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    Subject[PATCH 5.9 712/757] iomap: fix WARN_ON_ONCE() from unprivileged users
    Date
    From: Qian Cai <cai@lca.pw>

    [ Upstream commit a805c111650cdba6ee880f528abdd03c1af82089 ]

    It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
    unprivileged users which would taint the kernel, or worse - panic if
    panic_on_warn or panic_on_taint is set. Hence, just convert it to
    pr_warn_ratelimited() to let users know their workloads are racing.
    Thank Dave Chinner for the initial analysis of the racing reproducers.

    Signed-off-by: Qian Cai <cai@lca.pw>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/iomap/direct-io.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
    index c1aafb2ab9907..9519113ebc352 100644
    --- a/fs/iomap/direct-io.c
    +++ b/fs/iomap/direct-io.c
    @@ -388,6 +388,16 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
    return iomap_dio_bio_actor(inode, pos, length, dio, iomap);
    case IOMAP_INLINE:
    return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
    + case IOMAP_DELALLOC:
    + /*
    + * DIO is not serialised against mmap() access at all, and so
    + * if the page_mkwrite occurs between the writeback and the
    + * iomap_apply() call in the DIO path, then it will see the
    + * DELALLOC block that the page-mkwrite allocated.
    + */
    + pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %pD4 Comm: %.20s\n",
    + dio->iocb->ki_filp, current->comm);
    + return -EIO;
    default:
    WARN_ON_ONCE(1);
    return -EIO;
    --
    2.25.1


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