ARM: ptrace: Restore syscall skipping for tracers

[ Upstream commit 4697b58 ] Since commit 4e57a4d ("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. Cc: Russell King <[email protected]> Cc: Arnd Bergmann <[email protected]> Cc: Lecopzer Chen <[email protected]> Cc: Oleg Nesterov <[email protected]> Cc: [email protected] Fixes: 4e57a4d ("ARM: 9107/1: syscall: always store thread_info->abi_syscall") Reviewed-by: Arnd Bergmann <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Kees Cook <[email protected]> Signed-off-by: Sasha Levin <[email protected]>
OpenAMP · Sep 13, 2023 · e8e4d22 · e8e4d22
1 parent f8e97a0
commit e8e4d22
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Showing 2 changed files with 6 additions and 2 deletions.
diff --git 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/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;