[SRU][O][PATCH 1/1] seccomp: passthrough uretprobe systemcall without filtering

[Stefan Bader]

From: Eyal Birger <eyal.birger at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/2098759

When attaching uretprobes to processes running inside docker, the attached
process is segfaulted when encountering the retprobe.

The reason is that now that uretprobe is a system call the default seccomp
filters in docker block it as they only allow a specific set of known
syscalls. This is true for other userspace applications which use seccomp
to control their syscall surface.

Since uretprobe is a "kernel implementation detail" system call which is
not used by userspace application code directly, it is impractical and
there's very little point in forcing all userspace applications to
explicitly allow it in order to avoid crashing tracked processes.

Pass this systemcall through seccomp without depending on configuration.

Note: uretprobe is currently only x86_64 and isn't expected to ever be
supported in i386.

Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
Reported-by: Rafael Buchbinder <rafi at rbk.io>
Closes: https://lore.kernel.org/lkml/CAHsH6Gs3Eh8DFU0wq58c_LF8A4_+o6z456J7BidmcVY2AqOnHQ@mail.gmail.com/
Link: https://lore.kernel.org/lkml/20250121182939.33d05470@gandalf.local.home/T/#me2676c378eff2d6a33f3054fed4a5f3afa64e65b
Link: https://lore.kernel.org/lkml/20250128145806.1849977-1-eyal.birger@gmail.com/
Cc: stable at vger.kernel.org
Signed-off-by: Eyal Birger <eyal.birger at gmail.com>
Link: https://lore.kernel.org/r/20250202162921.335813-2-eyal.birger@gmail.com
[kees: minimized changes for easier backporting, tweaked commit log]
Signed-off-by: Kees Cook <kees at kernel.org>

Reported-by: Eyal Birger <eyal.birger at gmail.com>
(cherry picked from commit cf6cb56ef24410fb5308f9655087f1eddf4452e6)
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 kernel/seccomp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index f59381c4a2ff..7bbb408431eb 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -749,6 +749,15 @@ static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog,
 	if (WARN_ON_ONCE(!fprog))
 		return false;
 
+	/* Our single exception to filtering. */
+#ifdef __NR_uretprobe
+#ifdef SECCOMP_ARCH_COMPAT
+	if (sd->arch == SECCOMP_ARCH_NATIVE)
+#endif
+		if (sd->nr == __NR_uretprobe)
+			return true;
+#endif
+
 	for (pc = 0; pc < fprog->len; pc++) {
 		struct sock_filter *insn = &fprog->filter[pc];
 		u16 code = insn->code;
@@ -1023,6 +1032,9 @@ 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
 	-1, /* negative terminated */
 };
 
-- 
2.43.0