[PATCH 6.1.y] erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap()

[Sasha Levin <sashal@kernel.org>]

From: Junrui Luo <moonafterrain@outlook.com>

[ Upstream commit 21e161de2dc660b1bb70ef5b156ab8e6e1cca3ab ]

Some crafted images can have illegal (!partial_decoding &&
m_llen < m_plen) extents, and the LZ4 inplace decompression path
can be wrongly hit, but it cannot handle (outpages < inpages)
properly: "outpages - inpages" wraps to a large value and
the subsequent rq->out[] access reads past the decompressed_pages
array.

However, such crafted cases can correctly result in a corruption
report in the normal LZ4 non-inplace path.

Let's add an additional check to fix this for backporting.

Reproducible image (base64-encoded gzipped blob):

H4sIAJGR12kCA+3SPUoDQRgG4MkmkkZk8QRbRFIIi9hbpEjrHQI5ghfwCN5BLCzTGtLbBI+g
dilSJo1CnIm7GEXFxhT6PDDwfrs73/ywIQD/1ePD4r7Ou6ETsrq4mu7XcWfj++Pb58nJU/9i
PNtbjhan04/9GtX4qVYc814WDqt6FaX5s+ZwXXeq52lndT6IuVvlblytLMvh4Gzwaf90nsvz
2DF/21+20T/ldgp5s1jXRaN4t/8izsy/OUB6e/Qa79r+JwAAAAAAAL52vQVuGQAAAP6+my1w
ywAAAAAAAADwu14ATsEYtgBQAAA=

$ mount -t erofs -o cache_strategy=disabled foo.erofs /mnt
$ dd if=/mnt/data of=/dev/null bs=4096 count=1

Fixes: 598162d05080 ("erofs: support decompress big pcluster for lz4 backend")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
[ inverted condition to early-out `goto docopy` form and used `ctx->inpages`/`ctx->outpages` instead of `rq->` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/erofs/decompressor.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index e524c0b432f39..a63522e84fe0a 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -133,6 +133,7 @@ static void *z_erofs_lz4_handle_overlap(struct z_erofs_lz4_decompress_ctx *ctx,
 	if (rq->inplace_io) {
 		omargin = PAGE_ALIGN(ctx->oend) - ctx->oend;
 		if (rq->partial_decoding || !may_inplace ||
+		    ctx->outpages < ctx->inpages ||
 		    omargin < LZ4_DECOMPRESS_INPLACE_MARGIN(rq->inputsize))
 			goto docopy;
 
-- 
2.53.0