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From: Gao Xiang <hsiangkao@linux.alibaba.com>
To: linux-erofs@lists.ozlabs.org
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Roberto Sassu <roberto.sassu@huaweicloud.com>,
	syzkaller-bugs@googlegroups.com,
	Gao Xiang <hsiangkao@linux.alibaba.com>,
	syzbot+7bc44a489f0ef0670bd5@syzkaller.appspotmail.com
Subject: [PATCH] erofs: fix uninitialized page cache reported by KMSAN
Date: Mon,  4 Mar 2024 11:53:39 +0800
Message-Id: <20240304035339.425857-1-hsiangkao@linux.alibaba.com>
In-Reply-To: <ab2a337d-c2dd-437d-9ab8-e3b837f1ff1a@I-love.SAKURA.ne.jp>
References: <ab2a337d-c2dd-437d-9ab8-e3b837f1ff1a@I-love.SAKURA.ne.jp>
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syzbot reports a KMSAN reproducer [1] which generates a crafted
filesystem image and causes IMA to read uninitialized page cache.

Later, (rq->outputsize > rq->inputsize) will be formally supported
after either large uncompressed pclusters (> block size) or big
lclusters are landed.  However, currently there is no way to generate
such filesystems by using mkfs.erofs.

Thus, let's mark this condition as unsupported for now.

[1] https://lore.kernel.org/r/0000000000002be12a0611ca7ff8@google.com

Reported-by: syzbot+7bc44a489f0ef0670bd5@syzkaller.appspotmail.com
Fixes: 1ca01520148a ("erofs: refine z_erofs_transform_plain() for sub-page block support")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 fs/erofs/decompressor.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index d4cee95af14c..2ec9b2bb628d 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -323,7 +323,8 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
 	unsigned int cur = 0, ni = 0, no, pi, po, insz, cnt;
 	u8 *kin;
 
-	DBG_BUGON(rq->outputsize > rq->inputsize);
+	if (rq->outputsize > rq->inputsize)
+		return -EOPNOTSUPP;
 	if (rq->alg == Z_EROFS_COMPRESSION_INTERLACED) {
 		cur = bs - (rq->pageofs_out & (bs - 1));
 		pi = (rq->pageofs_in + rq->inputsize - cur) & ~PAGE_MASK;
-- 
2.39.3