The patch titled
Subject: mm: avoid data corruption on CoW fault into PFN-mapped VMA
has been added to the -mm tree. Its filename is
mm-avoid-data-corruption-on-cow-fault-into-pfn-mapped-vma.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-avoid-data-corruption-on-cow-fault-into-pfn-mapped-vma.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-avoid-data-corruption-on-cow-fault-into-pfn-mapped-vma.patch
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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
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------------------------------------------------------
From: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
Subject: mm: avoid data corruption on CoW fault into PFN-mapped VMA
Jeff Moyer has reported that one of xfstests triggers a warning when run
on DAX-enabled filesystem:
WARNING: CPU: 76 PID: 51024 at mm/memory.c:2317 wp_page_copy+0xc40/0xd50
...
wp_page_copy+0x98c/0xd50 (unreliable)
do_wp_page+0xd8/0xad0
__handle_mm_fault+0x748/0x1b90
handle_mm_fault+0x120/0x1f0
__do_page_fault+0x240/0xd70
do_page_fault+0x38/0xd0
handle_page_fault+0x10/0x30
The warning happens on failed __copy_from_user_inatomic() which tries to
copy data into a CoW page.
This happens because of race between MADV_DONTNEED and CoW page fault:
CPU0 CPU1
handle_mm_fault()
do_wp_page()
wp_page_copy()
do_wp_page()
madvise(MADV_DONTNEED)
zap_page_range()
zap_pte_range()
ptep_get_and_clear_full()
<TLB flush>
__copy_from_user_inatomic()
sees empty PTE and fails
WARN_ON_ONCE(1)
clear_page()
The solution is to re-try __copy_from_user_inatomic() under PTL after
checking that PTE is matches the orig_pte.
The second copy attempt can still fail, like due to non-readable PTE, but
there's nothing reasonable we can do about, except clearing the CoW page.
Link: http://lkml.kernel.org/r/20200218154151.13349-1-kirill.shutemov@xxxxxxxxxxxxxxx
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Tested-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Justin He <Justin.He@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memory.c | 35 +++++++++++++++++++++++++++--------
1 file changed, 27 insertions(+), 8 deletions(-)
--- a/mm/memory.c~mm-avoid-data-corruption-on-cow-fault-into-pfn-mapped-vma
+++ a/mm/memory.c
@@ -2257,7 +2257,7 @@ static inline bool cow_user_page(struct
bool ret;
void *kaddr;
void __user *uaddr;
- bool force_mkyoung;
+ bool locked = false;
struct vm_area_struct *vma = vmf->vma;
struct mm_struct *mm = vma->vm_mm;
unsigned long addr = vmf->address;
@@ -2282,11 +2282,11 @@ static inline bool cow_user_page(struct
* On architectures with software "accessed" bits, we would
* take a double page fault, so mark it accessed here.
*/
- force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf->orig_pte);
- if (force_mkyoung) {
+ if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
pte_t entry;
vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
+ locked = true;
if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
/*
* Other thread has already handled the fault
@@ -2310,18 +2310,37 @@ static inline bool cow_user_page(struct
* zeroes.
*/
if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
+ if (locked)
+ goto warn;
+
+ /* Re-validate under PTL if the page is still mapped */
+ vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
+ locked = true;
+ if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
+ /* The PTE changed under us. Retry page fault. */
+ ret = false;
+ goto pte_unlock;
+ }
+
/*
- * Give a warn in case there can be some obscure
- * use-case
+ * The same page can be mapped back since last copy attampt.
+ * Try to copy again under PTL.
*/
- WARN_ON_ONCE(1);
- clear_page(kaddr);
+ if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
+ /*
+ * Give a warn in case there can be some obscure
+ * use-case
+ */
+warn:
+ WARN_ON_ONCE(1);
+ clear_page(kaddr);
+ }
}
ret = true;
pte_unlock:
- if (force_mkyoung)
+ if (locked)
pte_unmap_unlock(vmf->pte, vmf->ptl);
kunmap_atomic(kaddr);
flush_dcache_page(dst);
_
Patches currently in -mm which might be from kirill@xxxxxxxxxxxxx are
mm-avoid-data-corruption-on-cow-fault-into-pfn-mapped-vma.patch
mm-filemap-fix-a-data-race-in-filemap_fault.patch
