Commit 77873803 authored by Dan Williams's avatar Dan Williams

net_dma: mark broken

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: <stable@vger.kernel.org>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Reported-by: default avatarDavid Whipple <whipple@securedatainnovations.ch>
Acked-by: default avatarDavid S. Miller <davem@davemloft.net>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
parent 0baf8f6a
...@@ -355,6 +355,7 @@ config NET_DMA ...@@ -355,6 +355,7 @@ config NET_DMA
bool "Network: TCP receive copy offload" bool "Network: TCP receive copy offload"
depends on DMA_ENGINE && NET depends on DMA_ENGINE && NET
default (INTEL_IOATDMA || FSL_DMA) default (INTEL_IOATDMA || FSL_DMA)
depends on BROKEN
help help
This enables the use of DMA engines in the network stack to This enables the use of DMA engines in the network stack to
offload receive copy-to-user operations, freeing CPU cycles. offload receive copy-to-user operations, freeing CPU cycles.
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment