ublk: clear VM_MAYWRITE on read-only ublk char device mmap

Summary

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.

Timeline

1. Sep 16, 2026, 9:54 AM UTCCVE published
2. Oct 19, 2026, 8:52 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 71f28f3136aff5890cd56de78abc673f8393cad9 · fixed in be41733c24be58e2a1ef718c80fafdfb98a40d5e; From 71f28f3136aff5890cd56de78abc673f8393cad9 · fixed in 5befd06a72216869b607cf7724a4f16c6a2d3999; From 71f28f3136aff5890cd56de78abc673f8393cad9 · fixed in e373c1acdbcf88cec533ece9f589020adaed0a78; From 71f28f3136aff5890cd56de78abc673f8393cad9 · fixed in fa5e1bc673ca59722608af67b27e65dba0c97926; From 71f28f3136aff5890cd56de78abc673f8393cad9 · fixed in 6e2b571b0a54755b06e092501913e1dfefe75d6c

• Linux Linux

  6.0

• KernelLinux

  From 6.0.0 · fixed in 6.6.157; From 6.7.0 · fixed in 6.12.110; From 6.13.0 · fixed in 6.18.51; From 6.19.0 · fixed in 7.2.5

References 5