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From:   Maurizio Lombardi <mlombard@redhat.com>
Date:   Wed, 6 Jul 2022 17:21:31 +0200
Message-ID: <CAFL455=bAEXeyUPjDPZm-hK-K8aKJSw7wRQ0CCYoKWO8VMJUqw@mail.gmail.com>
Subject: Re: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is
 bigger than PAGE_SIZE
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     =?UTF-8?B?5oSa5qCR?= <chen45464546@163.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>
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st 1. 6. 2022 v 17:05 odes=C3=ADlatel Alexander Duyck
<alexander.duyck@gmail.com> napsal:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index e6f211d..ac60a97 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5580,6 +5580,7 @@ void *page_frag_alloc_align(struct page_frag_cach=
e *nc,
> >                 /* reset page count bias and offset to start of new fra=
g */
> >                 nc->pagecnt_bias =3D PAGE_FRAG_CACHE_MAX_SIZE + 1;
> >                 offset =3D size - fragsz;
> > +               BUG_ON(offset < 0);
> >         }
> >
> >         nc->pagecnt_bias--;
> >
>
>
> I think I could be onboard with a patch like this. The test shouldn't
> add more than 1 instruction since it is essentially just a jump if
> signed test which will be performed after the size - fragsz check.

FYI, I hit this problem a few days ago with the nfp network driver, it uses
page_frag_alloc() with a frag size larger than PAGE_SIZE when MTU is
set to 9000,
this may result in memory corruptions when the system runs out of memory.

The solution I was working on was something like the following, this
makes the allocation
fail if fragsz is greater than the cache size.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4dc0d333279f..c6b40b85c55d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5544,12 +5544,17 @@ void *page_frag_alloc_align(struct page_frag_cache =
*nc,
                /* if size can vary use size else just use PAGE_SIZE */
                size =3D nc->size;
 #endif
-               /* OK, page count is 0, we can safely set it */
-               set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
-
                /* reset page count bias and offset to start of new frag */
                nc->pagecnt_bias =3D PAGE_FRAG_CACHE_MAX_SIZE + 1;
                offset =3D size - fragsz;
+               if (unlikely(offset < 0)) {
+                       free_the_page(page, compound_order(page));
+                       nc->va =3D NULL;
+                       return NULL;
+               }
+
+               /* OK, page count is 0, we can safely set it */
+               set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
        }

        nc->pagecnt_bias--;


Maurizio