On Tue, Jan 28, 2025 at 06:58:06AM -0800, Eyal Birger wrote:
> Note: uretprobe isn't supported in i386 and __NR_ia32_rt_tgsigqueueinfo
> uses the same number as __NR_uretprobe so the syscall isn't forced in the
> compat bitmap.
So a 64-bit tracer cannot use uretprobe on a 32-bit process? Also is
uretprobe strictly an x86_64 feature?
> [...]
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 385d48293a5f..23b594a68bc0 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -734,13 +734,13 @@ seccomp_prepare_user_filter(const char __user *user_filter)
>
> #ifdef SECCOMP_ARCH_NATIVE
> /**
> - * seccomp_is_const_allow - check if filter is constant allow with given data
> + * seccomp_is_filter_const_allow - check if filter is constant allow with given data
> * @fprog: The BPF programs
> * @sd: The seccomp data to check against, only syscall number and arch
> * number are considered constant.
> */
> -static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog,
> - struct seccomp_data *sd)
> +static bool seccomp_is_filter_const_allow(struct sock_fprog_kern *fprog,
> + struct seccomp_data *sd)
> {
> unsigned int reg_value = 0;
> unsigned int pc;
> @@ -812,6 +812,21 @@ static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog,
> return false;
> }
>
> +static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog,
> + struct seccomp_data *sd)
> +{
> +#ifdef __NR_uretprobe
> + if (sd->nr == __NR_uretprobe
> +#ifdef SECCOMP_ARCH_COMPAT
> + && sd->arch != SECCOMP_ARCH_COMPAT
> +#endif
I don't like this because it's not future-proof enough. __NR_uretprobe
may collide with other syscalls at some point. And if __NR_uretprobe_32
is ever implemented, the seccomp logic will be missing. I think this
will work now and in the future:
#ifdef __NR_uretprobe
# ifdef SECCOMP_ARCH_COMPAT
if (sd->arch == SECCOMP_ARCH_COMPAT) {
# ifdef __NR_uretprobe_32
if (sd->nr == __NR_uretprobe_32)
return true;
# endif
} else
# endif
if (sd->nr == __NR_uretprobe)
return true;
#endif
Instead of doing a function rename dance, I think you can just stick
the above into seccomp_is_const_allow() after the WARN().
Also please add a KUnit tests to cover this in
tools/testing/selftests/seccomp/seccomp_bpf.c
With at least these cases combinations below. Check each of:
- not using uretprobe passes
- using uretprobe passes (and validates that uretprobe did work)
in each of the following conditions:
- default-allow filter
- default-block filter
- filter explicitly blocking __NR_uretprobe and nothing else
- filter explicitly allowing __NR_uretprobe (and only other
required syscalls)
Hm, is uretprobe expected to work on mips? Because if so, you'll need to
do something similar to the mode1 checking in the !SECCOMP_ARCH_NATIVE
version of seccomp_cache_check_allow().
(You can see why I really dislike having policy baked into seccomp!)
> + )
> + return true;
> +#endif
> +
> + return seccomp_is_filter_const_allow(fprog, sd);
> +}
> +
> static void seccomp_cache_prepare_bitmap(struct seccomp_filter *sfilter,
> void *bitmap, const void *bitmap_prev,
> size_t bitmap_size, int arch)
> @@ -1023,6 +1038,9 @@ static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
> */
> static const int mode1_syscalls[] = {
> __NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
> +#ifdef __NR_uretprobe
> + __NR_uretprobe,
> +#endif
It'd be nice to update mode1_syscalls_32 with __NR_uretprobe_32 even
though it doesn't exist. (Is it _never_ planned to be implemented?) But
then, maybe the chances of a compat mode1 seccomp process running under
uretprobe is vanishingly small.
> -1, /* negative terminated */
> };
>
> --
> 2.43.0
>
-Kees
--
Kees Cook
