[PATCH] ARM: ptrace: Restore syscall skipping and restart while tracing

[Kees Cook]

Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
thread_info->abi_syscall"), the seccomp selftests "syscall_errno",
"syscall_faked", and "syscall_restart" have been broken. This was
related to two issues:

- 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().

- the syscall entry label "local_restart" is used for resuming syscalls
  interrupted by signals, but the updated syscall number (in scno) was
  not being stored in current_thread_info()->abi_syscall, causing traced
  syscall restarting to fail.

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

Move the AEABI-only assignment of current_thread_info()->abi_syscall
after the "local_restart" label to allow tracers to survive syscall
restarting.

Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxxxx>
Cc: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Note that I haven't tested OABI at all, and AEABI+OABI_COMPAT doesn't
work with seccomp. I booted an AEABI system under AEABI+OABI_COMPAT,
but I wasn't able to test tracing...
---
 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/entry-common.S | 5 +++--
 arch/arm/kernel/ptrace.c       | 5 +++--
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index dfeed440254a..fe4326d938c1 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h
@@ -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;
 }
 
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index bcc4c9ec3aa4..08bd624e4c6f 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -246,8 +246,6 @@ ENTRY(vector_swi)
 	bic	scno, scno, #0xff000000		@ mask off SWI op-code
 	str	scno, [tsk, #TI_ABI_SYSCALL]
 	eor	scno, scno, #__NR_SYSCALL_BASE	@ check OS number
-#else
-	str	scno, [tsk, #TI_ABI_SYSCALL]
 #endif
 	/*
 	 * Reload the registers that may have been corrupted on entry to
@@ -256,6 +254,9 @@ ENTRY(vector_swi)
  TRACE(	ldmia	sp, {r0 - r3}		)
 
 local_restart:
+#if defined(CONFIG_AEABI) && !defined(CONFIG_OABI_COMPAT)
+	str	scno, [tsk, #TI_ABI_SYSCALL]	@ store scno for syscall restart
+#endif
 	ldr	r10, [tsk, #TI_FLAGS]		@ check for syscall tracing
 	stmdb	sp!, {r4, r5}			@ push fifth and sixth args
 
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 2d8e2516906b..fef32d73f912 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -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