[PATCH] dma-buf/fence: Avoid use of uninitialised timestamp

[Gustavo Padovan]

2017-07-11 Chris Wilson <chris at chris-wilson.co.uk>:

> Quoting Chris Wilson (2017-02-14 12:40:01)
> > [  236.821534] WARNING: kmemcheck: Caught 64-bit read from uninitialized memory (ffff8802538683d0)
> > [  236.828642] 420000001e7f0000000000000000000000080000000000000000000000000000
> > [  236.839543]  i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
> > [  236.850420]                                  ^
> > [  236.854123] RIP: 0010:[<ffffffff81396f07>]  [<ffffffff81396f07>] fence_signal+0x17/0xd0
> > [  236.861313] RSP: 0018:ffff88024acd7ba0  EFLAGS: 00010282
> > [  236.865027] RAX: ffffffff812f6a90 RBX: ffff8802527ca800 RCX: ffff880252cb30e0
> > [  236.868801] RDX: ffff88024ac5d918 RSI: ffff880252f780e0 RDI: ffff880253868380
> > [  236.872579] RBP: ffff88024acd7bc0 R08: ffff88024acd7be0 R09: 0000000000000000
> > [  236.876407] R10: 0000000000000000 R11: 0000000000000000 R12: ffff880253868380
> > [  236.880185] R13: ffff8802538684d0 R14: ffff880253868380 R15: ffff88024cd48e00
> > [  236.883983] FS:  00007f1646d1a740(0000) GS:ffff88025d000000(0000) knlGS:0000000000000000
> > [  236.890959] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [  236.894702] CR2: ffff880251360318 CR3: 000000024ad21000 CR4: 00000000001406f0
> > [  236.898481]  [<ffffffff8130d1ad>] i915_gem_request_retire+0x1cd/0x230
> > [  236.902439]  [<ffffffff8130e2b3>] i915_gem_request_alloc+0xa3/0x2f0
> > [  236.906435]  [<ffffffff812fb1bd>] i915_gem_do_execbuffer.isra.41+0xb6d/0x18b0
> > [  236.910434]  [<ffffffff812fc265>] i915_gem_execbuffer2+0x95/0x1e0
> > [  236.914390]  [<ffffffff812ad625>] drm_ioctl+0x1e5/0x460
> > [  236.918275]  [<ffffffff8110d4cf>] do_vfs_ioctl+0x8f/0x5c0
> > [  236.922168]  [<ffffffff8110da3c>] SyS_ioctl+0x3c/0x70
> > [  236.926090]  [<ffffffff814b7a5f>] entry_SYSCALL_64_fastpath+0x17/0x93
> > [  236.930045]  [<ffffffffffffffff>] 0xffffffffffffffff
> 
> Ah something that I didn't take into account, and indeed gives this a bit
> more urgency than I realised, is that the timestamp is exposed to
> userspace. As such we are feeding it garbage, at best.
> 
> The trivial option is just to clear it in dma_fence_init(). I still have
> the slight preference for the extra complication here (for the reader) as
> it should be quicker for the more common path of signaling the fence.
> 
> > We only set the timestamp before we mark the fence as signaled. It is
> > done before to avoid observers having a window in which they may see the
> > fence as complete but no timestamp. Having it does incur a potential for
> > the timestamp to be written twice, and even for it to be corrupted if
> > the u64 write is not atomic. Instead use a new bit to record the
> > presence of the timestamp, and teach the readers to wait until it is set
> > if the fence is complete. There still remains a race where the timestamp
> > for the signaled fence may be shown before the fence is reported as
> > signaled, but that's a pre-existing error.
> 
> Now deserves a
> Reported-by: Rafael Antognolli <rafael.antognolli at intel.com>
> 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Sumit Semwal <sumit.semwal at linaro.org>
> > Cc: Gustavo Padovan <gustavo at padovan.org>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Rafael Antognolli <rafael.antognolli at intel.com>
> 
> > ---
> >  drivers/dma-buf/dma-fence.c  | 17 ++++++-----------
> >  drivers/dma-buf/sync_debug.c |  2 +-
> >  drivers/dma-buf/sync_file.c  |  8 +++++++-
> >  include/linux/dma-fence.h    |  2 ++
> >  4 files changed, 16 insertions(+), 13 deletions(-)

Finally pushed this one to drm-misc-fixes. Thanks.

	Gustavo