2021-04-23 19:53:42

by Vineet Gupta

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

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 <[email protected]>
Signed-off-by: Vineet Gupta <[email protected]>
---
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

2021-04-29 11:41:31

by Shahab Vahedi

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

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

2021-04-29 18:19:33

by Vineet Gupta

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

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