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

[Gustavo Padovan]

Hi Chris,

2017-02-14 Chris Wilson <chris at chris-wilson.co.uk>:

> [  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
> 
> 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.
> 
> 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>
> ---
>  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(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index d1f1f456f5c4..dd2d7b6d2831 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -74,11 +74,6 @@ int dma_fence_signal_locked(struct dma_fence *fence)
>  	if (WARN_ON(!fence))
>  		return -EINVAL;
>  
> -	if (!ktime_to_ns(fence->timestamp)) {
> -		fence->timestamp = ktime_get();
> -		smp_mb__before_atomic();
> -	}
> -
>  	if (test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
>  		ret = -EINVAL;
>  
> @@ -86,8 +81,11 @@ int dma_fence_signal_locked(struct dma_fence *fence)
>  		 * we might have raced with the unlocked dma_fence_signal,
>  		 * still run through all callbacks
>  		 */
> -	} else
> +	} else {
> +		fence->timestamp = ktime_get();
> +		set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
>  		trace_dma_fence_signaled(fence);
> +	}
>  
>  	list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
>  		list_del_init(&cur->node);
> @@ -114,14 +112,11 @@ int dma_fence_signal(struct dma_fence *fence)
>  	if (!fence)
>  		return -EINVAL;
>  
> -	if (!ktime_to_ns(fence->timestamp)) {
> -		fence->timestamp = ktime_get();
> -		smp_mb__before_atomic();
> -	}
> -
>  	if (test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>  		return -EINVAL;
>  
> +	fence->timestamp = ktime_get();
> +	set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
>  	trace_dma_fence_signaled(fence);
>  
>  	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) {
> diff --git a/drivers/dma-buf/sync_debug.c b/drivers/dma-buf/sync_debug.c
> index c769dc653b34..bfead12390f2 100644
> --- a/drivers/dma-buf/sync_debug.c
> +++ b/drivers/dma-buf/sync_debug.c
> @@ -84,7 +84,7 @@ static void sync_print_fence(struct seq_file *s,
>  		   show ? "_" : "",
>  		   sync_status_str(status));
>  
> -	if (status) {
> +	if (test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags)) {
>  		struct timespec64 ts64 =
>  			ktime_to_timespec64(fence->timestamp);

How about add this test_bit() to dma_fence_is_signaled_locked() so
we test both for DMA_FENCE_FLAG_SIGNALED_BIT and
DMA_FENCE_FLAG_TIMESTAMP_BIT there at the same time?

Gustavo