Messages in this thread | | | Date | Thu, 22 Sep 2022 09:46:35 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH] mm: hugetlb: fix UAF in hugetlb_handle_userfault |
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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
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