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

[Zhou Qingyang]

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")
Signed-off-by: Zhou Qingyang <zhou1615 at 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;
 }
-- 
2.25.1