Re: [syzbot] [zstd] UBSAN: array-index-out-of-bounds in FSE_decompress_wksp_body_bmi2

[Nick Terrell]

> On Oct 12, 2023, at 4:13 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
> !-------------------------------------------------------------------|
>  This Message Is From an External Sender
> 
> |-------------------------------------------------------------------!
> 
> On Thu, Oct 12, 2023 at 07:55:55PM +0000, Nick Terrell wrote:
>> 
>>> On Oct 9, 2023, at 1:29 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> 
>>> !-------------------------------------------------------------------|
>>> This Message Is From an External Sender
>>> 
>>> |-------------------------------------------------------------------!
>>> 
>>> On Sat, Oct 07, 2023 at 02:05:56PM -0700, Eric Biggers wrote:
>>>> Hi Nick,
>>>> 
>>>> On Wed, Aug 30, 2023 at 12:49:53AM -0700, syzbot wrote:
>>>>> UBSAN: array-index-out-of-bounds in lib/zstd/common/fse_decompress.c:345:30
>>>>> index 33 is out of range for type 'FSE_DTable[1]' (aka 'unsigned int[1]')
>>>> 
>>>> Zstandard needs to be converted to use C99 flex-arrays instead of length-1
>>>> arrays.  https://github.com/facebook/zstd/pull/3785 would fix this in upstream
>>>> Zstandard, though it doesn't work well with the fact that upstream Zstandard
>>>> supports C90.  Not sure how you want to handle this.
>>> 
>>> For the kernel, we just need:
>>> 
>>> diff --git a/lib/zstd/common/fse_decompress.c b/lib/zstd/common/fse_decompress.c
>>> index a0d06095be83..b11e87fff261 100644
>>> --- a/lib/zstd/common/fse_decompress.c
>>> +++ b/lib/zstd/common/fse_decompress.c
>>> @@ -312,7 +312,7 @@ size_t FSE_decompress_wksp(void* dst, size_t dstCapacity, const void* cSrc, size
>>> 
>>> typedef struct {
>>>    short ncount[FSE_MAX_SYMBOL_VALUE + 1];
>>> -    FSE_DTable dtable[1]; /* Dynamically sized */
>>> +    FSE_DTable dtable[]; /* Dynamically sized */
>>> } FSE_DecompressWksp;
>> 
>> Thanks Eric and Kees for the report and the fix! I am working on putting this
>> patch up now, just need to test the fix myself to ensure I can reproduce the
>> issue and the fix.
>> 
>> In your opinion does this worth trying to get this patch into v6.6, or should it
>> wait for v6.7?
> 
> For all these flex array conversions we're mostly on a "slow and steady"
> route, so there's no rush really. I think waiting for v6.7 is fine. If
> anyone ends up wanting to backport it, it should be pretty clean
> I imagine.

Sounds good, thanks for the context!

I’ll make sure the fix gets backported. Eric Biggers already has a PR up! [0]

[0] https://github.com/facebook/zstd/pull/3785

> Thanks for getting it all landed! :)
> 
> -Kees
> 
> -- 
> Kees Cook