[BUG] mm/ksm: KSM merge drops userfaultfd RWP markers

[shaos <shawnq1@163.com>]

Hello,

KSM can merge anonymous pages in a VMA registered for userfaultfd write-protect (RWP), then replace an armed PTE with an ordinary accessible PTE. The VMA remains registered, but a read of the replaced page completes without the expected UFFD_PAGEFAULT_FLAG_RWP event.

The RWP PTE is represented by PAGE_NONE plus the architecture's _PAGE_UFFD marker. In replace_page(), KSM creates the replacement with mk_pte(kpage, vma->vm_page_prot) and does not preserve that state from orig_pte. The PoC observes pages_sharing increase, reads the replaced page without an event, then re-arms it and receives the RWP event again. A non-merging control keeps the marker and continues to fault.

The same replacement path also loses soft-dirty state; that separate observable is documented at https://gist.github.com/shaos26/9db36768f01eb83c6aa14ee9fc057388. This report focuses on userfaultfd RWP tracking.

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/e887665d7868dc323d73a1d678cacd3d/raw/e3e39212903e5bd5ba12f55a94ba7ae5b25bb238/console-output.txt
kernel config: https://gist.githubusercontent.com/shaos26/e887665d7868dc323d73a1d678cacd3d/raw/3cd943ba8c6026b43ef016a37d07efd2533a317a/kernel-config.txt
reproducer: https://gist.githubusercontent.com/shaos26/e887665d7868dc323d73a1d678cacd3d/raw/7fc8685bb0cc01fca488fc096d4859b8b70af3fc/poc.c

Build the kernel from the revision above with the attached config. Build the static PoC with:

    gcc -static -O2 -pthread -Wall -Wextra -o poc poc.c

The program is intended to run as /init in a disposable initramfs and mounts procfs and sysfs itself. Boot it with:

    qemu-system-x86_64 -m 1G -smp 2 -machine q35 -nographic -no-reboot \
        -kernel build/arch/x86/boot/bzImage -initrd initramfs.cpio.gz \
        -append "console=ttyS0 rdinit=/init panic=-1 oops=panic loglevel=6"

Two fresh positive boots reported RESULT=KSM_RWP_MARKER_LOSS_CONFIRMED; the close non-merging control reported RESULT=KSM_CONTROL_TRAPS_PRESERVED. No privilege escalation, memory corruption, or cross-process access is claimed; the defect is loss of the caller's RWP access tracking. I checked the affected code against upstream master f47d223627f8; this source path appears unchanged there, but I have only reproduced the behavior on v7.3-rc7.

Thanks,

shaos