[PATCH] ublk: clear VM_MAYWRITE on read-only ublk char device mmap

[Kanishka De Silva <kpskanna1915@gmail.com>]

ublk_ch_mmap() rejects mmap requests with VM_WRITE set, but never
clears VM_MAYWRITE on the resulting read-only mapping. This allows
a userspace daemon to mmap the per-queue command buffer PROT_READ,
then upgrade it to PROT_WRITE via mprotect(), since VM_MAYWRITE was
never cleared.

The command buffer holds struct ublksrv_io_desc entries that are
kernel-written ABI; a writable mapping lets an unprivileged daemon
process corrupt fields such as addr, op_flags, nr_sectors, and
start_sector.

Same bug class as the drm/panthor and drm/vc4 VM_MAYWRITE fixes, and
the 2026-08-13 ptp/vmclock fix (a5edadbae57e).

Verified via mprotect() PoC: before the fix, a PROT_READ mapping can
be upgraded to PROT_READ|PROT_WRITE and a write into the command
buffer corrupts io_desc fields (confirmed under KASAN). After the
fix, mprotect() returns -EACCES.

Fixes: 3fee8d7599e1 ("ublk_drv: add io_uring based userspace block driver")
Cc: stable@vger.kernel.org
Signed-off-by: Kanishka De Silva <kpskanna1915@gmail.com>
---
 drivers/block/ublk_drv.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 6c5bec7..e5ba07d 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -2653,6 +2653,12 @@ static int ublk_ch_mmap(struct file *filp, struct vm_area_struct *vma)
 	if (vma->vm_flags & VM_WRITE)
 		return -EPERM;
 
+	/*
+	 * The per-queue command buffer is kernel-written ABI; prevent
+	 * the daemon from upgrading to writable via mprotect().
+	 */
+	vm_flags_clear(vma, VM_MAYWRITE);
+
 	end = UBLKSRV_CMD_BUF_OFFSET + ub->dev_info.nr_hw_queues * max_sz;
 	if (phys_off < UBLKSRV_CMD_BUF_OFFSET || phys_off >= end)
 		return -EINVAL;
-- 
2.53.0