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    Subject[PATCH 4.19 164/280] splice: dont merge into linked buffers
    Date
    4.19-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jann Horn <jannh@google.com>

    commit a0ce2f0aa6ad97c3d4927bf2ca54bcebdf062d55 upstream.

    Before this patch, it was possible for two pipes to affect each other after
    data had been transferred between them with tee():

    ============
    $ cat tee_test.c

    int main(void) {
    int pipe_a[2];
    if (pipe(pipe_a)) err(1, "pipe");
    int pipe_b[2];
    if (pipe(pipe_b)) err(1, "pipe");
    if (write(pipe_a[1], "abcd", 4) != 4) err(1, "write");
    if (tee(pipe_a[0], pipe_b[1], 2, 0) != 2) err(1, "tee");
    if (write(pipe_b[1], "xx", 2) != 2) err(1, "write");

    char buf[5];
    if (read(pipe_a[0], buf, 4) != 4) err(1, "read");
    buf[4] = 0;
    printf("got back: '%s'\n", buf);
    }
    $ gcc -o tee_test tee_test.c
    $ ./tee_test
    got back: 'abxx'
    $
    ============

    As suggested by Al Viro, fix it by creating a separate type for
    non-mergeable pipe buffers, then changing the types of buffers in
    splice_pipe_to_pipe() and link_pipe().

    Cc: <stable@vger.kernel.org>
    Fixes: 7c77f0b3f920 ("splice: implement pipe to pipe splicing")
    Fixes: 70524490ee2e ("[PATCH] splice: add support for sys_tee()")
    Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Jann Horn <jannh@google.com>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/pipe.c | 14 ++++++++++++++
    fs/splice.c | 4 ++++
    include/linux/pipe_fs_i.h | 1 +
    3 files changed, 19 insertions(+)

    --- a/fs/pipe.c
    +++ b/fs/pipe.c
    @@ -234,6 +234,14 @@ static const struct pipe_buf_operations
    .get = generic_pipe_buf_get,
    };

    +static const struct pipe_buf_operations anon_pipe_buf_nomerge_ops = {
    + .can_merge = 0,
    + .confirm = generic_pipe_buf_confirm,
    + .release = anon_pipe_buf_release,
    + .steal = anon_pipe_buf_steal,
    + .get = generic_pipe_buf_get,
    +};
    +
    static const struct pipe_buf_operations packet_pipe_buf_ops = {
    .can_merge = 0,
    .confirm = generic_pipe_buf_confirm,
    @@ -242,6 +250,12 @@ static const struct pipe_buf_operations
    .get = generic_pipe_buf_get,
    };

    +void pipe_buf_mark_unmergeable(struct pipe_buffer *buf)
    +{
    + if (buf->ops == &anon_pipe_buf_ops)
    + buf->ops = &anon_pipe_buf_nomerge_ops;
    +}
    +
    static ssize_t
    pipe_read(struct kiocb *iocb, struct iov_iter *to)
    {
    --- a/fs/splice.c
    +++ b/fs/splice.c
    @@ -1593,6 +1593,8 @@ retry:
    */
    obuf->flags &= ~PIPE_BUF_FLAG_GIFT;

    + pipe_buf_mark_unmergeable(obuf);
    +
    obuf->len = len;
    opipe->nrbufs++;
    ibuf->offset += obuf->len;
    @@ -1667,6 +1669,8 @@ static int link_pipe(struct pipe_inode_i
    */
    obuf->flags &= ~PIPE_BUF_FLAG_GIFT;

    + pipe_buf_mark_unmergeable(obuf);
    +
    if (obuf->len > len)
    obuf->len = len;

    --- a/include/linux/pipe_fs_i.h
    +++ b/include/linux/pipe_fs_i.h
    @@ -182,6 +182,7 @@ void generic_pipe_buf_get(struct pipe_in
    int generic_pipe_buf_confirm(struct pipe_inode_info *, struct pipe_buffer *);
    int generic_pipe_buf_steal(struct pipe_inode_info *, struct pipe_buffer *);
    void generic_pipe_buf_release(struct pipe_inode_info *, struct pipe_buffer *);
    +void pipe_buf_mark_unmergeable(struct pipe_buffer *buf);

    extern const struct pipe_buf_operations nosteal_pipe_buf_ops;


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