CVE-2026-63950

Summary

In the Linux kernel, the following vulnerability has been resolved:

mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one

Initialize nr_pages to 1 at the start of each loop iteration, like folio_referenced_one() does.

Without this, nr_pages computed by a previous folio_unmap_pte_batch() call can be reused on a later iteration that does not run folio_unmap_pte_batch() again.

mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call madvise(MADV_FREE), then make the last page device-exclusive via HMM_DMIRROR_EXCLUSIVE.

Trigger node reclaim through sysfs. Now, in try_to_unmap_one(), we will first clear the first 15 out of 16 entries mapping the lazyfree folio. This will set nr_pages to 15. In the next pvmw walk, this nr_pages gets reused on a device-exclusive pte, thus potentially corrupting folio refcount/mapcount.

At the moment, I have a userspace program which can make the kernel spit out a trace, but the blow up is in folio_referenced_one(), because there are existing bugs in the interaction between device-private and rmap (which too I am investigating). I did a one liner kernel change to avoid going into folio_referenced_one(), and the kernel blows up at folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted.

Note that the bug is there not since file folio batching but lazyfree folio batching, since device-exclusive only works for anonymous folios.

Userspace visible effect is simply kernel crashing somewhere due to refcount/mapcount corruption.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/0fcc34d0d8fefca4fea349e45c10e3a3d90350eb
• https://git.kernel.org/stable/c/f611db9b771b2b6775357555d2517af044fca4f0
• https://git.kernel.org/stable/c/3f8968e9cbf95d5d87d32218906cab0b9b9eddbe

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