[PATCH] fbdev: omap2: fix use-after-free in omapfb_mmap

[PATCH] fbdev: omap2: fix use-after-free in omapfb_mmap

[Hongling Zeng]

omapfb_mmap() has a race condition with OMAPFB_SETUP_PLANE ioctl that
can lead to use-after-free:

The fb_mmap() entry point holds mm_lock but not lock (fb_info->lock),
while ioctl handlers like OMAPFB_SETUP_PLANE hold lock but not mm_lock.
This allows concurrent execution.

In omapfb_mmap():
1. rg = omapfb_get_mem_region(ofbi->region);      // Get old region ref
2. start = omapfb_get_region_paddr(ofbi);          // Read from NEW region
3. len = fix->smem_len;                             // Read from NEW region
4. vm_iomap_memory(vma, start, len);               // Map NEW region memory
5. atomic_inc(&rg->map_count);                      // Increment OLD region!

Concurrently, OMAPFB_SETUP_PLANE can:
- Reassign ofbi->region = new_rg
- Update fix->smem_len
- OMAPFB_SETUP_MEM then checks NEW region's map_count (0!) and frees it

This leaves userspace with a mapping to freed physical memory.

The fix is to read all required values (start, len) from the same
region reference (rg) that will have its map_count incremented,
preventing the region from being freed while still mapped.

Cc: stable@vger.kernel.org
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
---
 drivers/video/fbdev/omap2/omapfb/omapfb-main.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/omap2/omapfb/omapfb-main.c b/drivers/video/fbdev/omap2/omapfb/omapfb-main.c
index d70deb6a9150..853bd55621ec 100644
--- a/drivers/video/fbdev/omap2/omapfb/omapfb-main.c
+++ b/drivers/video/fbdev/omap2/omapfb/omapfb-main.c
@@ -1099,8 +1099,11 @@ static int omapfb_mmap(struct fb_info *fbi, struct vm_area_struct *vma)
 
 	rg = omapfb_get_mem_region(ofbi->region);
 
-	start = omapfb_get_region_paddr(ofbi);
-	len = fix->smem_len;
+	if (ofbi->rotation_type == OMAP_DSS_ROT_VRFB)
+		start = rg->vrfb.paddr[0];
+	else
+		start = rg->paddr;
+	len = rg->size;
 
 	DBG("user mmap region start %lx, len %d, off %lx\n", start, len,
 			vma->vm_pgoff << PAGE_SHIFT);
-- 
2.25.1

Claude review: fbdev: omap2: fix use-after-free in omapfb_mmap

[Claude Code Review Bot]

Overall Series Review

Subject: fbdev: omap2: fix use-after-free in omapfb_mmap
Author: Hongling Zeng <zenghongling@kylinos.cn>
Patches: 1
Reviewed: 2026-06-04T13:05:53.596691

---

This is a single patch fixing a real use-after-free race condition in the omapfb driver's `mmap` path. The analysis in the commit message is accurate and well-explained: `omapfb_mmap()` takes a reference to `rg` via `omapfb_get_mem_region(ofbi->region)`, but then calls `omapfb_get_region_paddr(ofbi)` which re-dereferences `ofbi->region` — a pointer that can be changed concurrently by `OMAPFB_SETUP_PLANE`. This means `start` and `len` can come from a different region than the one whose `map_count` gets incremented, allowing the actual mapped region to be freed.

The fix is correct in principle: it reads `start` and `len` directly from `rg` (the locked region) rather than going through helpers that re-read `ofbi->region`. This ensures all values are consistent with the region being reference-counted.

**Verdict: The patch is correct and should be accepted**, with one minor observation below.

---
Generated by Claude Code Patch Reviewer

Claude review: fbdev: omap2: fix use-after-free in omapfb_mmap

[Claude Code Review Bot]

Patch Review

**Correctness: Good.** The patch correctly inlines the logic of `omapfb_get_region_paddr()` using the already-locked `rg` pointer instead of going through `ofbi`:

```c
-	start = omapfb_get_region_paddr(ofbi);
-	len = fix->smem_len;
+	if (ofbi->rotation_type == OMAP_DSS_ROT_VRFB)
+		start = rg->vrfb.paddr[0];
+	else
+		start = rg->paddr;
+	len = rg->size;
```

The original `omapfb_get_region_paddr()` does:
```c
static u32 omapfb_get_region_paddr(const struct omapfb_info *ofbi)
{
	if (ofbi->rotation_type == OMAP_DSS_ROT_VRFB)
		return ofbi->region->vrfb.paddr[0];
	else
		return ofbi->region->paddr;
}
```

The patch correctly replaces `ofbi->region->` with `rg->` for the paddr access, and replaces `fix->smem_len` (which is also updated by `set_fb_fix()` during the ioctl) with `rg->size` — both now derived from the locked region.

**Note on `ofbi->rotation_type`:** The condition still reads `ofbi->rotation_type` without holding a lock that protects it. However, `rotation_type` is set once during probe and is not changed by `OMAPFB_SETUP_PLANE`, so this is safe. It's a property of the fb_info, not of the region.

**Note on `fix->smem_len` vs `rg->size`:** The original code read `fix->smem_len` which gets updated by `set_fb_fix()` inside `omapfb_setup_plane()` to reflect the new region. Switching to `rg->size` is correct — it reads the size of the region we actually locked and will map-count.

**Commit message quality:** Excellent. The step-by-step race scenario is clearly laid out and accurate. The Cc: stable tag is appropriate for a use-after-free fix.

**Minor style observation:** The inlined code duplicates the body of `omapfb_get_region_paddr()`. An alternative would be to refactor that helper to take a `struct omapfb2_mem_region *` parameter instead of `struct omapfb_info *`. However, this is a legacy fbdev driver unlikely to see further development, so the pragmatic inline approach is fine and arguably clearer for a -stable backport.

**Overall: Reviewed-by worthy.** The fix is minimal, correct, and addresses a real race condition.

---
Generated by Claude Code Patch Reviewer