[PATCH net] ptp: vmclock: prevent read-only mappings from becoming writable

[PATCH net] ptp: vmclock: prevent read-only mappings from becoming writable

[Abdifatah Suruur]

vmclock_miscdev_mmap() rejects writable mappings of the shared vmclock
ABI page with -EROFS, but leaves VM_MAYWRITE set.  Userspace can map the
page read-only and then upgrade it to writable with mprotect(), after
which the guest can corrupt the host-written timekeeping data (sequence
counter, UTC time, TSC offset) that the vmclock ABI defines as read-only.

Clear VM_MAYWRITE on the read-only path so the mapping cannot be
upgraded, as i915 does for its read-only objects and as fixed in drm/vc4
(CVE-2026-68445) and drm/panthor (CVE-2024-53071).

Reported-by: Abdifatah Suruur <suruurism@gmail.com>

Fixes: 2050327242269 ("ptp: Add support for the AMZNC10C vmclock device")
Cc: stable@vger.kernel.org
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>

---
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -372,6 +372,12 @@
 	if ((vma->vm_flags & (VM_READ|VM_WRITE)) != VM_READ)
 		return -EROFS;
 
+	/*
+	 * Restrict the read-only mapping so it cannot be upgraded to
+	 * writable later with mprotect().
+	 */
+	vm_flags_clear(vma, VM_MAYWRITE);
+
 	if (vma->vm_end - vma->vm_start != PAGE_SIZE || vma->vm_pgoff)
 		return -EINVAL;
 

Re: [PATCH net] ptp: vmclock: prevent read-only mappings from becoming writable

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 13 Aug 2026 20:47:07 +0300 you wrote:
> vmclock_miscdev_mmap() rejects writable mappings of the shared vmclock
> ABI page with -EROFS, but leaves VM_MAYWRITE set.  Userspace can map the
> page read-only and then upgrade it to writable with mprotect(), after
> which the guest can corrupt the host-written timekeeping data (sequence
> counter, UTC time, TSC offset) that the vmclock ABI defines as read-only.
> 
> Clear VM_MAYWRITE on the read-only path so the mapping cannot be
> upgraded, as i915 does for its read-only objects and as fixed in drm/vc4
> (CVE-2026-68445) and drm/panthor (CVE-2024-53071).
> 
> [...]

Here is the summary with links:
  - [net] ptp: vmclock: prevent read-only mappings from becoming writable
    https://git.kernel.org/netdev/net/c/a5edadbae57e

You are awesome, thank you!
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