On Sun, Feb 02, 2025 at 08:29:20AM -0800, Eyal Birger wrote:

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 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.

Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe") Reported-by: Rafael Buchbinder rafi@rbk.io Link: https://lore.kernel.org/lkml/CAHsH6Gs3Eh8DFU0wq58c_LF8A4_+o6z456J7BidmcVY2Aq... Link: https://lore.kernel.org/lkml/20250121182939.33d05470@gandalf.local.home/T/#m... Link: https://lore.kernel.org/lkml/20250128145806.1849977-1-eyal.birger@gmail.com/ Cc: stable@vger.kernel.org Signed-off-by: Eyal Birger eyal.birger@gmail.com

---

v3: no change - deferring 32bit compat handling as there aren't plans to support this syscall in compat mode. v2: use action_cache bitmap and mode1 array to check the syscall

---

kernel/seccomp.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c index f59381c4a2ff..09b6f8e6db51 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

•   )
  

• 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)

I minimized the above to:

@@ -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 +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 -1, /* negative terminated */ }; -- 2.43.0

-Kees