[PATCH] erofs: fix uninitialized page cache reported by KMSAN

[Gao Xiang]

#syz test git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs.git dev-test

On 2024/3/4 11:53, Gao Xiang wrote:
> 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 at syzkaller.appspotmail.com
> Fixes: 1ca01520148a ("erofs: refine z_erofs_transform_plain() for sub-page block support")
> Signed-off-by: Gao Xiang <hsiangkao at 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;