Re: [Intel-gfx] [PATCH] drm/i915/gem: Fix a NULL pointer dereference in igt_request_rewind()

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 30/11/2021 14:15, Zhou Qingyang wrote:
> In igt_request_rewind(), mock_context(i915, "A") is assigned to ctx[0]
> and used in i915_gem_context_get_engine(). There is a dereference
> of ctx[0] in i915_gem_context_get_engine(), which could lead to a NULL
> pointer dereference on failure of mock_context(i915, "A") .
> 
> So as mock_context(i915, "B").
> 
> Although this bug is not serious for it belongs to testing code, it is
> better to be fixed to avoid unexpected failure in testing.
> 
> Fix this bugs by adding checks about ctx[0] and ctx[1].
> 
> This bug was found by a static analyzer. The analysis employs
> differential checking to identify inconsistent security operations
> (e.g., checks or kfrees) between two code paths and confirms that the
> inconsistent operations are not recovered in the current function or
> the callers, so they constitute bugs.
> 
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
> 
> Builds with CONFIG_DRM_I915_SELFTEST=y show no new warnings,
> and our static analyzer no longer warns about this code.
> 
> Fixes: ca883c304f54 ("drm/i915/selftests: Pass intel_context to mock_request")

I think it is this one instead:

591c0fb85d1c ("drm/i915: Exercise request cancellation using a mock selftest")

Fix looks correct so:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Thanks for the patch!

Regards,

Tvrtko

P.S.
Although Fixes: is probably a bit over the top since it is selftests only so I'll probably drop it while applying.

> Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
> ---
>   drivers/gpu/drm/i915/selftests/i915_request.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/selftests/i915_request.c b/drivers/gpu/drm/i915/selftests/i915_request.c
> index d67710d10615..d6fc7b892793 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_request.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_request.c
> @@ -209,6 +209,10 @@ static int igt_request_rewind(void *arg)
>   	int err = -EINVAL;
>   
>   	ctx[0] = mock_context(i915, "A");
> +	if (!ctx[0]) {
> +		err = -ENOMEM;
> +		goto err_ctx_0;
> +	}
>   
>   	ce = i915_gem_context_get_engine(ctx[0], RCS0);
>   	GEM_BUG_ON(IS_ERR(ce));
> @@ -223,6 +227,10 @@ static int igt_request_rewind(void *arg)
>   	i915_request_add(request);
>   
>   	ctx[1] = mock_context(i915, "B");
> +	if (!ctx[1]) {
> +		err = -ENOMEM;
> +		goto err_ctx_1;
> +	}
>   
>   	ce = i915_gem_context_get_engine(ctx[1], RCS0);
>   	GEM_BUG_ON(IS_ERR(ce));
> @@ -261,9 +269,11 @@ static int igt_request_rewind(void *arg)
>   	i915_request_put(vip);
>   err_context_1:
>   	mock_context_close(ctx[1]);
> +err_ctx_1:
>   	i915_request_put(request);
>   err_context_0:
>   	mock_context_close(ctx[0]);
> +err_ctx_0:
>   	mock_device_flush(i915);
>   	return err;
>   }
>