Re: [PATCH][next] dma-buf: Fix NULL pointer dereference in dma_fence_enable_sw_signaling()

[Gustavo A. R. Silva]

On 10/11/23 10:03, Kees Cook wrote:
> On Wed, Oct 11, 2023 at 08:03:43AM -0600, Gustavo A. R. Silva wrote:
> > Currently, a NULL pointer dereference will happen in function
> > `dma_fence_enable_sw_signaling()` (at line 615), in case `chain`
> > is not allocated in `mock_chain()` and this function returns
> > `NULL` (at line 86). See below:
> >
> > drivers/dma-buf/st-dma-fence-chain.c:
> >   86         chain = mock_chain(NULL, f, 1);
> >   87         if (!chain)
> >   88                 err = -ENOMEM;
> >   89
> >   90         dma_fence_enable_sw_signaling(chain);
>
> Instead of the larger patch, should line 88 here just do a "return
> -ENOMEM" instead?

Nope. I would have to add a `goto` to skip `dma_fence_enable_sw_signaling(chain)`.

I originally thought of that, but as other _signaling() functions have
sanity-checks inside, I decided to go with that solution.

This bug has been there since Sep 2022. So, adding a sanity check inside that
function should prevent any other issue of this same kind to enter the codebase
and stay there for years.

--
Gustavo

> -Kees
>
> > drivers/dma-buf/dma-fence.c:
> >   611 void dma_fence_enable_sw_signaling(struct dma_fence *fence)
> >   612 {
> >   613         unsigned long flags;
> >   614
> >   615         spin_lock_irqsave(fence->lock, flags);
> > 			       ^^^^^^^^^^^
> > 				    |
> > 			  NULL pointer reference
> > 			  if fence == NULL
> >
> >   616         __dma_fence_enable_signaling(fence);
> >   617         spin_unlock_irqrestore(fence->lock, flags);
> >   618 }
> >
> > Fix this by adding a NULL check before dereferencing `fence` in
> > `dma_fence_enable_sw_signaling()`. This will prevent any other NULL
> > pointer dereference when the `fence` passed as an argument is `NULL`.
> >
> > Addresses-Coverity: ("Dereference after null check")
> > Fixes: d62c43a953ce ("dma-buf: Enable signaling on fence for selftests")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> > ---
> >   drivers/dma-buf/dma-fence.c | 9 ++++++++-
> >   include/linux/dma-fence.h   | 2 +-
> >   2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > index 8aa8f8cb7071..4d2f13560d0f 100644
> > --- a/drivers/dma-buf/dma-fence.c
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -607,14 +607,21 @@ static bool __dma_fence_enable_signaling(struct dma_fence *fence)
> >    * This will request for sw signaling to be enabled, to make the fence
> >    * complete as soon as possible. This calls &dma_fence_ops.enable_signaling
> >    * internally.
> > + *
> > + * Returns 0 on success and a negative error value when @fence is NULL.
> >    */
> > -void dma_fence_enable_sw_signaling(struct dma_fence *fence)
> > +int dma_fence_enable_sw_signaling(struct dma_fence *fence)
> >   {
> >   	unsigned long flags;
> >   
> > +	if (!fence)
> > +		return -EINVAL;
> > +
> >   	spin_lock_irqsave(fence->lock, flags);
> >   	__dma_fence_enable_signaling(fence);
> >   	spin_unlock_irqrestore(fence->lock, flags);
> > +
> > +	return 0;
> >   }
> >   EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
> >   
> > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > index ebe78bd3d121..1e4025e925e6 100644
> > --- a/include/linux/dma-fence.h
> > +++ b/include/linux/dma-fence.h
> > @@ -399,7 +399,7 @@ int dma_fence_add_callback(struct dma_fence *fence,
> >   			   dma_fence_func_t func);
> >   bool dma_fence_remove_callback(struct dma_fence *fence,
> >   			       struct dma_fence_cb *cb);
> > -void dma_fence_enable_sw_signaling(struct dma_fence *fence);
> > +int dma_fence_enable_sw_signaling(struct dma_fence *fence);
> >   
> >   /**
> >    * dma_fence_is_signaled_locked - Return an indication if the fence
> > --
> > 2.34.1