FAILED: patch "[PATCH] arm64: ptrace: Fix seccomp of traced syscall -1 (NO_SYSCALL)" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From df84fe94708985cdfb78a83148322bcd0a699472 Mon Sep 17 00:00:00 2001
From: Timothy E Baldwin <[email protected]>
Date: Sat, 16 Jan 2021 15:18:54 +0000
Subject: [PATCH] arm64: ptrace: Fix seccomp of traced syscall -1 (NO_SYSCALL)

Since commit f086f67485c5 ("arm64: ptrace: add support for syscall emulation"), if system call number -1 is called and the process is being traced with PTRACE_SYSCALL, for example by strace, the seccomp check is skipped and -ENOSYS is returned unconditionally (unless altered by the tracer) rather than carrying out action specified in the seccomp filter.

The consequence of this is that it is not possible to reliably strace a seccomp based implementation of a foreign system call interface in which r7/x8 is permitted to be -1 on entry to a system call.

Also trace_sys_enter and audit_syscall_entry are skipped if a system call is skipped.

Fix by removing the in_syscall(regs) check restoring the previous behaviour which is like AArch32, x86 (which uses generic code) and everything else.

index 3d5c8afca75b..170f42fd6101 100644
@@ -1797,7 +1797,7 @@ int syscall_trace_enter(struct pt_regs *regs)
 
 	if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
-		if (!in_syscall(regs) || (flags & _TIF_SYSCALL_EMU))
+		if (flags & _TIF_SYSCALL_EMU)
 			return NO_SYSCALL;
 	}
 

Hi Timothy,

Looks like simple context mismatch here, the change itself applies easily. Would you be able to send the backport for v5.4 separately ?

Regards,
Sudeep

commit df84fe94708985cdfb78a83148322bcd0a699472 upstream.

Backported to Linux 5.4 by changing "return NO_SYSCALL" to "return -1" in patch context.

Since commit f086f67485c5 ("arm64: ptrace: add support for syscall emulation"), if system call number -1 is called and the process is being traced with PTRACE_SYSCALL, for example by strace, the seccomp check is skipped and -ENOSYS is returned unconditionally (unless altered by the tracer) rather than carrying out action specified in the seccomp filter.

The consequence of this is that it is not possible to reliably strace a seccomp based implementation of a foreign system call interface in which r7/x8 is permitted to be -1 on entry to a system call.

Also trace_sys_enter and audit_syscall_entry are skipped if a system call is skipped.

Fix by removing the in_syscall(regs) check restoring the previous behaviour which is like AArch32, x86 (which uses generic code) and everything else.

---
 arch/arm64/kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 30b877f8b85e..0cfd68577489 100644
@@ -1844,7 +1844,7 @@ int syscall_trace_enter(struct pt_regs *regs)
 
 	if (flags & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE)) {
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
-		if (!in_syscall(regs) || (flags & _TIF_SYSCALL_EMU))
+		if (flags & _TIF_SYSCALL_EMU)
 			return -1;
 	}
 

2.27.0

Thanks for the backport, now queued up.

greg k-h