Hi,

Thanks for the review!

On Tue, Jan 28, 2025 at 5:41 PM Kees Cook [off-list ref] wrote:

On Tue, Jan 28, 2025 at 06:58:06AM -0800, Eyal Birger wrote:

quoted

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?

My understanding is that they'd be able to do so, but use the int3 trap
instead of the uretprobe syscall.

quoted

[...]

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.

I'm not sure I got this 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

I don't know if implementing uretprobe syscall for compat binaries is
planned or makes sense - I'd appreciate Jiri's and others opinion on that.
That said, I don't mind adding this code for the sake of future proofing.

Instead of doing a function rename dance, I think you can just stick
the above into seccomp_is_const_allow() after the WARN().

My motivation for the renaming dance was that you mentioned we might add
new syscalls to this as well, so I wanted to avoid cluttering the existing
function which seems to be well defined.

Also please add a KUnit tests to cover this in
tools/testing/selftests/seccomp/seccomp_bpf.c

I think this would mean that this test suite would need to run as
privileged. Is that Ok? or maybe it'd be better to have a new suite?

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)

Ok.

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().

I don't know if uretprobe syscall is expected to run on mips. Personally
I'd avoid adding this dead code.

(You can see why I really dislike having policy baked into seccomp!)

I definitely understand :)

quoted

+        )
+             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.

It seems to me very unlikely. BTW, when I tested the "strict" mode change
my program was killed by seccomp. The reason wasn't the uretprobe syscall
(which I added to the list), it was actually the exit_group syscall which
libc uses instead of the exit syscall.

Thanks again,
Eyal.