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    Subject[PATCH 5.5 123/367] erofs: fix out-of-bound read for shifted uncompressed block
    Date
    From: Gao Xiang <gaoxiang25@huawei.com>

    commit 4d2024370d877f9ac8b98694bcff666da6a5d333 upstream.

    rq->out[1] should be valid before accessing. Otherwise,
    in very rare cases, out-of-bound dirty onstack rq->out[1]
    can equal to *in and lead to unintended memmove behavior.

    Link: https://lore.kernel.org/r/20200107022546.19432-1-gaoxiang25@huawei.com
    Fixes: 7fc45dbc938a ("staging: erofs: introduce generic decompression backend")
    Cc: <stable@vger.kernel.org> # 5.3+
    Reviewed-by: Chao Yu <yuchao0@huawei.com>
    Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/erofs/decompressor.c | 22 ++++++++++------------
    1 file changed, 10 insertions(+), 12 deletions(-)

    --- a/fs/erofs/decompressor.c
    +++ b/fs/erofs/decompressor.c
    @@ -306,24 +306,22 @@ static int z_erofs_shifted_transform(con
    }

    src = kmap_atomic(*rq->in);
    - if (!rq->out[0]) {
    - dst = NULL;
    - } else {
    + if (rq->out[0]) {
    dst = kmap_atomic(rq->out[0]);
    memcpy(dst + rq->pageofs_out, src, righthalf);
    + kunmap_atomic(dst);
    }

    - if (rq->out[1] == *rq->in) {
    - memmove(src, src + righthalf, rq->pageofs_out);
    - } else if (nrpages_out == 2) {
    - if (dst)
    - kunmap_atomic(dst);
    + if (nrpages_out == 2) {
    DBG_BUGON(!rq->out[1]);
    - dst = kmap_atomic(rq->out[1]);
    - memcpy(dst, src + righthalf, rq->pageofs_out);
    + if (rq->out[1] == *rq->in) {
    + memmove(src, src + righthalf, rq->pageofs_out);
    + } else {
    + dst = kmap_atomic(rq->out[1]);
    + memcpy(dst, src + righthalf, rq->pageofs_out);
    + kunmap_atomic(dst);
    + }
    }
    - if (dst)
    - kunmap_atomic(dst);
    kunmap_atomic(src);
    return 0;
    }

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