mm/mprotect: Fix soft-dirty check in can_change_pte_writable()

The check wanted to make sure when soft-dirty tracking is enabled we won't grant write bit by accident, as a page fault is needed for dirty tracking. The intention is correct but we didn't check it right because VM_SOFTDIRTY set actually means soft-dirty tracking disabled. Fix it.

It wasn't a bug for a long time because we used to only optimize the write bit settings in change_pte_range() for page caches, and since we've got a higher level check in vma_wants_writenotify(), we will never set the bit MM_CP_TRY_CHANGE_WRITABLE for soft-dirty enabled page caches, hence even if we checked with the wrong value of VM_SOFTDIRTY in change_pte_range() it'll just be an no-op. Functionally it was still correct, even if cpu cycles wasted.

                  MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);

However after the recent work of anonymous page optimization on exclusive pages we'll start to make it wrong because anonymous page does not require the check in vma_wants_writenotify() hence it'll suffer from the wrong check here in can_change_pte_writable().

We can easily verify this with any exclusive anonymous page, like program below:

=======8<======
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <assert.h>
#include <inttypes.h>
#include <stdint.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdbool.h>

  #define BIT_ULL(nr)                   (1ULL << (nr))
  #define PM_SOFT_DIRTY                 BIT_ULL(55)

unsigned int psize;
char *page;

  uint64_t pagemap_read_vaddr(int fd, void *vaddr)
  {
      uint64_t value;
      int ret;

      ret = pread(fd, &value, sizeof(uint64_t),
                  ((uint64_t)vaddr >> 12) * sizeof(uint64_t));
      assert(ret == sizeof(uint64_t));

      return value;

}

  void clear_refs_write(void)
  {
      int fd = open("/proc/self/clear_refs", O_RDWR);

      assert(fd >= 0);
      write(fd, "4", 2);
      close(fd);

}

  #define  check_soft_dirty(str, expect)  do {                            \
          bool dirty = pagemap_read_vaddr(fd, page) & PM_SOFT_DIRTY;      \
          if (dirty != expect) {                                          \
              printf("ERROR: %s, soft-dirty=%d (expect: %d)\n", str, dirty, expect); \
              exit(-1);                                                   \
          }                                                               \

} while (0)

  int main(void)
  {
      int fd = open("/proc/self/pagemap", O_RDONLY);

      assert(fd >= 0);
      psize = getpagesize();
      page = mmap(NULL, psize, PROT_READ|PROT_WRITE,
                  MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
      assert(page != MAP_FAILED);

      *page = 1;
      check_soft_dirty("Just faulted in page", 1);
      clear_refs_write();
      check_soft_dirty("Clear_refs written", 0);
      mprotect(page, psize, PROT_READ);
      check_soft_dirty("Marked RO", 0);
      mprotect(page, psize, PROT_READ|PROT_WRITE);
      check_soft_dirty("Marked RW", 0);
      *page = 2;
      check_soft_dirty("Wrote page again", 1);

      munmap(page, psize);
      close(fd);
      printf("Test passed.\n");

      return 0;

}

=======8<======

So even if commit 64fe24a3e05e kept the old behavior and didn't attempt to change the behavior here, the bug will only be able to be triggered after commit 64fe24a3e05e because only anonymous page will suffer from it.

Fixes 64fe24a3e05e ("mm/mprotect: try avoiding write faults for exclusive anonymous pages when changing protection")

Signed-off-by Peter Xu <[email protected]>

---
 mm/mprotect.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

mm/mprotect.c +0 -0

index 0420c3ed936c..804807ab14e6 100644
@@ -48,8 +48,11 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 	if (pte_protnone(pte) || !pte_dirty(pte))

mm/mprotect.c +0 -0

 	/* Do we need write faults for uffd-wp tracking? */

Modifying your test to map page from a file MAP_SHARED gives me under 5.18.11-100.fc35.x86_64:

53,54d52
< FILE *file = fopen("tmpfile", "w+");
< int file_fd;
56d53
< assert(file);
59,61d55
<
< file_fd = fileno(file);
< ftruncate(file_fd, psize);
63c57
< MAP_SHARED, file_fd, 0);
---

Quoted 1 line

                  MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);

t480s: ~ $ sudo ./tmp
ERROR: Wrote page again, soft-dirty=0 (expect: 1

IMHO, while the check in vma_wants_writenotify() is correct and makes sure that the pages are kept R/O by the generic machinery. We get vma_wants_writenotify(), so we activate MM_CP_TRY_CHANGE_WRITABLE. The wrong logic in can_change_pte_writable(), however, maps the page writable again without caring about softdirty.

At least that would be my explanation for the failure. But maybe I messes up something else :)

Can we turn that into a vm selftest in tools/testing/selftests/vm/soft-dirty.c, and also extend it by MAP_SHARED froma file as above?

Quoted 4 lines

mm/mprotect.c +0 -0

@@ -48,8 +48,11 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 	if (pte_protnone(pte) || !pte_dirty(pte))
 		return false;
 

mm/mprotect.c static inline bool can_change_pte_writable(struct vm_area_struct *vma, · line 56

Quoted 8 lines

mm/mprotect.c +5 -2

-	/* Do we need write faults for softdirty tracking? */
-	if ((vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte))
+	/*
+	 * Do we need write faults for softdirty tracking?  Note,
+	 * soft-dirty is enabled when !VM_SOFTDIRTY.
+	 */
+	if (!(vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte))
 		return false;

I wonder if we now want, just as in vma_wants_writenotify(), an early check for IS_ENABLED(CONFIG_MEM_SOFT_DIRTY), to optimize this out completely.