[v4] mm/userfaultfd: fix memory corruption due to writeprotect

Commit Message

Nadav Amit March 4, 2021, 9:54 a.m. UTC

From: Nadav Amit <[e-mail]>

Userfaultfd self-test fails occasionally, indicating a memory
corruption.

Analyzing this problem indicates that there is a real bug since
mmap_lock is only taken for read in mwriteprotect_range() and defers
flushes, and since there is insufficient consideration of concurrent
deferred TLB flushes in wp_page_copy(). Although the PTE is flushed from
the TLBs in wp_page_copy(), this flush takes place after the copy has
already been performed, and therefore changes of the page are possible
between the time of the copy and the time in which the PTE is flushed.

To make matters worse, memory-unprotection using userfaultfd also poses
a problem. Although memory unprotection is logically a promotion of PTE
permissions, and therefore should not require a TLB flush, the current
userrfaultfd code might actually cause a demotion of the architectural
PTE permission: when userfaultfd_writeprotect() unprotects memory
region, it unintentionally *clears* the RW-bit if it was already set.
Note that this unprotecting a PTE that is not write-protected is a valid
use-case: the userfaultfd monitor might ask to unprotect a region that
holds both write-protected and write-unprotected PTEs.

The scenario that happens in selftests/vm/userfaultfd is as follows:

cpu0				cpu1			cpu2
----				----			----
							[ Writable PTE
							  cached in TLB ]
userfaultfd_writeprotect()
[ write-*unprotect* ]
mwriteprotect_range()
mmap_read_lock()
change_protection()

change_protection_range()
...
change_pte_range()
[ *clear* “write”-bit ]
[ defer TLB flushes ]
				[ page-fault ]
				...
				wp_page_copy()
				 cow_user_page()
				  [ copy page ]
							[ write to old
							  page ]
				...
				 set_pte_at_notify()

A similar scenario can happen:

cpu0		cpu1		cpu2		cpu3
----		----		----		----
						[ Writable PTE
				  		  cached in TLB ]
userfaultfd_writeprotect()
[ write-protect ]
[ deferred TLB flush ]
		userfaultfd_writeprotect()
		[ write-unprotect ]
		[ deferred TLB flush]
				[ page-fault ]
				wp_page_copy()
				 cow_user_page()
				 [ copy page ]
				 ...		[ write to page ]
				set_pte_at_notify()

This race exists since commit 292924b26024 ("userfaultfd: wp: apply
_PAGE_UFFD_WP bit"). Yet, as Yu Zhao pointed, these races became
apparent since commit 09854ba94c6a ("mm: do_wp_page() simplification")
which made wp_page_copy() more likely to take place, specifically if
page_count(page) > 1.

To resolve the aforementioned races, check whether there are pending
flushes on uffd-write-protected VMAs, and if there are, perform a flush
before doing the COW.

Further optimizations will follow to avoid during uffd-write-unprotect
unnecassary PTE write-protection and TLB flushes.

Cc: Andrea Arcangeli <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: [e-mail] # 5.9+
Suggested-by: Yu Zhao <[e-mail]>
Reviewed-by: Peter Xu <[e-mail]>
Tested-by: Peter Xu <[e-mail]>
Fixes: 09854ba94c6a ("mm: do_wp_page() simplification")
Signed-off-by: Nadav Amit <[e-mail]>

---
v3->v4:
* Fix the "Fixes" tag for real [Peter Xu]
* Reviewed-by, suggested-by tags [Peter Xu]
* Adding unlikely() [Peter Xu]

v2->v3:
* Do not acquire mmap_lock for write, flush conditionally instead [Yu]
* Change the fixes tag to the patch that made the race apparent [Yu]
* Removing patch to avoid write-protect on uffd unprotect. More
  comprehensive solution to follow (and avoid the TLB flush as well).
---
 mm/memory.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Patch

diff --git a/mm/memory.c b/mm/memory.c
index 9e8576a83147..79253cb3bcd5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3092,6 +3092,14 @@  static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		return handle_userfault(vmf, VM_UFFD_WP);
 	}
 
+	/*
+	 * Userfaultfd write-protect can defer flushes. Ensure the TLB
+	 * is flushed in this case before copying.
+	 */
+	if (unlikely(userfaultfd_wp(vmf->vma) &&
+		     mm_tlb_flush_pending(vmf->vma->vm_mm)))
+		flush_tlb_page(vmf->vma, vmf->address);
+
 	vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
 	if (!vmf->page) {
 		/*