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authorVlastimil Babka <vbabka@suse.cz>2017-11-01 10:21:25 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2017-11-01 18:09:58 +0300
commitcb0631fd3cf9e989cd48293fe631cbc402aec9a9 (patch)
tree5e53bbe7addecdfcf046603314fe964b4af607fb /arch/x86/mm
parent89db69d670a11274c323af48479841d3d765bd49 (diff)
downloadlinux-cb0631fd3cf9e989cd48293fe631cbc402aec9a9.tar.xz
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 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>
Diffstat (limited to 'arch/x86/mm')
-rw-r--r--arch/x86/mm/fault.c11
1 files changed, 10 insertions, 1 deletions
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index e2baeaa053a5..7101c281c7ce 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1440,7 +1440,17 @@ 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;
@@ -1467,7 +1477,6 @@ 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);