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    Subject[PATCH 3.2 050/115] splice: handle zero nr_pages in splice_to_pipe()
    3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Rabin Vincent <rabin@rab.in>

    commit d6785d9152147596f60234157da2b02540c3e60f upstream.

    Running the following command:

    busybox cat /sys/kernel/debug/tracing/trace_pipe > /dev/null

    with any tracing enabled pretty very quickly leads to various NULL
    pointer dereferences and VM BUG_ON()s, such as these:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
    IP: [<ffffffff8119df6c>] generic_pipe_buf_release+0xc/0x40
    Call Trace:
    [<ffffffff811c48a3>] splice_direct_to_actor+0x143/0x1e0
    [<ffffffff811c42e0>] ? generic_pipe_buf_nosteal+0x10/0x10
    [<ffffffff811c49cf>] do_splice_direct+0x8f/0xb0
    [<ffffffff81196869>] do_sendfile+0x199/0x380
    [<ffffffff81197600>] SyS_sendfile64+0x90/0xa0
    [<ffffffff8192cbee>] entry_SYSCALL_64_fastpath+0x12/0x6d

    page dumped because: VM_BUG_ON_PAGE(atomic_read(&page->_count) == 0)
    kernel BUG at include/linux/mm.h:367!
    invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
    RIP: [<ffffffff8119df9c>] generic_pipe_buf_release+0x3c/0x40
    Call Trace:
    [<ffffffff811c48a3>] splice_direct_to_actor+0x143/0x1e0
    [<ffffffff811c42e0>] ? generic_pipe_buf_nosteal+0x10/0x10
    [<ffffffff811c49cf>] do_splice_direct+0x8f/0xb0
    [<ffffffff81196869>] do_sendfile+0x199/0x380
    [<ffffffff81197600>] SyS_sendfile64+0x90/0xa0
    [<ffffffff8192cd1e>] tracesys_phase2+0x84/0x89

    (busybox's cat uses sendfile(2), unlike the coreutils version)

    This is because tracing_splice_read_pipe() can call splice_to_pipe()
    with spd->nr_pages == 0. spd_pages underflows in splice_to_pipe() and
    we fill the page pointers and the other fields of the pipe_buffers with
    garbage.

    All other callers of splice_to_pipe() avoid calling it when nr_pages ==
    0, and we could make tracing_splice_read_pipe() do that too, but it
    seems reasonable to have splice_to_page() handle this condition
    gracefully.

    Signed-off-by: Rabin Vincent <rabin@rab.in>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/splice.c | 3 +++
    1 file changed, 3 insertions(+)

    --- a/fs/splice.c
    +++ b/fs/splice.c
    @@ -188,6 +188,9 @@ ssize_t splice_to_pipe(struct pipe_inode
    unsigned int spd_pages = spd->nr_pages;
    int ret, do_wakeup, page_nr;

    + if (!spd_pages)
    + return 0;
    +
    ret = 0;
    do_wakeup = 0;
    page_nr = 0;
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