Re: [PATCH v6] drm/i915: Prevent overflow of execbuf.buffer_count and num_cliprects

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On 16/11/2017 10:50, Chris Wilson wrote:
We check whether the multiplies will overflow prior to calling
kmalloc_array so that we can respond with -EINVAL for the invalid user
arguments rather than treating it as an -ENOMEM that would otherwise
occur. However, as Dan Carpenter pointed out, we did an addition on the
unsigned int prior to passing to kmalloc_array where it would be
promoted to size_t for the calculation, thereby allowing it to overflow
and underallocate.

v2: buffer_count is currently limited to INT_MAX because we treat it as
signaled variable for LUT_HANDLE in eb_lookup_vma
v3: Move common checks for eb1/eb2 into the same function
v4: Put the check back for nfence*sizeof(user_fence) overflow
v5: access_ok uses ULONG_MAX but kvmalloc_array uses SIZE_MAX
v6: size_t and unsigned long are not type-equivalent on 32b

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 66 +++++++++++++++++++-----------
  1 file changed, 43 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 435ed95df144..53ccb27bfe91 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -2074,23 +2074,27 @@ static struct drm_syncobj **
  get_fence_array(struct drm_i915_gem_execbuffer2 *args,
  		struct drm_file *file)
  {
-	const unsigned int nfences = args->num_cliprects;
+	const unsigned long nfences = args->num_cliprects;
  	struct drm_i915_gem_exec_fence __user *user;
  	struct drm_syncobj **fences;
-	unsigned int n;
+	unsigned long n;
  	int err;
if (!(args->flags & I915_EXEC_FENCE_ARRAY))
  		return NULL;
- if (nfences > SIZE_MAX / sizeof(*fences))
+	/* Check multiplication overflow for access_ok() and kvmalloc_array() */
+	BUILD_BUG_ON(sizeof(size_t) > sizeof(unsigned long));
+	if (nfences > min_t(unsigned long,
+			    ULONG_MAX / sizeof(*user),
+			    SIZE_MAX / sizeof(*fences)))
  		return ERR_PTR(-EINVAL);
user = u64_to_user_ptr(args->cliprects_ptr);
-	if (!access_ok(VERIFY_READ, user, nfences * 2 * sizeof(u32)))
+	if (!access_ok(VERIFY_READ, user, nfences * sizeof(*user)))
  		return ERR_PTR(-EFAULT);
- fences = kvmalloc_array(args->num_cliprects, sizeof(*fences),
+	fences = kvmalloc_array(nfences, sizeof(*fences),
  				__GFP_NOWARN | GFP_KERNEL);
  	if (!fences)
  		return ERR_PTR(-ENOMEM);
@@ -2447,6 +2451,26 @@ i915_gem_do_execbuffer(struct drm_device *dev,
  	return err;
  }
+static size_t eb_element_size(void)
+{
+	return (sizeof(struct drm_i915_gem_exec_object2) +
+		sizeof(struct i915_vma *) +
+		sizeof(unsigned int));
+}
+
+static bool check_buffer_count(size_t count)
+{
+	const size_t sz = eb_element_size();
+
+	/*
+	 * When using LUT_HANDLE, we impose a limit of INT_MAX for the lookup
+	 * array size (see eb_create()). Otherwise, we can accept an array as
+	 * large as can be addressed (though use large arrays at your peril)!
+	 */
+
+	return !(count < 1 || count > INT_MAX || count > SIZE_MAX / sz - 1);
+}
+
  /*
   * Legacy execbuffer just creates an exec2 list from the original exec object
   * list array and passes it to the real function.
@@ -2455,18 +2479,16 @@ int
  i915_gem_execbuffer(struct drm_device *dev, void *data,
  		    struct drm_file *file)
  {
-	const size_t sz = (sizeof(struct drm_i915_gem_exec_object2) +
-			   sizeof(struct i915_vma *) +
-			   sizeof(unsigned int));
  	struct drm_i915_gem_execbuffer *args = data;
  	struct drm_i915_gem_execbuffer2 exec2;
  	struct drm_i915_gem_exec_object *exec_list = NULL;
  	struct drm_i915_gem_exec_object2 *exec2_list = NULL;
+	const size_t count = args->buffer_count;
  	unsigned int i;
  	int err;
- if (args->buffer_count < 1 || args->buffer_count > SIZE_MAX / sz - 1) {
-		DRM_DEBUG("execbuf2 with %d buffers\n", args->buffer_count);
+	if (!check_buffer_count(count)) {
+		DRM_DEBUG("execbuf2 with %zd buffers\n", count);
  		return -EINVAL;
  	}
@@ -2485,9 +2507,9 @@ i915_gem_execbuffer(struct drm_device *dev, void *data,
  		return -EINVAL;
/* Copy in the exec list from userland */
-	exec_list = kvmalloc_array(args->buffer_count, sizeof(*exec_list),
+	exec_list = kvmalloc_array(count, sizeof(*exec_list),
  				   __GFP_NOWARN | GFP_KERNEL);
-	exec2_list = kvmalloc_array(args->buffer_count + 1, sz,
+	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
  				    __GFP_NOWARN | GFP_KERNEL);
  	if (exec_list == NULL || exec2_list == NULL) {
  		DRM_DEBUG("Failed to allocate exec list for %d buffers\n",
@@ -2498,7 +2520,7 @@ i915_gem_execbuffer(struct drm_device *dev, void *data,
  	}
  	err = copy_from_user(exec_list,
  			     u64_to_user_ptr(args->buffers_ptr),
-			     sizeof(*exec_list) * args->buffer_count);
+			     sizeof(*exec_list) * count);
  	if (err) {
  		DRM_DEBUG("copy %d exec entries failed %d\n",
  			  args->buffer_count, err);
@@ -2548,16 +2570,14 @@ int
  i915_gem_execbuffer2(struct drm_device *dev, void *data,
  		     struct drm_file *file)
  {
-	const size_t sz = (sizeof(struct drm_i915_gem_exec_object2) +
-			   sizeof(struct i915_vma *) +
-			   sizeof(unsigned int));
  	struct drm_i915_gem_execbuffer2 *args = data;
  	struct drm_i915_gem_exec_object2 *exec2_list;
  	struct drm_syncobj **fences = NULL;
+	const size_t count = args->buffer_count;
  	int err;
- if (args->buffer_count < 1 || args->buffer_count > SIZE_MAX / sz - 1) {
-		DRM_DEBUG("execbuf2 with %d buffers\n", args->buffer_count);
+	if (!check_buffer_count(count)) {
+		DRM_DEBUG("execbuf2 with %zd buffers\n", count);
  		return -EINVAL;
  	}
@@ -2565,17 +2585,17 @@ i915_gem_execbuffer2(struct drm_device *dev, void *data,
  		return -EINVAL;
/* Allocate an extra slot for use by the command parser */
-	exec2_list = kvmalloc_array(args->buffer_count + 1, sz,
+	exec2_list = kvmalloc_array(count + 1, eb_element_size(),
  				    __GFP_NOWARN | GFP_KERNEL);
  	if (exec2_list == NULL) {
-		DRM_DEBUG("Failed to allocate exec list for %d buffers\n",
-			  args->buffer_count);
+		DRM_DEBUG("Failed to allocate exec list for %zd buffers\n",
+			  count);
  		return -ENOMEM;
  	}
  	if (copy_from_user(exec2_list,
  			   u64_to_user_ptr(args->buffers_ptr),
-			   sizeof(*exec2_list) * args->buffer_count)) {
-		DRM_DEBUG("copy %d exec entries failed\n", args->buffer_count);
+			   sizeof(*exec2_list) * count)) {
+		DRM_DEBUG("copy %zd exec entries failed\n", count);
  		kvfree(exec2_list);
  		return -EFAULT;
  	}


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

Regards,

Tvrtko
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