Re: [BUG] mm: userfaultfd RWP drops a populated shmem PMD

[David Hildenbrand (Arm)]

On 10/20/26 13:07, shaos wrote:
> Hello,
> 
> UFFDIO_RWPROTECT_MODE_RWP can return success for a present shmem THP mapping while dropping the PMD mapping instead of leaving the populated page resident and write-protected. The next read refaults the page and completes without a UFFD_PAGEFAULT_FLAG_RWP event.
> 
> The PoC verifies ShmemPmdMapped=2048 kB immediately before the ioctl. After the successful ioctl, smaps reports Rss=0 and ShmemPmdMapped=0; the read completes with the original byte and no event. Re-arming the refaulted PTE and a separate 4 KiB shmem control both produce the expected RWP event. The observed issue is loss of the mapping and protection tracking, not corruption of the file contents.
> 
> The source path splits and clears the non-anonymous PMD for MM_CP_UFFD_RWP, but the retry does not repopulate a PTE table because pgtable_populate_needed() only recognizes MM_CP_UFFD_WP. The ioctl nevertheless returns success.
> 
> HEAD commit:    24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
> source version: Linux 7.3.0-rc7
> architecture:   x86-64
> compiler:       gcc 11.4.0, GNU ld 2.38
> QEMU:           6.2.0
> 
> console output: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/da13df3db0c05df09952f74bedb457f57c53b78b/console-output.txt
> kernel config: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/baa5ee61ce748667104c480560ab5116a100e3fd/kernel-config.txt
> reproducer: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/420975783e9d5dd88bc4a69190d8ef194faf5fdc/poc.c
> 
> Build the kernel from the revision above with the attached config. Build the static PoC and initramfs from poc.c; the program is /init and creates the shmem mapping itself. A fresh-boot run used:
> 
>     timeout 300 qemu-system-x86_64 -m 2G -smp 2 \
>         -kernel build/arch/x86/boot/bzImage -initrd initramfs.cpio.gz \
>         -append "console=ttyS0 panic=1 oops=panic loglevel=7 kfence.sample_interval=0" \
>         -nographic -no-reboot -monitor none
> 
> Two fresh boots reported RESULT=RWP_SHMEM_PMD_BYPASS_CONFIRMED and exited cleanly. The affected code appears unchanged in upstream master f47d223627f8; I have only reproduced it on v7.3-rc7. No privilege escalation, memory corruption, or guest escape is claimed.
> 
> Thanks,
> 
> shaos
> 

https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@xxxxxxxxx/

-- 
Cheers,

David