mm: BUG_ON to avoid NULL deference while __GFP_NOFAIL fails

mm: BUG_ON to avoid NULL deference while __GFP_NOFAIL fails

We have cases we still fail though callers might have __GFP_NOFAIL.  Since
they don't check the return, we are exposed to the security risks for NULL
deference.

Though BUG_ON() is not encouraged by Linus, this is an unrecoverable
situation.

Christoph Hellwig:
The whole freaking point of __GFP_NOFAIL is that callers don't handle
allocation failures.  So in fact a straight BUG is the right thing
here.

Vlastimil Babka:
It's just not a recoverable situation (WARN_ON is for recoverable
situations). The caller cannot handle allocation failure and at the same
time asked for an impossible allocation. BUG_ON() is a guaranteed oops
with stracktrace etc. We don't need to hope for the later NULL pointer
dereference (which might if really unlucky happen from a different
context where it's no longer obvious what lead to the allocation failing).

Michal Hocko:
Linus tends to be against adding new BUG() calls unless the failure is
absolutely unrecoverable (e.g. corrupted data structures etc.). I am
not sure how he would look at simply incorrect memory allocator usage to
blow up the kernel. Now the argument could be made that those failures
could cause subtle memory corruptions or even be exploitable which might
be a sufficient reason to stop them early.

Link: https://lkml.kernel.org/r/20240731000155.109583-4-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
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: 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>
Cc: "Eugenio Pérez" <eperezma@redhat.com>
Cc: Hailong.Liu <hailong.liu@oppo.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>