erofs: fix uninitialized page cache reported by KMSAN

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-and-tested-by: syzbot+7bc44a489f0ef0670bd5@syzkaller.appspotmail.com
Fixes: 1ca0152 ("erofs: refine z_erofs_transform_plain() for sub-page block support")
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240304035339.425857-1-hsiangkao@linux.alibaba.com

Author: hsiangkao

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;