[PATCH] drm/panthor: set __GFP_SKIP_KASAN

[Chia-I Wu via B4 Relay <devnull+olvaffe.gmail.com@kernel.org>]

From: Chia-I Wu <olvaffe@gmail.com>

Pages that can be swapped out should be allocated with __GFP_SKIP_KASAN.
Rather than setting the flag directly, replace GFP_HIGHUSER by
(GFP_HIGHUSER_MOVABLE & ~__GFP_MOVABLE) instead, which should match the
preceding comment better.

On a CONFIG_KASAN_HW_TAGS=y system, without __GFP_SKIP_KASAN, the page
allocator assigns a valid tag to both the kernel mapping and MTE,
instead of assigning the match-all KASAN_TAG_KERNEL tag to the kernel
mapping. If userspace also maps the page with PROT_MTE and modifies the
MTE tag, accessing the page via the kernel mapping results in KASAN
invalid-access, such as

  BUG: KASAN: invalid-access in swap_writepage+0xb0/0x21c
  Read at addr f5ffff81aa71dff8 by task WM.task-4/6956
  Pointer tag: [f5], memory tag: [f9]

While userspace cannot map drm gem objects with PROT_MTE, the problem is
shmem_swapin_cluster. When it swaps in a cluster of pages using our gfp
flags, some of the pages might belong to other mappings that have
PROT_MTE.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
---
The latest snapdragons appear to have MTE support. drm/msm might need
the same treatment.
---
 drivers/gpu/drm/panthor/panthor_gem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/panthor/panthor_gem.c b/drivers/gpu/drm/panthor/panthor_gem.c
index 13295d7a593df..08c03aa0db2f7 100644
--- a/drivers/gpu/drm/panthor/panthor_gem.c
+++ b/drivers/gpu/drm/panthor/panthor_gem.c
@@ -1013,7 +1013,8 @@ panthor_gem_create(struct drm_device *dev, size_t size, uint32_t flags,
 	 * going to pin these pages.
 	 */
 	mapping_set_gfp_mask(bo->base.filp->f_mapping,
-			     GFP_HIGHUSER | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
+			     (GFP_HIGHUSER_MOVABLE & ~__GFP_MOVABLE) |
+				     __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
 
 	ret = drm_gem_create_mmap_offset(&bo->base);
 	if (ret)

---
base-commit: 6101f78b684895d5860a96322e607e0f46f433ad
change-id: 20260512-panthor-kasan-10477239bad1

Best regards,
--  
Chia-I Wu <olvaffe@gmail.com>