From f78d92ec155c038a8b475bbf163b70c2d6d961fc Mon Sep 17 00:00:00 2001
From: Barry Song <v-songbaohua@oppo.com>
Date: Wed, 31 Jul 2024 12:01:55 +1200
Subject: [PATCH] mm: prohibit NULL deference exposed for unsupported
 non-blockable __GFP_NOFAIL
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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.

Link: https://lkml.kernel.org/r/20240731000155.109583-5-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
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>
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>
---
 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 d2c37f8f8d09..fb5850ecd3ae 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4399,11 +4399,11 @@ nopage:
 	 */
 	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);
 
 		/*
 		 * PF_MEMALLOC request from this context is rather bizarre
@@ -4434,7 +4434,7 @@ nopage:
 		cond_resched();
 		goto retry;
 	}
-fail:
+
 	warn_alloc(gfp_mask, ac->nodemask,
 			"page allocation failure: order:%u", order);
 got_pg:
-- 
2.50.1