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From:   Ryusuke Konishi <konishi.ryusuke@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-nilfs@vger.kernel.org,
        syzbot <syzbot+0ad741797f4565e7e2d2@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: [PATCH] nilfs2: fix general protection fault in nilfs_lookup_dirty_data_buffers()
Date:   Sat,  5 Aug 2023 22:20:38 +0900
Message-Id: <20230805132038.6435-1-konishi.ryusuke@gmail.com>
In-Reply-To: <0000000000002930a705fc32b231@google.com>
References: <0000000000002930a705fc32b231@google.com>
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A syzbot stress test reported that create_empty_buffers() called from
nilfs_lookup_dirty_data_buffers() can cause a general protection fault.

Analysis using its reproducer revealed that the back reference "mapping"
from a page/folio has been changed to NULL after dirty page/folio gang
lookup in nilfs_lookup_dirty_data_buffers().

Fix this issue by excluding pages/folios from being collected if, after
acquiring a lock on each page/folio, its back reference "mapping"
differs from the pointer to the address space struct that held the
page/folio.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+0ad741797f4565e7e2d2@syzkaller.appspotmail.com
Closes: https://lkml.kernel.org/r/0000000000002930a705fc32b231@google.com
Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
---
 fs/nilfs2/segment.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 581691e4be49..7ec16879756e 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -725,6 +725,11 @@ static size_t nilfs_lookup_dirty_data_buffers(struct inode *inode,
 		struct folio *folio = fbatch.folios[i];
 
 		folio_lock(folio);
+		if (unlikely(folio->mapping != mapping)) {
+			/* Exclude folios removed from the address space */
+			folio_unlock(folio);
+			continue;
+		}
 		head = folio_buffers(folio);
 		if (!head) {
 			create_empty_buffers(&folio->page, i_blocksize(inode), 0);
-- 
2.34.1