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Subject[PATCH] drm/i915: Minimize uaccess exposure in i915_gem_execbuffer2_ioctl()
Date
With CONFIG_CC_OPTIMIZE_FOR_SIZE, objtool reports:

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x5b7: call to gen8_canonical_addr() with UACCESS enabled

This means i915_gem_execbuffer2_ioctl() is calling gen8_canonical_addr()
-- and indirectly, sign_extend64() -- from the user_access_begin/end
critical region (i.e, with SMAP disabled).

While it's probably harmless in this case, in general we like to avoid
extra function calls in SMAP-disabled regions because it can open up
inadvertent security holes.

Fix it by moving the gen8_canonical_addr() conversion to a separate loop
before user_access_begin() is called.

Note that gen8_canonical_addr() is now called *before* masking off the
PIN_OFFSET_MASK bits. That should be ok because it just does a sign
extension and ignores the masked lower bits anyway.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index d5a0f5ae4a8b..183cab13e028 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -2947,6 +2947,13 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
u64_to_user_ptr(args->buffers_ptr);
unsigned int i;

+ /*
+ * Do the call to gen8_canonical_addr() outside the
+ * uaccess-enabled region to minimize uaccess exposure.
+ */
+ for (i = 0; i < args->buffer_count; i++)
+ exec2_list[i].offset = gen8_canonical_addr(exec2_list[i].offset);
+
/* Copy the new buffer offsets back to the user's exec list. */
/*
* Note: count * sizeof(*user_exec_list) does not overflow,
@@ -2962,9 +2969,7 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
if (!(exec2_list[i].offset & UPDATE))
continue;

- exec2_list[i].offset =
- gen8_canonical_addr(exec2_list[i].offset & PIN_OFFSET_MASK);
- unsafe_put_user(exec2_list[i].offset,
+ unsafe_put_user(exec2_list[i].offset & PIN_OFFSET_MASK,
&user_exec_list[i].offset,
end_user);
}
--
2.21.1
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