Document some more-or-less surprising things about seccomp. I'm not sure whether changing the example code like that is a good idea - maybe that part of the patch should be left out? Demo code for the X32 issue: https://gist.github.com/thejh/c5b670a816bbb9791a6d Demo code for full 64bit registers being visible in seccomp if the i386 ABI is used on a 64bit system: https://gist.github.com/thejh/c37b27aefc44ab775db5 --- man2/seccomp.2 | 72 +++++++++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 66 insertions(+), 6 deletions(-) diff --git a/man2/seccomp.2 b/man2/seccomp.2 index 702ceb8..307a408 100644 --- a/man2/seccomp.2 +++ b/man2/seccomp.2 @@ -223,6 +223,47 @@ struct seccomp_data { .fi .in +Because the numbers of system calls vary between architectures and +some architectures (e.g. X86-64) allow user-space code to use +the calling conventions of multiple architectures, it is usually +necessary to verify the value of the +.IR arch +field. + +The +.IR arch +field is not unique for all calling conventions. The X86-64 ABI and +the X32 ABI both use +.BR AUDIT_ARCH_X86_64 +as +.IR arch , +and they run on the same processors. Instead, the mask +.BR __X32_SYSCALL_BIT +is used on the system call number to tell the two ABIs apart. +This means that in order to create a seccomp-based +blacklist for system calls performed through the X86-64 ABI, +it is necessary to not only check that +.IR arch +equals +.BR AUDIT_ARCH_X86_64 , +but also to explicitly reject all syscalls that contain +.BR __X32_SYSCALL_BIT +in +.IR nr . + +When checking values from +.IR args +against a blacklist, keep in mind that arguments are often +silently truncated before being processed, but after the seccomp +check. For example, this happens if the i386 ABI is used on an +X86-64 kernel: Although the kernel will normally not look beyond +the 32 lowest bits of the arguments, the values of the full +64-bit registers will be present in the seccomp data. A less +surprising example is that if any 64-bit ABI is used to perform +a syscall that takes an argument of type int, the +more-significant half of the argument register is ignored by +the syscall, but visible in the seccomp data. + A seccomp filter returns a 32-bit value consisting of two parts: the most significant 16 bits (corresponding to the mask defined by the constant @@ -584,38 +625,57 @@ cecilia #include <linux/seccomp.h> #include <sys/prctl.h> +#define X32_SYSCALL_BIT 0x40000000 + static int install_filter(int syscall_nr, int t_arch, int f_errno) { + int forbidden_bitmask = 0; + /* assume that AUDIT_ARCH_X86_64 means the normal X86-64 ABI */ + if (t_arch == AUDIT_ARCH_X86_64) + forbidden_bitmask = X32_SYSCALL_BIT; + struct sock_filter filter[] = { /* [0] Load architecture from 'seccomp_data' buffer into accumulator */ BPF_STMT(BPF_LD | BPF_W | BPF_ABS, (offsetof(struct seccomp_data, arch))), - /* [1] Jump forward 4 instructions if architecture does not + /* [1] Jump forward 7 instructions if architecture does not match 't_arch' */ - BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 4), + BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, t_arch, 0, 7), /* [2] Load system call number from 'seccomp_data' buffer into accumulator */ BPF_STMT(BPF_LD | BPF_W | BPF_ABS, (offsetof(struct seccomp_data, nr))), - /* [3] Jump forward 1 instruction if system call number + /* [3] Determine ABI from system call number - only needed for X86-64 + in blacklist usecases */ + BPF_STMT(BPF_ALU | BPF_AND | BPF_K, forbidden_bitmask), + + /* [4] Check ABI - only needed for X86-64 in blacklist usecases */ + BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, 0, 0, 4), + + /* [5] Load system call number from 'seccomp_data' buffer into + accumulator */ + BPF_STMT(BPF_LD | BPF_W | BPF_ABS, + (offsetof(struct seccomp_data, nr))), + + /* [6] Jump forward 1 instruction if system call number does not match 'syscall_nr' */ BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscall_nr, 0, 1), - /* [4] Matching architecture and system call: don't execute + /* [7] Matching architecture and system call: don't execute the system call, and return 'f_errno' in 'errno' */ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ERRNO | (f_errno & SECCOMP_RET_DATA)), - /* [5] Destination of system call number mismatch: allow other + /* [8] Destination of system call number mismatch: allow other system calls */ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW), - /* [6] Destination of architecture mismatch: kill process */ + /* [9] Destination of architecture mismatch: kill process */ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL), }; -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
