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

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

[Vineet Gupta]

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

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

[Shahab Vahedi]

On 4/23/21 9:52 PM, Vineet Gupta wrote:
> 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

Same problem exists in "syscall Tracing" section:

; Do the Sys Call as we normally would.
; Validate the Sys Call number
cmp     r8,  NR_syscalls
mov.hi  r0, -ENOSYS
bhi     tracesys_exit


-- 
Shahab

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

[Vineet Gupta]

On 4/29/21 4:39 AM, Shahab Vahedi wrote:
> On 4/23/21 9:52 PM, Vineet Gupta wrote:
>> 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
> Same problem exists in "syscall Tracing" section:
>
> ; Do the Sys Call as we normally would.
> ; Validate the Sys Call number
> cmp     r8,  NR_syscalls
> mov.hi  r0, -ENOSYS
> bhi     tracesys_exit

Thanks for spotting this. I'll fix this up and repush (after the ongoing 
merge window).

-Vineet