Re: [PATCH v2 4/4] mm: prohibit NULL deference exposed for unsupported non-blockable __GFP_NOFAIL

[Barry Song <21cnbao@gmail.com>]

On Wed, Jul 31, 2024 at 6:55 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 7/31/24 2:01 AM, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > When users allocate memory with the __GFP_NOFAIL flag, they might
> > incorrectly use it alongside GFP_ATOMIC, GFP_NOWAIT, etc. This kind
> > of non-blockable __GFP_NOFAIL is not supported and is pointless. If
> > we attempt and still fail to allocate memory for these users, we have
> > two choices:
> >
> >     1. We could busy-loop and hope that some other direct reclamation or
> >     kswapd rescues the current process. However, this is unreliable
> >     and could ultimately lead to hard or soft lockups, which might not
> >     be well supported by some architectures.
> >
> >     2. We could use BUG_ON to trigger a reliable system crash, avoiding
> >     exposing NULL dereference.
> >
> > This patch chooses the second option because the first is unreliable. Even
> > if the process incorrectly using __GFP_NOFAIL is sometimes rescued, the
> > long latency might be unacceptable, especially considering that misusing
> > GFP_ATOMIC and __GFP_NOFAIL is likely to occur in atomic contexts with
> > strict timing requirements.
> >
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Roman Gushchin <roman.gushchin@linux.dev>
> > Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Kees Cook <kees@kernel.org>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  mm/page_alloc.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index cc179c3e68df..ed1bd8f595bd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4439,11 +4439,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >        */
> >       if (gfp_mask & __GFP_NOFAIL) {
> >               /*
> > -              * All existing users of the __GFP_NOFAIL are blockable, so warn
> > -              * of any new users that actually require GFP_NOWAIT
> > +              * All existing users of the __GFP_NOFAIL are blockable
> > +              * otherwise we introduce a busy loop with inside the page
> > +              * allocator from non-sleepable contexts
> >                */
> > -             if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> > -                     goto fail;
> > +             BUG_ON(!can_direct_reclaim);
>
> We might get more useful output if here we did just "if
> (!can_direct_reclaim) goto fail; and let warn_alloc() print it, and then
> there would be a BUG_ON(gfp_mask & __GFP_NOFAIL)?
> Additionally we could mask out __GFP_NOWARN from gfp_mask before the goto,
> as a __GFP_NOWARN would suppress the output in a non-recoverable situation
> so it would be wrong.

If we use BUG_ON, it seems like we don't need to do anything else, as the BUG_ON
report gives developers all the information they need. If we go with
approach 1—doing
a busy loop until rescued or a lockup occurs—I agree it might be
better to add more
warnings.

>
> >
> >               /*
> >                * PF_MEMALLOC request from this context is rather bizarre
> > @@ -4474,7 +4474,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >               cond_resched();
> >               goto retry;
> >       }
> > -fail:
> > +
> >       warn_alloc(gfp_mask, ac->nodemask,
> >                       "page allocation failure: order:%u", order);
> >  got_pg:
>