Re: [syzbot] [crypto?] general protection fault in scatterwalk_copychunks (5)

[Chengming Zhou]

On 2023/12/27 14:25, Barry Song wrote:
> On Wed, Dec 27, 2023 at 4:51 PM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>
>> On 2023/12/27 08:23, Nhat Pham wrote:
>>> On Tue, Dec 26, 2023 at 3:30 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>>>>
>>>> Again, sorry I was looking at the decompression side rather than the
>>>> compression side. The compression side does not even offer a safe
>>>> version of the compression function.
>>>> That seems to be dangerous. It seems for now we should make the zswap
>>>> roll back to 2 page buffer until we have a safe way to do compression
>>>> without overwriting the output buffers.
>>>
>>> Unfortunately, I think this is the way - at least until we rework the
>>> crypto/compression API (if that's even possible?).
>>> I still think the 2 page buffer is dumb, but it is what it is :(
>>
>> Hi,
>>
>> I think it's a bug in `scomp_acomp_comp_decomp()`, which doesn't use
>> the caller passed "src" and "dst" scatterlist. Instead, it uses its own
>> per-cpu "scomp_scratch", which have 128KB src and dst.
>>
>> When compression done, it uses the output req->dlen to copy scomp_scratch->dst
>> to our dstmem, which has only one page now, so this problem happened.
>>
>> I still don't know why the alg->compress(src, slen, dst, &dlen) doesn't
>> check the dlen? It seems an obvious bug, right?
>>
>> As for this problem in `scomp_acomp_comp_decomp()`, this patch below
>> should fix it. I will set up a few tests to check later.
>>
>> Thanks!
>>
>> diff --git a/crypto/scompress.c b/crypto/scompress.c
>> index 442a82c9de7d..e654a120ae5a 100644
>> --- a/crypto/scompress.c
>> +++ b/crypto/scompress.c
>> @@ -117,6 +117,7 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
>>         struct crypto_scomp *scomp = *tfm_ctx;
>>         void **ctx = acomp_request_ctx(req);
>>         struct scomp_scratch *scratch;
>> +       unsigned int dlen;
>>         int ret;
>>
>>         if (!req->src || !req->slen || req->slen > SCOMP_SCRATCH_SIZE)
>> @@ -128,6 +129,8 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
>>         if (!req->dlen || req->dlen > SCOMP_SCRATCH_SIZE)
>>                 req->dlen = SCOMP_SCRATCH_SIZE;
>>
>> +       dlen = req->dlen;
>> +
>>         scratch = raw_cpu_ptr(&scomp_scratch);
>>         spin_lock(&scratch->lock);
>>
>> @@ -145,6 +148,9 @@ static int scomp_acomp_comp_decomp(struct acomp_req *req, int dir)
>>                                 ret = -ENOMEM;
>>                                 goto out;
>>                         }
>> +               } else if (req->dlen > dlen) {
>> +                       ret = -ENOMEM;
>> +                       goto out;
>>                 }
> 
> This can't fix the problem as crypto_scomp_compress() has written overflow data.

No, crypto_scomp_compress() writes to its own scomp_scratch->dst memory, then copy
to our dstmem.

> 
> BTW, in many cases, hardware-accelerators drivers/crypto can do compression and
> decompression by off-loading CPU;
> we won't have a chance to let hardware check the dst buffer size. so
> giving the dst buffer
> with enough length to the hardware's dma engine is the right way. I
> mean, we shouldn't
> change dst from 2pages to 1page.

But how do we know 2 pages is enough for any compression algorithm?

Thanks.

> 
>>                 scatterwalk_map_and_copy(scratch->dst, req->dst, 0, req->dlen,
>>                                          1);
> 
> 
> Thanks
> Barry