed indirectly, via a pointer to user
memory. They therefore cannot be inspected by seccomp.
11. Some valid system calls are not listed in any table in the kernel source.
For example, __ARM_NR_cacheflush is an "ARM private" system call. It does
not appear in any *.tbl file.
12. Conditional branches are limited to relative jumps of at most 256
instructions forward.
13. Prior to Linux 4.8, any process able to spawn another process and call
ptrace could bypass seccomp restrictions.
To address (1), (2), and (3), we include preprocessor checks to identify the
native architecture value, and reject all system calls that don't use the
native architecture.
To address (4), we use the AC_C_BIGENDIAN autoconf check to conditionally
define WORDS_BIGENDIAN, and match up the proper portions of any uint64 we test
for with the value in the accumulator being tested against.
To address (5) and (6), we use system call pinning. That is, we hardcode a
snapshot of all the valid system call numbers at the time of writing, and
reject any system call numbers not in the recorded set. A set is recorded for
every architecture+ABI combo, and the native one is chosen at compile-time.
This ensures that not only are non-native architectures rejected, but so are
non-native ABIs. For the sake of conciseness, we represent these sets as sets
of disjoint ranges. Due to (7), checking each range in turn could add a lot
of overhead to each system call, so we instead binary search through the
ranges. Due to (8), this binary search has to be fully unrolled, so we do
that too.
It can be tedious and error-prone to manually produce the syscall ranges by
looking at linux's *.tbl files, since the gaps are often small and
uncommented. To address this, a script, build-aux/extract-syscall-ranges.sh,
is added that will produce them given a *.tbl filename and an ABI regex (some
tables seem to abuse the ABI field with strange values like "memfd_secret").
Note that producing the final values still requires looking at the proper
asm/unistd.h file to find any private numbers and to identify any offsets and
ABI variants used.
(10) used to have no good solution, but in the past decade most architectures
have gained dedicated system call alternatives to at least socketcall, so we
can (hopefully) just block it entirely.
To address (13), we block ptrace also.
* build-aux/extract-syscall-ranges.sh: new script.
* Makefile.am (EXTRA_DIST): register it.
* config-daemon.ac: use AC_C_BIGENDIAN.
* nix/libutil/spawn.cc (setNoNewPrivsAction, addSeccompFilterAction): new
functions.
* nix/libutil/spawn.hh (setNoNewPrivsAction, addSeccompFilterAction): new
declarations.
(SpawnContext)[setNoNewPrivs, addSeccompFilter]: new fields.
* nix/libutil/seccomp.hh: new header file.
* nix/libutil/seccomp.cc: new file.
* nix/local.mk (libutil_a_SOURCES, libutil_headers): register them.
* nix/libstore/build.cc (slirpSeccompFilter, writeSeccompFilterDot):
new functions.
(spawnSlirp4netns): use them, set seccomp filter for slirp4netns.
Change-Id: Ic92c7f564ab12596b87ed0801b22f88fbb543b95
Signed-off-by: John Kehayias <john.kehayias@protonmail.com>
This commit is contained in:
Reepca Russelstein
John Kehayias
parent
fb42611b8f
commit
c659f977bb
9 changed files with 738 additions and 2 deletions
222
nix/libutil/seccomp.hh
Normal file
222
nix/libutil/seccomp.hh
Normal file
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@ -0,0 +1,222 @@
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#pragma once
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#include "util.hh"
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#include <linux/audit.h> /* For AUDIT_ARCH_* */
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#include <linux/seccomp.h>
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#include <linux/filter.h>
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/* This file provides two preprocessor macros (among other things):
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1. AUDIT_ARCH_NATIVE, which evaluates to whichever of the AUDIT_ARCH_*
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values best represents the target system. Linux's internal headers have
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a SECCOMP_ARCH_NATIVE since 2020, but it's not user-visible. Detection
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of this is based on src/arch.c in libseccomp.
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2. NATIVE_SYSCALL_RANGES, an array initializer for an array of two-element
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objects, the first of which is an integral number representing the
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start (inclusive) of a range of valid syscall numbers, and the second
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of which is an integral number representing the end (inclusive) of that
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range of valid syscall numbers. The ranges provided are all
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non-overlapping and strictly ascending (that is, the start of a range is
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strictly higher than any of the numbers in any of the ranges that
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precede it). All numbers involved fit into a long.
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These ranges were generated from the various syscall.tbl,
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syscall_32.tbl, and syscall_64.tbl files lying around in the linux
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kernel source. Some were derived from
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include/uapi/asm-generic/unistd.h. The kernel source used was commit
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b3ee1e460951 of https://github.com/torvalds/linux.git, read on
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2025-04-23. Not all of the gaps in the files have any comments pointing
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them out, so I recommend using build-aux/extract-syscall-ranges.sh for
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the *.tbl files.
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The intent behind saving these ranges is to be able to use a
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default-allow seccomp policy that nevertheless disallows future
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syscalls. This ensures that our security analysis can work with a
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static, well-defined set of system calls that won't grow in the future
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unless someone explicitly revisits the system call tables to consider
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the implications of the new additions. */
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/* Both ends are inclusive. Some of the .tbl files use strange entries for
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* the "abi" field, check arch/$ARCH/kernel/Makefile.syscalls to see what it
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* specifies for syscall_abis_32 and syscall_abis_64 in addition to 32 or 64
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* and "common" (added in Makefile.asm-headers). Also check what, if
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* anything, the makefile uses as the --offset flag to syscallhdr.sh. And
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* look at arch/$ARCH/include/uapi/asm/unistd.h to see what value the offset
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* takes in what configurations. */
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#ifndef AUDIT_ARCH_NATIVE
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#if __i386__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_I386
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#define NATIVE_SYSCALL_RANGES { {0, 221}, {224, 250}, {252, 284}, {286, 386}, \
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{393, 414}, {416, 466} }
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#elif __x86_64__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_X86_64
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#ifdef __ILP32__
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#include <asm/unistd.h>
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#define X32RANGE(low, high) { (low | __X32_SYSCALL_BIT), (high | __X32_SYSCALL_BIT) }
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#define NATIVE_SYSCALL_RANGES \
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{ X32RANGE(0, 12), X32RANGE(14, 14), X32RANGE(17, 18), X32RANGE(21, 44), X32RANGE(48, 53), \
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X32RANGE(56, 58), X32RANGE(60, 100), X32RANGE(102, 126), X32RANGE(130, 130), \
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X32RANGE(132, 133), X32RANGE(135, 155), X32RANGE(157, 173), X32RANGE(175, 176), \
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X32RANGE(179, 179), X32RANGE(181, 204), X32RANGE(207, 208), X32RANGE(210, 210), \
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X32RANGE(212, 213), X32RANGE(216, 221), X32RANGE(223, 235), X32RANGE(237, 243), \
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X32RANGE(245, 245), X32RANGE(248, 272), X32RANGE(275, 277), X32RANGE(280, 294), \
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X32RANGE(298, 298), X32RANGE(300, 306), X32RANGE(308, 309), X32RANGE(312, 321), \
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X32RANGE(323, 326), X32RANGE(329, 335), X32RANGE(424, 466), X32RANGE(512, 547) }
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#else
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#define NATIVE_SYSCALL_RANGES { {0, 335}, {424, 466} }
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#endif
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#elif __arm__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_ARM
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/* Note: there are at present 6 extra ARM syscall numbers not listed in
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arch/arm/tools/syscall.tbl, namely __ARM_NR_breakpoint through
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__ARM_NR_get_tls. */
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#ifdef __ARM_EABI__
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#include <asm/unistd.h>
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 6}, {8, 12}, {14, 16}, {19, 21}, {23, 24}, {26, 26}, {29, 29}, \
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{33, 34}, {36, 43}, {45, 47}, {49, 52}, {54, 55}, {57, 57}, {60, 67}, \
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{70, 75}, {77, 81}, {83, 83}, {85, 88}, {91, 97}, {99, 100}, {103, 108}, \
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{111, 111}, {114, 116}, {118, 122}, {124, 126}, {128, 129}, {131, 136}, \
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{138, 165}, {168, 187}, {190, 221}, {224, 253}, {256, 401}, {403, 414}, \
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{416, 446}, {448, 466}, {(__ARM_NR_BASE + 1), (__ARM_NR_BASE + 6)} }
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#else
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#include <asm/unistd.h>
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#define OABIRANGE(low, high) { (low | __NR_OABI_SYSCALL_BASE), (high | __NR_OABI_SYSCALL_BASE) }
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#define NATIVE_SYSCALL_RANGES \
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{ OABIRANGE(0, 6), OABIRANGE(8, 16), OABIRANGE(19, 27), OABIRANGE(29, 30), \
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OABIRANGE(33, 34), OABIRANGE(36, 43), OABIRANGE(45, 47), OABIRANGE(49, 52), \
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OABIRANGE(54, 55), OABIRANGE(57, 57), OABIRANGE(60, 67), OABIRANGE(70, 83), \
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OABIRANGE(85, 97), OABIRANGE(99, 100), OABIRANGE(102, 108), \
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OABIRANGE(111, 111), OABIRANGE(113, 122), OABIRANGE(124, 126), \
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OABIRANGE(128, 129), OABIRANGE(131, 136), OABIRANGE(138, 165), \
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OABIRANGE(168, 187), OABIRANGE(190, 221), OABIRANGE(224, 253), \
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OABIRANGE(256, 401), OABIRANGE(403, 414), OABIRANGE(416, 446), \
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OABIRANGE(448, 466), {(__ARM_NR_BASE + 1), (__ARM_NR_BASE + 6)} }
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#endif
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#elif __aarch64__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_AARCH64
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/* extract-syscall-ranges.sh $LINUXSOURCE/arch/arm64/tools/syscall_64.tbl \
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'64|renameat|rlimit|memfd_secret'
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the extra ABIs are taken from arch/arm64/kernel/Makefile.syscalls and
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scripts/Makefile.asm-headers */
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#define NATIVE_SYSCALL_RANGES { {0, 243}, {260, 294}, {424, 466} }
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/* To my knowledge there is no x32 equivalent for aarch64 in mainline linux */
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#elif __mips__ && _MIPS_SIM == _MIPS_SIM_ABI32
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/* o32 abi in both endianness cases */
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#include <asm/unistd.h>
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#define SYSRANGE(low, high) {(low) + __NR_Linux, (high) + __NR_Linux}
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#define NATIVE_SYSCALL_RANGES \
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{ SYSRANGE(0, 278), SYSRANGE(280, 368), SYSRANGE(393, 414), \
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SYSRANGE(416, 446), SYSRANGE(448, 466) }
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#if __MIPSEB__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPS;
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#elif __MIPSEL__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPSEL
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#endif
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#elif __mips__ && _MIPS_SIM == _MIPS_SIM_ABI64
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/* n64 abi in both endianness cases */
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#include <asm/unistd.h>
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#define SYSRANGE(low, high) {(low) + __NR_Linux, (high) + __NR_Linux}
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#define NATIVE_SYSCALL_RANGES \
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{ SYSRANGE(0, 237), SYSRANGE(239, 328), SYSRANGE(424, 446), SYSRANGE(448, 466) }
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#if __MIPSEB__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPS64
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#elif __MIPSEL__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPSEL64
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#endif /* _MIPS_SIM_ABI64 */
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#elif __mips__ && _MIPS_SIM == _MIPS_SIM_NABI32
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/* n32 abi in both endianness cases */
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#include <asm/unistd.h>
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#define SYSRANGE(low, high) {(low) + __NR_Linux, (high) + __NR_Linux}
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#define NATIVE_SYSCALL_RANGES \
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{ SYSRANGE(0, 241), SYSRANGE(243, 332), SYSRANGE(403, 414), \
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SYSRANGE(416, 446), SYSRANGE(448, 466) }
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#if __MIPSEB__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPS64N32
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#elif __MIPSEL__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_MIPSEL64N32
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#endif /* _MIPS_SIM_NABI32 */
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#elif __hppa64__ /* hppa64 must be checked before hppa */
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 101}, {103, 126}, {128, 129}, {131, 136}, {138, 166}, \
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{168, 168}, {170, 195}, {198, 202}, {206, 212}, {215, 219}, \
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{222, 262}, {264, 302}, {304, 356}, {424, 446}, {448, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_PARISC64
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#elif __hppa__
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 101}, {103, 126}, {128, 129}, {131, 136}, {138, 166}, \
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{168, 168}, {170, 195}, {198, 202}, {206, 212}, {215, 219}, \
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{222, 262}, {264, 302}, {304, 356}, {403, 414}, {416, 446}, {448, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_PARISC
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#elif __PPC64__
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 191}, {198, 203}, {205, 223}, {225, 225}, {227, 253}, \
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{255, 256}, {258, 365}, {378, 388}, {392, 402}, {424, 446}, {448, 466} }
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#ifdef __BIG_ENDIAN__
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_PPC64
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#else
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_PPC64LE
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#endif
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#elif __PPC__
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 223}, {225, 256}, {258, 365}, {378, 388}, {393, 414}, \
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{416, 446}, {448, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_PPC
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#elif __s390x__ /* s390x must be checked before s390 */
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#define NATIVE_SYSCALL_RANGES \
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{ {1, 12}, {14, 15}, {19, 22}, {26, 27}, {29, 30}, {33, 34}, \
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{36, 43}, {45, 45}, {48, 48}, {51, 52}, {54, 55}, {57, 57}, \
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{60, 67}, {72, 75}, {77, 79}, {83, 83}, {85, 94}, {96, 97}, \
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{99, 100}, {102, 108}, {110, 112}, {114, 122}, {124, 137}, \
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{141, 163}, {167, 169}, {172, 181}, {183, 191}, {198, 220}, \
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{222, 222}, {224, 241}, {243, 262}, {265, 386}, {392, 402}, {424, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_S390X
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#elif __s390__
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#define NATIVE_SYSCALL_RANGES \
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{ {1, 16}, {19, 27}, {29, 30}, {33, 34}, {36, 43}, {45, 52}, \
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{54, 55}, {57, 57}, {60, 67}, {70, 81}, {83, 83}, {85, 97}, \
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{99, 108}, {110, 112}, {114, 122}, {124, 165}, {167, 241}, \
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{243, 262}, {264, 386}, {393, 414}, {416, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_S390
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#elif __riscv && __riscv_xlen == 64
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#define NATIVE_SYSCALL_RANGES { {0, 37}, {39, 243}, {258, 294}, {424, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_RISCV64
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#elif __riscv && __riscv_xlen == 32
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#define NATIVE_SYSCALL_RANGES \
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{ {0, 3}, {5, 37}, {39, 71}, {74, 78}, {81, 85}, {89, 97}, {99, 100}, \
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{102, 107}, {109, 109}, {111, 111}, {116, 126}, {128, 136}, {138, 162}, \
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{165, 168}, {172, 181}, {184, 191}, {193, 242}, {258, 259}, {261, 265}, \
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{267, 291}, {293, 294}, {403, 414}, {416, 466} }
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#define AUDIT_ARCH_NATIVE AUDIT_ARCH_RISCV32
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#else
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#error cannot determine which AUDIT_ARCH_* value to use for AUDIT_ARCH_NATIVE
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#endif
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#else
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#ifndef NATIVE_SYSCALL_RANGES
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/* Fall back to default-allow if the user specified (with
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-DAUDIT_ARCH_NATIVE=...) an arch but not NATIVE_SYSCALL_RANGES */
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#define NATIVE_SYSCALL_RANGES {}
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#endif
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#endif /* #ifndef AUDIT_ARCH_NATIVE */
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namespace nix {
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struct Uint32RangeAction {
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uint32_t low; /* inclusive */
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uint32_t high; /* inclusive */
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std::vector<struct sock_filter> instructions;
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};
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std::vector<struct sock_filter> rangeActionsToFilter(std::vector<Uint32RangeAction> & ranges);
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std::vector<struct sock_filter>
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seccompMatchu64(std::vector<struct sock_filter> & out,
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uint64_t value,
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std::vector<struct sock_filter> instructions,
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uint32_t offset);
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}
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