ARM: ptrace: Restore syscall skipping and restart while tracing

Hi,

Fix tracing on arm since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall:
always store thread_info->abi_syscall"). This was seen with the broken seccomp tests "syscall_errno", "syscall_faked", and "syscall_restart".

Thanks!

-Kees

v2:
- split fixes
- move scno store into the actual restart path
v1: https://lore.kernel.org/lkml/[email protected]

Kees Cook (2):
ARM: ptrace: Restore syscall restart tracing
ARM: ptrace: Restore syscall skipping for tracers

 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/entry-common.S | 1 +
 arch/arm/kernel/ptrace.c       | 5 +++--
 3 files changed, 7 insertions(+), 2 deletions(-)

2.34.1

Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall"), the seccomp selftests "syscall_restart" has been broken. This was caused by the restart syscall not being stored to "abi_syscall" during restart setup before branching to the "local_restart" label. Tracers would see the wrong syscall, and scno would get overwritten while returning from the TIF_WORK path. Add the missing store.

---
 arch/arm/kernel/entry-common.S | 1 +
 1 file changed, 1 insertion(+)

index bcc4c9ec3aa4..5c31e9de7a60 100644
@@ -90,6 +90,7 @@ slow_work_pending:
 	cmp	r0, #0
 	beq	no_work_pending
 	movlt	scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)
+	str	scno, [tsk, #TI_ABI_SYSCALL]	@ make sure tracers see update
 	ldmia	sp, {r0 - r6}			@ have to reload r0 - r6
 	b	local_restart			@ ... and off we go
 ENDPROC(ret_fast_syscall)

2.34.1

Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall"), the seccomp selftests "syscall_errno" and "syscall_faked" have been broken. Both seccomp and PTRACE depend on using the special value of "-1" for skipping syscalls. This value wasn't working because it was getting masked by __NR_SYSCALL_MASK in both PTRACE_SET_SYSCALL and get_syscall_nr().

Explicitly test for -1 in PTRACE_SET_SYSCALL and get_syscall_nr(), leaving it exposed when present, allowing tracers to skip syscalls again.

---
 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/ptrace.c       | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

index dfeed440254a..fe4326d938c1 100644
@@ -25,6 +25,9 @@ static inline int syscall_get_nr(struct task_struct *task,
 	if (IS_ENABLED(CONFIG_AEABI) && !IS_ENABLED(CONFIG_OABI_COMPAT))
 		return task_thread_info(task)->abi_syscall;
 
+	if (task_thread_info(task)->abi_syscall == -1)
+		return -1;
+
 	return task_thread_info(task)->abi_syscall & __NR_SYSCALL_MASK;
 }
 

index 2d8e2516906b..fef32d73f912 100644
@@ -783,8 +783,9 @@ long arch_ptrace(struct task_struct *child, long request,
 			break;
 
 		case PTRACE_SET_SYSCALL:
-			task_thread_info(child)->abi_syscall = data &
-							__NR_SYSCALL_MASK;
+			if (data != -1)
+				data &= __NR_SYSCALL_MASK;
+			task_thread_info(child)->abi_syscall = data;
 			ret = 0;
 			break;
 

2.34.1

I'd like to get this fixed in v6.5. Russell, do you have a fixes tree that you'll be sending Linus's way before the next -rc? I can send it via my seccomp-fixes tree if that'd be easier.

Thanks!

-Kees

Kees Cook

Applied to for-next/seccomp, thanks!

[1/2] ARM: ptrace: Restore syscall restart tracing

      https://git.kernel.org/kees/c/cf007647475b
[2/2] ARM: ptrace: Restore syscall skipping for tracers
      https://git.kernel.org/kees/c/4697b5848bd9

Take care,

Kees Cook