[PATCH] ARC: entry: fix off-by-one error in syscall number validation

From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: linux-snps-arc@lists.infradead.org
Cc: linux-kernel@vger.kernel.org,
	Vineet Gupta <Vineet.Gupta1@synopsys.com>,
	Shahab Vahedi <Shahab.Vahedi@synopsys.com>
Subject: [PATCH] ARC: entry: fix off-by-one error in syscall number validation
Date: Fri, 23 Apr 2021 12:52:57 -0700	[thread overview]
Message-ID: <20210423195257.892560-1-vgupta@synopsys.com> (raw)

We have NR_syscall syscalls from [0 .. NR_syscall-1].
However the check for invalid syscall number is "> NR_syscall".
This off-by-one error erronesously allows "NR_syscall" itself as valid
and when passed causes out-of-bounds syscall-call table access leading
to crash.

This problem showed up when testing glibc 2.33 (v5.10 kernel capable,
includng faccessat2 syscall (439) against a v5.6 kernel with
NR_syscalls=439 (0 to 438). Due to the bug, 439 was not returned with
-ENOSYS but processed leading to a crash.

Link: https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/48
Reported-by: Shahab Vahedi <shahab@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
 arch/arc/kernel/entry.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index 1743506081da..aea9b558993d 100644
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -255,7 +255,7 @@ ENTRY(EV_Trap)
 	;============ Normal syscall case
 
 	; syscall num shd not exceed the total system calls avail
-	cmp     r8,  NR_syscalls
+	cmp     r8,  NR_syscalls - 1
 	mov.hi  r0, -ENOSYS
 	bhi     .Lret_from_system_call
 
-- 
2.25.1

