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

[Chengming Zhou]

On 2023/12/27 15:01, Barry Song wrote:
> On Wed, Dec 27, 2023 at 7:38 PM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>
>> 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?
>>
> 
> There is no guarrette 2 pages is enough.
> 
> We can invent our own compression algorithm, in our algorithm, we can
> add a lot of metadata, for example, longer than 4KB when the source data
> is uncompress-able. then dst can be larger than 2 * PAGE_SIZE.  but this
> is not the case :-) This kind of algorithm may never succeed.
> 
> For those in-tree algorithms, we have a WORST_COMPR_FACTOR. in
> ubifs, zram and zswap, we all use "2" as the worst comp factor.

Thanks for your explanation! Maybe it's best for us to return to 2 pages
if no other people's comments. And this really need more documentation :-)
since there is no any comment or check in the acomp compress interface.

/*
 * @src:	Source Data
 * @dst:	Destination data
 * @slen:	Size of the input buffer
 * @dlen:	Size of the output buffer and number of bytes produced
 * @flags:	Internal flags
 * @__ctx:	Start of private context data
 */
struct acomp_req {
	struct crypto_async_request base;
	struct scatterlist *src;
	struct scatterlist *dst;
	unsigned int slen;
	unsigned int dlen;
	u32 flags;
	void *__ctx[] CRYPTO_MINALIGN_ATTR;
};

> 
> /*
>  * Some compressors, like LZO, may end up with more data then the input buffer.
>  * So UBIFS always allocates larger output buffer, to be sure the compressor
>  * will not corrupt memory in case of worst case compression.
>  */
> #define WORST_COMPR_FACTOR 2
> 
> #ifdef CONFIG_FS_ENCRYPTION
> #define UBIFS_CIPHER_BLOCK_SIZE FSCRYPT_CONTENTS_ALIGNMENT
> #else
> #define UBIFS_CIPHER_BLOCK_SIZE 0
> #endif
> 
> /*
>  * How much memory is needed for a buffer where we compress a data node.
>  */
> #define COMPRESSED_DATA_NODE_BUF_SZ \
>         (UBIFS_DATA_NODE_SZ + UBIFS_BLOCK_SIZE * WORST_COMPR_FACTOR)
> 
> 
> For years, we have never seen 2 pages that can be a problem. but 1
> page is definitely
> not enough, I remember I once saw many cases where accelerators' dmaengine
> can write more than 1 page.
> 
>> Thanks.
>>
>>>
>>>>                 scatterwalk_map_and_copy(scratch->dst, req->dst, 0, req->dlen,
>>>>                                          1);
>>>
>>>
> 
> Thanks
> Barry