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.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store thread_info->abi_syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/arm/include/asm/syscall.h | 3 +++
 arch/arm/kernel/ptrace.c       | 5 +++--
 2 files changed, 6 insertions(+), 2 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/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

On Thu, Aug 10, 2023, at 21:54, Kees Cook wrote:
> 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.
>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Arnd Bergmann <arnd@kernel.org>
> Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store 
> thread_info->abi_syscall")
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>