drm/shmem-helper: Fix BUG_ON() on mmap(PROT_WRITE, MAP_PRIVATE)

From: "Wachowski, Karol" <[email protected]>

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;

[Artem: bp to fix CVE-2024-39497, in order to adapt this patch to branch 5.10 add header file mm/internal.h]

---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 5 +++++
 1 file changed, 5 insertions(+)

index e8f07305e279..37f347f39c88 100644
@@ -17,6 +17,8 @@
 #include <drm/drm_prime.h>
 #include <drm/drm_print.h>
 
+#include "../../../mm/internal.h"   /* is_cow_mapping() */
+
 /**
  * DOC: overview
  *

@@ -630,6 +632,9 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 		return ret;
 	}
 
+	if (is_cow_mapping(vma->vm_flags))
+		return -EINVAL;
+
 	shmem = to_drm_gem_shmem_obj(obj);
 
 	ret = drm_gem_shmem_get_pages(shmem);

2.43.0