[3.13.y-ckt stable] Patch "splice: handle zero nr_pages in splice_to_pipe()" has been added to the 3.13.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    splice: handle zero nr_pages in splice_to_pipe()

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt38.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

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

>From 79ac90478c9237cc9ba020af5c64f7729398c7a9 Mon Sep 17 00:00:00 2001
From: Rabin Vincent <rabin at rab.in>
Date: Thu, 10 Mar 2016 21:19:06 +0100
Subject: splice: handle zero nr_pages in splice_to_pipe()

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 at rab.in>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/splice.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/splice.c b/fs/splice.c
index bef92b4..2cf3994 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -189,6 +189,9 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
 	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;
--
2.7.4