[PATCH 4.9 006/138] x86/mm: fix use-after-free of vma during userfaultfd fault

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Vlastimil Babka 

commit cb0631fd3cf9e989cd48293fe631cbc402aec9a9 upstream.

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

  mmap(&(0x7f00/0xfff000)=nil, 0xfff000, 0x3, 0x32, 0x, 
0x0)
  mmap(&(0x7f011000/0x3000)=nil, 0x3000, 0x1, 0x32, 0x, 0x0)
  r0 = userfaultfd(0x0)
  ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f002000-0x18)={0xaa, 0x0, 0x0})
  ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, 
&(0x7f019000)={{&(0x7f012000/0x2000)=nil, 0x2000}, 0x1, 0x0})
  r1 = gettid()
  
syz_open_dev$evdev(&(0x7f013000-0x12)="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 a3c4fb7c9c2e ("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 

Reported-by: Dmitry Vyukov 
Suggested-by: Kirill A. Shutemov 
Fixes: 3c4fb7c9c2e ("x86/mm: Fix fault error path using unsafe vma pointer")
Reviewed-by: Andrea Arcangeli 
Signed-off-by: Vlastimil Babka 
Signed-off-by: Linus Torvalds 
Cc: Eric Biggers 
Signed-off-by: Greg Kroah-Hartman 

---
 arch/x86/mm/fault.c |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1393,7 +1393,17 @@ good_area:
 * make sure we exit gracefully rather than endlessly redo
 * the fault.  Since we never set FAULT_FLAG_RETRY_NOWAIT, if
 * 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);
major |= fault & VM_FAULT_MAJOR;
 
@@ -1420,7 +1430,6 @@ good_area:
return;
}
 
-   pkey = vma_pkey(vma);
up_read(>mmap_sem);
if (unlikely(fault & VM_FAULT_ERROR)) {
mm_fault_error(regs, error_code, address, , fault);

[PATCH 4.9 006/138] x86/mm: fix use-after-free of vma during userfaultfd fault

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Vlastimil Babka 

commit cb0631fd3cf9e989cd48293fe631cbc402aec9a9 upstream.

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

  mmap(&(0x7f00/0xfff000)=nil, 0xfff000, 0x3, 0x32, 0x, 
0x0)
  mmap(&(0x7f011000/0x3000)=nil, 0x3000, 0x1, 0x32, 0x, 0x0)
  r0 = userfaultfd(0x0)
  ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f002000-0x18)={0xaa, 0x0, 0x0})
  ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, 
&(0x7f019000)={{&(0x7f012000/0x2000)=nil, 0x2000}, 0x1, 0x0})
  r1 = gettid()
  
syz_open_dev$evdev(&(0x7f013000-0x12)="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 a3c4fb7c9c2e ("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 

Reported-by: Dmitry Vyukov 
Suggested-by: Kirill A. Shutemov 
Fixes: 3c4fb7c9c2e ("x86/mm: Fix fault error path using unsafe vma pointer")
Reviewed-by: Andrea Arcangeli 
Signed-off-by: Vlastimil Babka 
Signed-off-by: Linus Torvalds 
Cc: Eric Biggers 
Signed-off-by: Greg Kroah-Hartman 

---
 arch/x86/mm/fault.c |   11 ++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1393,7 +1393,17 @@ good_area:
 * make sure we exit gracefully rather than endlessly redo
 * the fault.  Since we never set FAULT_FLAG_RETRY_NOWAIT, if
 * 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);
major |= fault & VM_FAULT_MAJOR;
 
@@ -1420,7 +1430,6 @@ good_area:
return;
}
 
-   pkey = vma_pkey(vma);
up_read(>mmap_sem);
if (unlikely(fault & VM_FAULT_ERROR)) {
mm_fault_error(regs, error_code, address, , fault);