[PATCH 6.9 257/281] drm/shmem-helper: Fix BUG_ON() on mmap(PROT_WRITE, MAP_PRIVATE)

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6.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wachowski, Karol <karol.wachowski@xxxxxxxxx>

commit 39bc27bd688066a63e56f7f64ad34fae03fbe3b8 upstream.

Lack of check for copy-on-write (COW) mapping in drm_gem_shmem_mmap
allows users to call mmap with PROT_WRITE and MAP_PRIVATE flag
causing a kernel panic due to BUG_ON in vmf_insert_pfn_prot:
BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));

Return -EINVAL early if COW mapping is detected.

This bug affects all drm drivers using default shmem helpers.
It can be reproduced by this simple example:
void *ptr = mmap(0, size, PROT_WRITE, MAP_PRIVATE, fd, mmap_offset);
ptr[0] = 0;

Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Cc: Noralf Trønnes <noralf@xxxxxxxxxxx>
Cc: Eric Anholt <eric@xxxxxxxxxx>
Cc: Rob Herring <robh@xxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx> # v5.2+
Signed-off-by: Wachowski, Karol <karol.wachowski@xxxxxxxxx>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@xxxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20240520100514.925681-1-jacek.lawrynowicz@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -610,6 +610,9 @@ int drm_gem_shmem_mmap(struct drm_gem_sh
 		return ret;
 	}
 
+	if (is_cow_mapping(vma->vm_flags))
+		return -EINVAL;
+
 	dma_resv_lock(shmem->base.resv, NULL);
 	ret = drm_gem_shmem_get_pages(shmem);
 	dma_resv_unlock(shmem->base.resv);





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