linux-kernel - Re: [syzbot] [crypto?] general protection fault in scatterwalk

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Date: Thu, 28 Dec 2023 09:47:43 +0800
From: Barry Song <21cnbao@...il.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Chengming Zhou <zhouchengming@...edance.com>, Chris Li <chrisl@...nel.org>, 
	syzbot <syzbot+3eff5e51bf1db122a16e@...kaller.appspotmail.com>, 
	akpm@...ux-foundation.org, davem@...emloft.net, herbert@...dor.apana.org.au, 
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com, yosryahmed@...gle.com
Subject: Re: [syzbot] [crypto?] general protection fault in
 scatterwalk_copychunks (5)

On Thu, Dec 28, 2023 at 9:43 AM Barry Song <21cnbao@...il.com> wrote:
>
> On Thu, Dec 28, 2023 at 7:26 AM Nhat Pham <nphamcs@...il.com> wrote:
> >
> > On Wed, Dec 27, 2023 at 3:10 AM Barry Song <21cnbao@...il.com> wrote:
> > >
> > > On Wed, Dec 27, 2023 at 5:16 PM Chengming Zhou
> > > <zhouchengming@...edance.com> wrote:
> > > >
> > > > 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 :-)
> >
> > Fine by me. Hmm we're basically wasting one extra page per CPU (since
> > these buffers are per-CPU), correct? That's not ideal, but not *too*
> > bad for now I suppose...
> >
> > >
> > > I agree. we need some doc.
> > >
> > > besides, i actually think we can skip zswap frontend if
> > > over-compression is really
> > > happening.
> >
> > IIUC, zsmalloc already checked that - and most people are (or should
> > be) using zsmalloc for zswap anyway. I wouldn't be opposed to adding
> > an added layer of protection on the zswap side, but not super high
> > priority I'd say.
>
> Thanks for this info. I guess you mean the below ?
> unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
> {
>         ...
>
>         if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
>                 return (unsigned long)ERR_PTR(-EINVAL);

BTW, do you think zsmalloc is worth a patch to change the ret from
EINVAL to ENOSPC?
This seems more sensible and matches the behaviour of zswap, and zbud, z3fold.

        ret = zpool_malloc(zpool, dlen, gfp, &handle);
        if (ret == -ENOSPC) {
                zswap_reject_compress_poor++;
                goto put_dstmem;
        }
        if (ret) {
                zswap_reject_alloc_fail++;
                goto put_dstmem;
        }
        buf = z

>
> }
>
> i find zbud also has similar code:
> static int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
>                         unsigned long *handle)
> {
>         int chunks, i, freechunks;
>         struct zbud_header *zhdr = NULL;
>         enum buddy bud;
>         struct page *page;
>
>         if (!size || (gfp & __GFP_HIGHMEM))
>                 return -EINVAL;
>         if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
>                 return -ENOSPC;
>
> and z3fold,
>
> static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>                         unsigned long *handle)
> {
>         int chunks = size_to_chunks(size);
>         struct z3fold_header *zhdr = NULL;
>         struct page *page = NULL;
>         enum buddy bud;
>         bool can_sleep = gfpflags_allow_blocking(gfp);
>
>         if (!size || (gfp & __GFP_HIGHMEM))
>                 return -EINVAL;
>
>         if (size > PAGE_SIZE)
>                 return -ENOSPC;
>
>
> Thus, I agree that another layer to check size in zswap isn't necessary now.

Thanks
Barry