Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration entries

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jul 8, 2025 at 8:57 AM Sasha Levin <sashal@kernel.org> wrote:
>
> On Tue, Jul 08, 2025 at 08:39:47AM -0700, Suren Baghdasaryan wrote:
> >On Tue, Jul 8, 2025 at 8:33 AM Sasha Levin <sashal@kernel.org> wrote:
> >>
> >> On Tue, Jul 08, 2025 at 05:10:44PM +0200, David Hildenbrand wrote:
> >> >On 01.07.25 02:57, Andrew Morton wrote:
> >> >>On Sun, 29 Jun 2025 23:19:58 -0400 Sasha Levin <sashal@kernel.org> wrote:
> >> >>
> >> >>>When handling non-swap entries in move_pages_pte(), the error handling
> >> >>>for entries that are NOT migration entries fails to unmap the page table
> >> >>>entries before jumping to the error handling label.
> >> >>>
> >> >>>This results in a kmap/kunmap imbalance which on CONFIG_HIGHPTE systems
> >> >>>triggers a WARNING in kunmap_local_indexed() because the kmap stack is
> >> >>>corrupted.
> >> >>>
> >> >>>Example call trace on ARM32 (CONFIG_HIGHPTE enabled):
> >> >>>   WARNING: CPU: 1 PID: 633 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
> >> >>>   Call trace:
> >> >>>     kunmap_local_indexed from move_pages+0x964/0x19f4
> >> >>>     move_pages from userfaultfd_ioctl+0x129c/0x2144
> >> >>>     userfaultfd_ioctl from sys_ioctl+0x558/0xd24
> >> >>>
> >> >>>The issue was introduced with the UFFDIO_MOVE feature but became more
> >> >>>frequent with the addition of guard pages (commit 7c53dfbdb024 ("mm: add
> >> >>>PTE_MARKER_GUARD PTE marker")) which made the non-migration entry code
> >> >>>path more commonly executed during userfaultfd operations.
> >> >>>
> >> >>>Fix this by ensuring PTEs are properly unmapped in all non-swap entry
> >> >>>paths before jumping to the error handling label, not just for migration
> >> >>>entries.
> >> >>
> >> >>I don't get it.
> >> >>
> >> >>>--- a/mm/userfaultfd.c
> >> >>>+++ b/mm/userfaultfd.c
> >> >>>@@ -1384,14 +1384,15 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> >> >>>             entry = pte_to_swp_entry(orig_src_pte);
> >> >>>             if (non_swap_entry(entry)) {
> >> >>>+                    pte_unmap(src_pte);
> >> >>>+                    pte_unmap(dst_pte);
> >> >>>+                    src_pte = dst_pte = NULL;
> >> >>>                     if (is_migration_entry(entry)) {
> >> >>>-                            pte_unmap(src_pte);
> >> >>>-                            pte_unmap(dst_pte);
> >> >>>-                            src_pte = dst_pte = NULL;
> >> >>>                             migration_entry_wait(mm, src_pmd, src_addr);
> >> >>>                             err = -EAGAIN;
> >> >>>-                    } else
> >> >>>+                    } else {
> >> >>>                             err = -EFAULT;
> >> >>>+                    }
> >> >>>                     goto out;
> >> >>
> >> >>where we have
> >> >>
> >> >>out:
> >> >>      ...
> >> >>      if (dst_pte)
> >> >>              pte_unmap(dst_pte);
> >> >>      if (src_pte)
> >> >>              pte_unmap(src_pte);
> >> >
> >> >AI slop?
> >>
> >> Nah, this one is sadly all me :(
> >>
> >> I was trying to resolve some of the issues found with linus-next on
> >> LKFT, and misunderstood the code. Funny enough, I thought that the
> >> change above "fixed" it by making the warnings go away, but clearly is
> >> the wrong thing to do so I went back to the drawing table...
> >>
> >> If you're curious, here's the issue: https://qa-reports.linaro.org/lkft/sashal-linus-next/build/v6.13-rc7-43418-g558c6dd4d863/testrun/29030370/suite/log-parser-test/test/exception-warning-cpu-pid-at-mmhighmem-kunmap_local_indexed/details/
> >
> >Any way to symbolize that Call trace? I can't find build artefacts to
> >extract vmlinux image...
>
> The build artifacts are at
> https://storage.tuxsuite.com/public/linaro/lkft/builds/2zSrTao2x4P640QKIx18JUuFdc1/
> but I couldn't get it to do the right thing. I'm guessing that I need
> some magical arm32 toolchain bits that I don't carry:
>
> cat tr.txt | ./scripts/decode_stacktrace.sh vmlinux
> <4>[   38.566145] ------------[ cut here ]------------
> <4>[ 38.566392] WARNING: CPU: 1 PID: 637 at mm/highmem.c:622 kunmap_local_indexed+0x198/0x1a4
> <4>[   38.569398] Modules linked in: nfnetlink ip_tables x_tables
> <4>[   38.570481] CPU: 1 UID: 0 PID: 637 Comm: uffd-unit-tests Not tainted 6.16.0-rc4 #1 NONE
> <4>[   38.570815] Hardware name: Generic DT based system
> <4>[   38.571073] Call trace:
> <4>[ 38.571239] unwind_backtrace from show_stack (arch/arm64/kernel/stacktrace.c:465)
> <4>[ 38.571602] show_stack from dump_stack_lvl (lib/dump_stack.c:118 (discriminator 1))
> <4>[ 38.571805] dump_stack_lvl from __warn (kernel/panic.c:791)
> <4>[ 38.572002] __warn from warn_slowpath_fmt+0xa8/0x174
> <4>[ 38.572290] warn_slowpath_fmt from kunmap_local_indexed+0x198/0x1a4
> <4>[ 38.572520] kunmap_local_indexed from move_pages_pte+0xc40/0xf48
> <4>[ 38.572970] move_pages_pte from move_pages+0x428/0x5bc
> <4>[ 38.573189] move_pages from userfaultfd_ioctl+0x900/0x1ec0
> <4>[ 38.573376] userfaultfd_ioctl from sys_ioctl+0xd24/0xd90
> <4>[ 38.573581] sys_ioctl from ret_fast_syscall+0x0/0x5c
> <4>[   38.573810] Exception stack(0xf9d69fa8 to 0xf9d69ff0)
> <4>[   38.574546] 9fa0:                   00001000 00000005 00000005 c028aa05 b2d3ecd8 b2d3ecc8
> <4>[   38.574919] 9fc0: 00001000 00000005 b2d3ece0 00000036 b2d3ed84 b2d3ed50 b2d3ed7c b2d3ed58
> <4>[   38.575131] 9fe0: 00000036 b2d3ecb0 b6df1861 b6d5f736
> <4>[   38.575511] ---[ end trace 0000000000000000 ]---

Ah, I know what's going on. 6.13.rc7 which is used in this test does
not have my fix 927e926d72d9 ("userfaultfd: fix PTE unmapping
stack-allocated PTE copies") (see
https://elixir.bootlin.com/linux/v6.13.7/source/mm/userfaultfd.c#L1284).
It was backported into 6.13.rc8. So, it tries to unmap a copy of a
mapped PTE, which will fail when CONFIG_HIGHPTE is enabled. So, it
makes sense that it is failing on arm32.

>
> --
> Thanks,
> Sasha