CVE-2026-89793

Description

In the Linux kernel, the following vulnerability has been resolved: ublk: clear VM_MAYWRITE on read-only ublk char device mmap 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.

References

• https://git.kernel.org/stable/c/5befd06a72216869b607cf7724a4f16c6a2d3999
• https://git.kernel.org/stable/c/6e2b571b0a54755b06e092501913e1dfefe75d6c
• https://git.kernel.org/stable/c/be41733c24be58e2a1ef718c80fafdfb98a40d5e
• https://git.kernel.org/stable/c/e373c1acdbcf88cec533ece9f589020adaed0a78
• https://git.kernel.org/stable/c/fa5e1bc673ca59722608af67b27e65dba0c97926