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    Subject[PATCH 3.19.y-ckt 026/160] fuse: break infinite loop in fuse_fill_write_pages()
    Date
    3.19.8-ckt13 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Roman Gushchin <klamm@yandex-team.ru>

    commit 3ca8138f014a913f98e6ef40e939868e1e9ea876 upstream.

    I got a report about unkillable task eating CPU. Further
    investigation shows, that the problem is in the fuse_fill_write_pages()
    function. If iov's first segment has zero length, we get an infinite
    loop, because we never reach iov_iter_advance() call.

    Fix this by calling iov_iter_advance() before repeating an attempt to
    copy data from userspace.

    A similar problem is described in 124d3b7041f ("fix writev regression:
    pan hanging unkillable and un-straceable"). If zero-length segmend
    is followed by segment with invalid address,
    iov_iter_fault_in_readable() checks only first segment (zero-length),
    iov_iter_copy_from_user_atomic() skips it, fails at second and
    returns zero -> goto again without skipping zero-length segment.

    Patch calls iov_iter_advance() before goto again: we'll skip zero-length
    segment at second iteraction and iov_iter_fault_in_readable() will detect
    invalid address.

    Special thanks to Konstantin Khlebnikov, who helped a lot with the commit
    description.

    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Maxim Patlasov <mpatlasov@parallels.com>
    Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>
    Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
    Fixes: ea9b9907b82a ("fuse: implement perform_write")
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    fs/fuse/file.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/fs/fuse/file.c b/fs/fuse/file.c
    index 760b2c5..60788e9 100644
    --- a/fs/fuse/file.c
    +++ b/fs/fuse/file.c
    @@ -1043,6 +1043,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
    tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);
    flush_dcache_page(page);

    + iov_iter_advance(ii, tmp);
    if (!tmp) {
    unlock_page(page);
    page_cache_release(page);
    @@ -1055,7 +1056,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
    req->page_descs[req->num_pages].length = tmp;
    req->num_pages++;

    - iov_iter_advance(ii, tmp);
    count += tmp;
    pos += tmp;
    offset += tmp;
    --
    1.9.1

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