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Message-ID: <aG0_-79QiMEk3N-R@lappy>
Date: Tue, 8 Jul 2025 11:57:47 -0400
From: Sasha Levin <sashal@...nel.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>, peterx@...hat.com,
aarcange@...hat.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration
entries
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@...nel.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@...nel.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 ]---
--
Thanks,
Sasha
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