Re: [PATCH] mm: hugetlb: fix UAF in hugetlb_handle_userfault

[David Hildenbrand]

On 22.09.22 01:57, Mike Kravetz wrote:
> On 09/21/22 10:48, Mike Kravetz wrote:
> > On 09/21/22 16:34, Liu Shixin wrote:
> > > The vma_lock and hugetlb_fault_mutex are dropped before handling
> > > userfault and reacquire them again after handle_userfault(), but
> > > reacquire the vma_lock could lead to UAF[1] due to the following
> > > race,
> > >
> > > hugetlb_fault
> > >    hugetlb_no_page
> > >      /*unlock vma_lock */
> > >      hugetlb_handle_userfault
> > >        handle_userfault
> > >          /* unlock mm->mmap_lock*/
> > >                                             vm_mmap_pgoff
> > >                                               do_mmap
> > >                                                 mmap_region
> > >                                                   munmap_vma_range
> > >                                                     /* clean old vma */
> > >          /* lock vma_lock again  <--- UAF */
> > >      /* unlock vma_lock */
> > >
> > > Since the vma_lock will unlock immediately after hugetlb_handle_userfault(),
> > > let's drop the unneeded lock and unlock in hugetlb_handle_userfault() to fix
> > > the issue.
> >
> > Thank you very much!
> >
> > When I saw this report, the obvious fix was to do something like what you have
> > done below.  That looks fine with a few minor comments.
> >
> > One question I have not yet answered is, "Does this same issue apply to
> > follow_hugetlb_page()?".  I believe it does.  follow_hugetlb_page calls
> > hugetlb_fault which could result in the fault being processed by userfaultfd.
> > If we experience the race above, then the associated vma could no longer be
> > valid when returning from hugetlb_fault.  follow_hugetlb_page and callers
> > have a flag (locked) to deal with dropping mmap lock.  However, I am not sure
> > if it is handled correctly WRT userfaultfd.  I think this needs to be answered
> > before fixing.  And, if the follow_hugetlb_page code needs to be fixed it
> > should be done at the same time.
>
> To at least verify this code path, I added userfaultfd handling to the gup_test
> program in kernel selftests.  When doing basic gup test on a hugetlb page in
> a userfaultfd registered range, I hit this warning:
>
> [ 6939.867796] FAULT_FLAG_ALLOW_RETRY missing 1
> [ 6939.871503] CPU: 2 PID: 5720 Comm: gup_test Not tainted 6.0.0-rc6-next-20220921+ #72
> [ 6939.874562] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1.fc35 04/01/2014
> [ 6939.877707] Call Trace:
> [ 6939.878745]  <TASK>
> [ 6939.879779]  dump_stack_lvl+0x6c/0x9f
> [ 6939.881199]  handle_userfault.cold+0x14/0x1e
> [ 6939.882830]  ? find_held_lock+0x2b/0x80
> [ 6939.884370]  ? __mutex_unlock_slowpath+0x45/0x280
> [ 6939.886145]  hugetlb_handle_userfault+0x90/0xf0
> [ 6939.887936]  hugetlb_fault+0xb7e/0xda0
> [ 6939.889409]  ? vprintk_emit+0x118/0x3a0
> [ 6939.890903]  ? _printk+0x58/0x73
> [ 6939.892279]  follow_hugetlb_page.cold+0x59/0x145
> [ 6939.894116]  __get_user_pages+0x146/0x750
> [ 6939.895580]  __gup_longterm_locked+0x3e9/0x680
> [ 6939.897023]  ? seqcount_lockdep_reader_access.constprop.0+0xa5/0xb0
> [ 6939.898939]  ? lockdep_hardirqs_on+0x7d/0x100
> [ 6939.901243]  gup_test_ioctl+0x320/0x6e0
> [ 6939.902202]  __x64_sys_ioctl+0x87/0xc0
> [ 6939.903220]  do_syscall_64+0x38/0x90
> [ 6939.904233]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> [ 6939.905423] RIP: 0033:0x7fbb53830f7b
>
> This is because userfaultfd is expecting FAULT_FLAG_ALLOW_RETRY which is not
> set in this path.

Right. Without being able to drop the mmap lock, we cannot continue. And 
we don't know if we can drop it without FAULT_FLAG_ALLOW_RETRY.

FAULT_FLAG_ALLOW_RETRY is only set when we can communicate to the caller 
that we dropped the mmap lock [e.g., int *locked parameter].

All code paths that pass NULL won't be able to handle --  especially 
surprisingly also pin_user_pages_fast() -- cannot trigger usefaultfd and 
will result in this warning.


A "sane" example is access via /proc/self/mem via ptrace: we don't want 
to trigger userfaultfd, but instead simply fail the GUP get/pin.


Now, this is just a printed *warning* (not a WARN/BUG/taint) that tells 
us that there is a GUP user that isn't prepared for userfaultfd. So it 
rather points out a missing GUP adaption -- incomplete userfaultfd 
support. And we seem to have plenty of that judging that 
pin_user_pages_fast_only().

Maybe the printed stack trace is a bit too much and makes this look very 
scary.

> Adding John, Peter and David on Cc: as they are much more fluent in all the
> fault and FOLL combinations and might have immediate suggestions.  It is going
> to take me a little while to figure out:
> 1) How to make sure we get the right flags passed to handle_userfault

This is a GUP caller problem -- or rather, how GUP has to deal with 
userfaultfd.

> 2) How to modify follow_hugetlb_page as userfaultfd can certainly drop
>     mmap_lock.  So we can not assume vma still exists upon return.

Again, we have to communicate to the GUP caller that we dropped the mmap 
lock. And that requires GUP caller changes.

--
Thanks,

David / dhildenb