[3.8.y.z extended stable] Patch "net_dma: mark broken" has been added to staging queue

[Kamal Mostafa]

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

    net_dma: mark broken

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.16.

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

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 640c334c23b091f0ac35c6698a41a13a76a4c575 Mon Sep 17 00:00:00 2001
From: Dan Williams <dan.j.williams at intel.com>
Date: Tue, 17 Dec 2013 10:09:32 -0800
Subject: net_dma: mark broken

commit 77873803363c9e831fc1d1e6895c084279090c22 upstream.

net_dma can cause data to be copied to a stale mapping if a
copy-on-write fault occurs during dma.  The application sees missing
data.

The following trace is triggered by modifying the kernel to WARN if it
ever triggers copy-on-write on a page that is undergoing dma:

 WARNING: CPU: 24 PID: 2529 at lib/dma-debug.c:485 debug_dma_assert_idle+0xd2/0x120()
 ioatdma 0000:00:04.0: DMA-API: cpu touching an active dma mapped page [pfn=0x16bcd9]
 Modules linked in: iTCO_wdt iTCO_vendor_support ioatdma lpc_ich pcspkr dca
 CPU: 24 PID: 2529 Comm: linbug Tainted: G        W    3.13.0-rc1+ #353
  00000000000001e5 ffff88016f45f688 ffffffff81751041 ffff88017ab0ef70
  ffff88016f45f6d8 ffff88016f45f6c8 ffffffff8104ed9c ffffffff810f3646
  ffff8801768f4840 0000000000000282 ffff88016f6cca10 00007fa2bb699349
 Call Trace:
  [<ffffffff81751041>] dump_stack+0x46/0x58
  [<ffffffff8104ed9c>] warn_slowpath_common+0x8c/0xc0
  [<ffffffff810f3646>] ? ftrace_pid_func+0x26/0x30
  [<ffffffff8104ee86>] warn_slowpath_fmt+0x46/0x50
  [<ffffffff8139c062>] debug_dma_assert_idle+0xd2/0x120
  [<ffffffff81154a40>] do_wp_page+0xd0/0x790
  [<ffffffff811582ac>] handle_mm_fault+0x51c/0xde0
  [<ffffffff813830b9>] ? copy_user_enhanced_fast_string+0x9/0x20
  [<ffffffff8175fc2c>] __do_page_fault+0x19c/0x530
  [<ffffffff8175c196>] ? _raw_spin_lock_bh+0x16/0x40
  [<ffffffff810f3539>] ? trace_clock_local+0x9/0x10
  [<ffffffff810fa1f4>] ? rb_reserve_next_event+0x64/0x310
  [<ffffffffa0014c00>] ? ioat2_dma_prep_memcpy_lock+0x60/0x130 [ioatdma]
  [<ffffffff8175ffce>] do_page_fault+0xe/0x10
  [<ffffffff8175c862>] page_fault+0x22/0x30
  [<ffffffff81643991>] ? __kfree_skb+0x51/0xd0
  [<ffffffff813830b9>] ? copy_user_enhanced_fast_string+0x9/0x20
  [<ffffffff81388ea2>] ? memcpy_toiovec+0x52/0xa0
  [<ffffffff8164770f>] skb_copy_datagram_iovec+0x5f/0x2a0
  [<ffffffff8169d0f4>] tcp_rcv_established+0x674/0x7f0
  [<ffffffff816a68c5>] tcp_v4_do_rcv+0x2e5/0x4a0
  [..]
 ---[ end trace e30e3b01191b7617 ]---
 Mapped at:
  [<ffffffff8139c169>] debug_dma_map_page+0xb9/0x160
  [<ffffffff8142bf47>] dma_async_memcpy_pg_to_pg+0x127/0x210
  [<ffffffff8142cce9>] dma_memcpy_pg_to_iovec+0x119/0x1f0
  [<ffffffff81669d3c>] dma_skb_copy_datagram_iovec+0x11c/0x2b0
  [<ffffffff8169d1ca>] tcp_rcv_established+0x74a/0x7f0:

...the problem is that the receive path falls back to cpu-copy in
several locations and this trace is just one of the areas.  A few
options were considered to fix this:

1/ sync all dma whenever a cpu copy branch is taken

2/ modify the page fault handler to hold off while dma is in-flight

Option 1 adds yet more cpu overhead to an "offload" that struggles to compete
with cpu-copy.  Option 2 adds checks for behavior that is already documented as
broken when using get_user_pages().  At a minimum a debug mode is warranted to
catch and flag these violations of the dma-api vs get_user_pages().

Thanks to David for his reproducer.

Cc: Dave Jiang <dave.jiang at intel.com>
Cc: Vinod Koul <vinod.koul at intel.com>
Cc: Alexander Duyck <alexander.h.duyck at intel.com>
Reported-by: David Whipple <whipple at securedatainnovations.ch>
Acked-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Dan Williams <dan.j.williams at intel.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/dma/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
index d4c1218..11bbcca 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -333,6 +333,7 @@ config NET_DMA
 	bool "Network: TCP receive copy offload"
 	depends on DMA_ENGINE && NET
 	default (INTEL_IOATDMA || FSL_DMA)
+	depends on BROKEN
 	help
 	  This enables the use of DMA engines in the network stack to
 	  offload receive copy-to-user operations, freeing CPU cycles.
--
1.8.3.2