Commit cb0631fd authored Nov 01, 2017 by Vlastimil Babka Committed by Linus Torvalds Nov 01, 2017

x86/mm: fix use-after-free of vma during userfaultfd fault



Syzkaller with KASAN has reported a use-after-free of vma->vm_flags in
__do_page_fault() with the following reproducer:

  mmap(&(0x7f0000000000/0xfff000)=nil, 0xfff000, 0x3, 0x32, 0xffffffffffffffff, 0x0)
  mmap(&(0x7f0000011000/0x3000)=nil, 0x3000, 0x1, 0x32, 0xffffffffffffffff, 0x0)
  r0 = userfaultfd(0x0)
  ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f0000002000-0x18)={0xaa, 0x0, 0x0})
  ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, &(0x7f0000019000)={{&(0x7f0000012000/0x2000)=nil, 0x2000}, 0x1, 0x0})
  r1 = gettid()
  syz_open_dev$evdev(&(0x7f0000013000-0x12)="2f6465762f696e7075742f6576656e742300", 0x0, 0x0)
  tkill(r1, 0x7)

The vma should be pinned by mmap_sem, but handle_userfault() might (in a
return to userspace scenario) release it and then acquire again, so when
we return to __do_page_fault() (with other result than VM_FAULT_RETRY),
the vma might be gone.

Specifically, per Andrea the scenario is
 "A return to userland to repeat the page fault later with a
  VM_FAULT_NOPAGE retval (potentially after handling any pending signal
  during the return to userland). The return to userland is identified
  whenever FAULT_FLAG_USER|FAULT_FLAG_KILLABLE are both set in
  vmf->flags"

However, since commit a3c4fb7c ("x86/mm: Fix fault error path using
unsafe vma pointer") there is a vma_pkey() read of vma->vm_flags after
that point, which can thus become use-after-free.  Fix this by moving
the read before calling handle_mm_fault().

Reported-by: syzbot <bot+6a5269ce759a7bb12754ed9622076dc93f65a1f6@syzkaller.appspotmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Suggested-by: Kirill A. Shutemov <kirill@shutemov.name>
Fixes: 3c4fb7c9c2e ("x86/mm: Fix fault error path using unsafe vma pointer")
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 89db69d6

arch/x86/mm/fault.c

+10 −1

Original line number	Original line	Diff line number	Diff line
	@@ -1440,7 +1440,17 @@ good_area:
	* make sure we exit gracefully rather than endlessly redo		* make sure we exit gracefully rather than endlessly redo
	* the fault. Since we never set FAULT_FLAG_RETRY_NOWAIT, if		* the fault. Since we never set FAULT_FLAG_RETRY_NOWAIT, if
	* we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.		* we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
			*
			* Note that handle_userfault() may also release and reacquire mmap_sem
			* (and not return with VM_FAULT_RETRY), when returning to userland to
			* repeat the page fault later with a VM_FAULT_NOPAGE retval
			* (potentially after handling any pending signal during the return to
			* userland). The return to userland is identified whenever
			* FAULT_FLAG_USER\|FAULT_FLAG_KILLABLE are both set in flags.
			* Thus we have to be careful about not touching vma after handling the
			* fault, so we read the pkey beforehand.
	*/		*/
			pkey = vma_pkey(vma);
	fault = handle_mm_fault(vma, address, flags);		fault = handle_mm_fault(vma, address, flags);
	major \|= fault & VM_FAULT_MAJOR;		major \|= fault & VM_FAULT_MAJOR;

	@@ -1467,7 +1477,6 @@ good_area:
	return;		return;
	}		}

	pkey = vma_pkey(vma);
	up_read(&mm->mmap_sem);		up_read(&mm->mmap_sem);
	if (unlikely(fault & VM_FAULT_ERROR)) {		if (unlikely(fault & VM_FAULT_ERROR)) {
	mm_fault_error(regs, error_code, address, &pkey, fault);		mm_fault_error(regs, error_code, address, &pkey, fault);

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