Patch "ARM: ptrace: Restore syscall skipping for tracers" has been added to the 6.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    ARM: ptrace: Restore syscall skipping for tracers

to the 6.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm-ptrace-restore-syscall-skipping-for-tracers.patch
and it can be found in the queue-6.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit a605f0b49fbd521349da18fc16fb71322a3ea3bc
Author: Kees Cook <keescook@xxxxxxxxxxxx>
Date:   Thu Aug 10 12:54:19 2023 -0700

    ARM: ptrace: Restore syscall skipping for tracers
    
    [ Upstream commit 4697b5848bd933f68ebd04836362c8de0cacaf71 ]
    
    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@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")
    Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
    Link: https://lore.kernel.org/r/20230810195422.2304827-2-keescook@xxxxxxxxxxxx
    Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index dfeed440254a8..fe4326d938c18 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 2d8e2516906b6..fef32d73f9120 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;