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mm: page_alloc: move mlocked flag clearance into free_pages_prepare()
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commit 66edc3a upstream.

Syzbot reported a bad page state problem caused by a page being freed
using free_page() still having a mlocked flag at free_pages_prepare()
stage:

  BUG: Bad page state in process syz.5.504  pfn:61f45
  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x61f45
  flags: 0xfff00000080204(referenced|workingset|mlocked|node=0|zone=1|lastcpupid=0x7ff)
  raw: 00fff00000080204 0000000000000000 dead000000000122 0000000000000000
  raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
  page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
  page_owner tracks the page as allocated
  page last allocated via order 0, migratetype Unmovable, gfp_mask 0x400dc0(GFP_KERNEL_ACCOUNT|__GFP_ZERO), pid 8443, tgid 8442 (syz.5.504), ts 201884660643, free_ts 201499827394
   set_page_owner include/linux/page_owner.h:32 [inline]
   post_alloc_hook+0x1f3/0x230 mm/page_alloc.c:1537
   prep_new_page mm/page_alloc.c:1545 [inline]
   get_page_from_freelist+0x303f/0x3190 mm/page_alloc.c:3457
   __alloc_pages_noprof+0x292/0x710 mm/page_alloc.c:4733
   alloc_pages_mpol_noprof+0x3e8/0x680 mm/mempolicy.c:2265
   kvm_coalesced_mmio_init+0x1f/0xf0 virt/kvm/coalesced_mmio.c:99
   kvm_create_vm virt/kvm/kvm_main.c:1235 [inline]
   kvm_dev_ioctl_create_vm virt/kvm/kvm_main.c:5488 [inline]
   kvm_dev_ioctl+0x12dc/0x2240 virt/kvm/kvm_main.c:5530
   __do_compat_sys_ioctl fs/ioctl.c:1007 [inline]
   __se_compat_sys_ioctl+0x510/0xc90 fs/ioctl.c:950
   do_syscall_32_irqs_on arch/x86/entry/common.c:165 [inline]
   __do_fast_syscall_32+0xb4/0x110 arch/x86/entry/common.c:386
   do_fast_syscall_32+0x34/0x80 arch/x86/entry/common.c:411
   entry_SYSENTER_compat_after_hwframe+0x84/0x8e
  page last free pid 8399 tgid 8399 stack trace:
   reset_page_owner include/linux/page_owner.h:25 [inline]
   free_pages_prepare mm/page_alloc.c:1108 [inline]
   free_unref_folios+0xf12/0x18d0 mm/page_alloc.c:2686
   folios_put_refs+0x76c/0x860 mm/swap.c:1007
   free_pages_and_swap_cache+0x5c8/0x690 mm/swap_state.c:335
   __tlb_batch_free_encoded_pages mm/mmu_gather.c:136 [inline]
   tlb_batch_pages_flush mm/mmu_gather.c:149 [inline]
   tlb_flush_mmu_free mm/mmu_gather.c:366 [inline]
   tlb_flush_mmu+0x3a3/0x680 mm/mmu_gather.c:373
   tlb_finish_mmu+0xd4/0x200 mm/mmu_gather.c:465
   exit_mmap+0x496/0xc40 mm/mmap.c:1926
   __mmput+0x115/0x390 kernel/fork.c:1348
   exit_mm+0x220/0x310 kernel/exit.c:571
   do_exit+0x9b2/0x28e0 kernel/exit.c:926
   do_group_exit+0x207/0x2c0 kernel/exit.c:1088
   __do_sys_exit_group kernel/exit.c:1099 [inline]
   __se_sys_exit_group kernel/exit.c:1097 [inline]
   __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1097
   x64_sys_call+0x2634/0x2640 arch/x86/include/generated/asm/syscalls_64.h:232
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  Modules linked in:
  CPU: 0 UID: 0 PID: 8442 Comm: syz.5.504 Not tainted 6.12.0-rc6-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
  Call Trace:
   <TASK>
   __dump_stack lib/dump_stack.c:94 [inline]
   dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
   bad_page+0x176/0x1d0 mm/page_alloc.c:501
   free_page_is_bad mm/page_alloc.c:918 [inline]
   free_pages_prepare mm/page_alloc.c:1100 [inline]
   free_unref_page+0xed0/0xf20 mm/page_alloc.c:2638
   kvm_destroy_vm virt/kvm/kvm_main.c:1327 [inline]
   kvm_put_kvm+0xc75/0x1350 virt/kvm/kvm_main.c:1386
   kvm_vcpu_release+0x54/0x60 virt/kvm/kvm_main.c:4143
   __fput+0x23f/0x880 fs/file_table.c:431
   task_work_run+0x24f/0x310 kernel/task_work.c:239
   exit_task_work include/linux/task_work.h:43 [inline]
   do_exit+0xa2f/0x28e0 kernel/exit.c:939
   do_group_exit+0x207/0x2c0 kernel/exit.c:1088
   __do_sys_exit_group kernel/exit.c:1099 [inline]
   __se_sys_exit_group kernel/exit.c:1097 [inline]
   __ia32_sys_exit_group+0x3f/0x40 kernel/exit.c:1097
   ia32_sys_call+0x2624/0x2630 arch/x86/include/generated/asm/syscalls_32.h:253
   do_syscall_32_irqs_on arch/x86/entry/common.c:165 [inline]
   __do_fast_syscall_32+0xb4/0x110 arch/x86/entry/common.c:386
   do_fast_syscall_32+0x34/0x80 arch/x86/entry/common.c:411
   entry_SYSENTER_compat_after_hwframe+0x84/0x8e
  RIP: 0023:0xf745d579
  Code: Unable to access opcode bytes at 0xf745d54f.
  RSP: 002b:00000000f75afd6c EFLAGS: 00000206 ORIG_RAX: 00000000000000fc
  RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 00000000ffffff9c RDI: 00000000f744cff4
  RBP: 00000000f717ae61 R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
   </TASK>

The problem was originally introduced by commit b109b87 ("mm/munlock:
replace clear_page_mlock() by final clearance"): it was focused on
handling pagecache and anonymous memory and wasn't suitable for lower
level get_page()/free_page() API's used for example by KVM, as with this
reproducer.

Fix it by moving the mlocked flag clearance down to free_page_prepare().

The bug itself if fairly old and harmless (aside from generating these
warnings), aside from a small memory leak - "bad" pages are stopped from
being allocated again.

Link: https://lkml.kernel.org/r/20241106195354.270757-1-roman.gushchin@linux.dev
Fixes: b109b87 ("mm/munlock: replace clear_page_mlock() by final clearance")
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reported-by: syzbot+e985d3026c4fd041578e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6729f475.050a0220.701a.0019.GAE@google.com
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@rgushchin @gregkh
rgushchin authored and gregkh committed Dec 14, 2024
1 parent 67a1023 commit 81ad32b
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Showing 2 changed files with 15 additions and 20 deletions.
15 changes: 15 additions & 0 deletions mm/page_alloc.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1082,12 +1082,27 @@ static __always_inline bool free_pages_prepare(struct page *page,
int bad = 0;
bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
bool init = want_init_on_free();
struct folio *folio = page_folio(page);

VM_BUG_ON_PAGE(PageTail(page), page);

trace_mm_page_free(page, order);
kmsan_free_page(page, order);

/*
* In rare cases, when truncation or holepunching raced with
* munlock after VM_LOCKED was cleared, Mlocked may still be
* found set here. This does not indicate a problem, unless
* "unevictable_pgs_cleared" appears worryingly large.
*/
if (unlikely(folio_test_mlocked(folio))) {
long nr_pages = folio_nr_pages(folio);

__folio_clear_mlocked(folio);
zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
}

if (unlikely(PageHWPoison(page)) && !order) {
/*
* Do not let hwpoison pages hit pcplists/buddy
Expand Down
20 changes: 0 additions & 20 deletions mm/swap.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -89,14 +89,6 @@ static void __page_cache_release(struct folio *folio)
__folio_clear_lru_flags(folio);
unlock_page_lruvec_irqrestore(lruvec, flags);
}
/* See comment on folio_test_mlocked in release_pages() */
if (unlikely(folio_test_mlocked(folio))) {
long nr_pages = folio_nr_pages(folio);

__folio_clear_mlocked(folio);
zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
}
}

static void __folio_put_small(struct folio *folio)
Expand Down Expand Up @@ -1021,18 +1013,6 @@ void release_pages(release_pages_arg arg, int nr)
__folio_clear_lru_flags(folio);
}

/*
* In rare cases, when truncation or holepunching raced with
* munlock after VM_LOCKED was cleared, Mlocked may still be
* found set here. This does not indicate a problem, unless
* "unevictable_pgs_cleared" appears worryingly large.
*/
if (unlikely(folio_test_mlocked(folio))) {
__folio_clear_mlocked(folio);
zone_stat_sub_folio(folio, NR_MLOCK);
count_vm_event(UNEVICTABLE_PGCLEARED);
}

list_add(&folio->lru, &pages_to_free);
}
if (lruvec)
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