The patch titled Subject: mm/userfaultfd: propagate uffd-wp bit when PTE-mapping the huge zeropage has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-userfaultfd-propagate-uffd-wp-bit-when-pte-mapping-the-huge-zeropage.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Hildenbrand david@redhat.com Subject: mm/userfaultfd: propagate uffd-wp bit when PTE-mapping the huge zeropage Date: Thu, 2 Mar 2023 18:54:23 +0100

Currently, we'd lose the userfaultfd-wp marker when PTE-mapping a huge zeropage, resulting in the next write faults in the PMD range not triggering uffd-wp events.

Various actions (partial MADV_DONTNEED, partial mremap, partial munmap, partial mprotect) could trigger this. However, most importantly, un-protecting a single sub-page from the userfaultfd-wp handler when processing a uffd-wp event will PTE-map the shared huge zeropage and lose the uffd-wp bit for the remainder of the PMD.

Let's properly propagate the uffd-wp bit to the PMDs.

#define _GNU_SOURCE #include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <stdbool.h> #include <inttypes.h> #include <fcntl.h> #include <unistd.h> #include <errno.h> #include <poll.h> #include <pthread.h> #include <sys/mman.h> #include <sys/syscall.h> #include <sys/ioctl.h> #include <linux/userfaultfd.h>

static size_t pagesize; static int uffd; static volatile bool uffd_triggered;

#define barrier() __asm__ __volatile__("": : :"memory")

static void uffd_wp_range(char *start, size_t size, bool wp) { struct uffdio_writeprotect uffd_writeprotect;

uffd_writeprotect.range.start = (unsigned long) start; uffd_writeprotect.range.len = size; if (wp) { uffd_writeprotect.mode = UFFDIO_WRITEPROTECT_MODE_WP; } else { uffd_writeprotect.mode = 0; } if (ioctl(uffd, UFFDIO_WRITEPROTECT, &uffd_writeprotect)) { fprintf(stderr, "UFFDIO_WRITEPROTECT failed: %d\n", errno); exit(1); } }

static void *uffd_thread_fn(void *arg) { static struct uffd_msg msg; ssize_t nread;

while (1) { struct pollfd pollfd; int nready;

pollfd.fd = uffd; pollfd.events = POLLIN; nready = poll(&pollfd, 1, -1); if (nready == -1) { fprintf(stderr, "poll() failed: %d\n", errno); exit(1); }

nread = read(uffd, &msg, sizeof(msg)); if (nread <= 0) continue;

if (msg.event != UFFD_EVENT_PAGEFAULT || !(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP)) { printf("FAIL: wrong uffd-wp event fired\n"); exit(1); }

/* un-protect the single page. */ uffd_triggered = true; uffd_wp_range((char *)(uintptr_t)msg.arg.pagefault.address, pagesize, false); } return arg; }

static int setup_uffd(char *map, size_t size) { struct uffdio_api uffdio_api; struct uffdio_register uffdio_register; pthread_t thread;

uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY); if (uffd < 0) { fprintf(stderr, "syscall() failed: %d\n", errno); return -errno; }

uffdio_api.api = UFFD_API; uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP; if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) { fprintf(stderr, "UFFDIO_API failed: %d\n", errno); return -errno; }

if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) { fprintf(stderr, "UFFD_FEATURE_WRITEPROTECT missing\n"); return -ENOSYS; }

uffdio_register.range.start = (unsigned long) map; uffdio_register.range.len = size; uffdio_register.mode = UFFDIO_REGISTER_MODE_WP; if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) { fprintf(stderr, "UFFDIO_REGISTER failed: %d\n", errno); return -errno; }

pthread_create(&thread, NULL, uffd_thread_fn, NULL);

return 0; }

int main(void) { const size_t size = 4 * 1024 * 1024ull; char *map, *cur;

pagesize = getpagesize();

map = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0); if (map == MAP_FAILED) { fprintf(stderr, "mmap() failed\n"); return -errno; }

if (madvise(map, size, MADV_HUGEPAGE)) { fprintf(stderr, "MADV_HUGEPAGE failed\n"); return -errno; }

if (setup_uffd(map, size)) return 1;

/* Read the whole range, populating zeropages. */ madvise(map, size, MADV_POPULATE_READ);

/* Write-protect the whole range. */ uffd_wp_range(map, size, true);

/* Make sure uffd-wp triggers on each page. */ for (cur = map; cur < map + size; cur += pagesize) { uffd_triggered = false;

barrier(); /* Trigger a write fault. */ *cur = 1; barrier();

if (!uffd_triggered) { printf("FAIL: uffd-wp did not trigger\n"); return 1; } }

printf("PASS: uffd-wp triggered\n"); return 0; }

Link: https://lkml.kernel.org/r/20230302175423.589164-1-david@redhat.com Fixes: e06f1e1dd499 ("userfaultfd: wp: enabled write protection in userfaultfd API") Signed-off-by: David Hildenbrand david@redhat.com Acked-by: Peter Xu peterx@redhat.com Cc: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Jerome Glisse jglisse@redhat.com Cc: Shaohua Li shli@fb.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

--- a/mm/huge_memory.c~mm-userfaultfd-propagate-uffd-wp-bit-when-pte-mapping-the-huge-zeropage +++ a/mm/huge_memory.c @@ -2037,7 +2037,7 @@ static void __split_huge_zero_page_pmd(s { struct mm_struct *mm = vma->vm_mm; pgtable_t pgtable; - pmd_t _pmd; + pmd_t _pmd, old_pmd; int i;

/* @@ -2048,7 +2048,7 @@ static void __split_huge_zero_page_pmd(s * * See Documentation/mm/mmu_notifier.rst */ - pmdp_huge_clear_flush(vma, haddr, pmd); + old_pmd = pmdp_huge_clear_flush(vma, haddr, pmd);

pgtable = pgtable_trans_huge_withdraw(mm, pmd); pmd_populate(mm, &_pmd, pgtable); @@ -2057,6 +2057,8 @@ static void __split_huge_zero_page_pmd(s pte_t *pte, entry; entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot); entry = pte_mkspecial(entry); + if (pmd_uffd_wp(old_pmd)) + entry = pte_mkuffd_wp(entry); pte = pte_offset_map(&_pmd, haddr); VM_BUG_ON(!pte_none(*pte)); set_pte_at(mm, haddr, pte, entry); _

Patches currently in -mm which might be from david@redhat.com are

mm-userfaultfd-propagate-uffd-wp-bit-when-pte-mapping-the-huge-zeropage.patch