linux-kernel - [PATCH] drm/i915: Minimize uaccess exposure in i915_gem_execbuffer2

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Date:   Thu, 27 Feb 2020 16:08:26 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        intel-gfx@...ts.freedesktop.org
Subject: [PATCH] drm/i915: Minimize uaccess exposure in i915_gem_execbuffer2_ioctl()

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@...radead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.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