linux-kernel - [PATCH] bcachefs: add GFP_ZERO flag in btree_bounce

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Message-ID: <20240916224746.692973-1-pZ010001011111@proton.me>
Date: Mon, 16 Sep 2024 22:47:57 +0000
From: Piotr Zalewski <pZ010001011111@...ton.me>
To: kent.overstreet@...ux.dev, linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: skhan@...uxfoundation.org, Piotr Zalewski <pZ010001011111@...ton.me>
Subject: [PATCH] bcachefs: add GFP_ZERO flag in btree_bounce_alloc

Add __GFP_ZERO flag to kvmalloc call in btree_bounce_alloc to mitigate
later uinit-value use KMSAN warning[1].

After applying the patch reproducer still triggers stack overflow[2] but
it seems unrelated to the uninit-value use warning. After further
investigation it was found that stack overflow occurs because KMSAN adds
additional function calls. Backtrace of where the stack magic number gets 
smashed was added as a reply to syzkaller thread[3].

I confirmed that task's stack magic number gets smashed after the code path
where KSMAN detects uninit-value use is executed, so it can be assumed that
it doesn't contribute in any way to uninit-value use detection.

[1] https://syzkaller.appspot.com/bug?extid=6f655a60d3244d0c6718
[2] https://lore.kernel.org/lkml/66e57e46.050a0220.115905.0002.GAE@google.com
[3] https://lore.kernel.org/all/rVaWgPULej8K7HqMPNIu8kVNyXNjjCiTB-QBtItLFBmk0alH6fV2tk4joVPk97Evnuv4ZRDd8HB5uDCkiFG6u81xKdzDj-KrtIMJSlF6Kt8=@proton.me

Signed-off-by: Piotr Zalewski <pZ010001011111@...ton.me>
---
 fs/bcachefs/btree_io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/bcachefs/btree_io.c b/fs/bcachefs/btree_io.c
index 56ea9a77cd4a..3ac8b37f97d7 100644
--- a/fs/bcachefs/btree_io.c
+++ b/fs/bcachefs/btree_io.c
@@ -121,7 +121,7 @@ static void *btree_bounce_alloc(struct bch_fs *c, size_t size,
 	BUG_ON(size > c->opts.btree_node_size);

 	*used_mempool = false;
-	p = kvmalloc(size, __GFP_NOWARN|GFP_NOWAIT);
+	p = kvmalloc(size, __GFP_ZERO|__GFP_NOWARN|GFP_NOWAIT);
 	if (!p) {
 		*used_mempool = true;
 		p = mempool_alloc(&c->btree_bounce_pool, GFP_NOFS);
-- 
2.46.0