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Subject[PATCH 4.9 006/138] x86/mm: fix use-after-free of vma during userfaultfd fault
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vlastimil Babka <vbabka@suse.cz>

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(&(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 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 <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>
Cc: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

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