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 b109b87050df ("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@xxxxxxxxx
Fixes: b109b87050df ("mm/munlock: replace clear_page_mlock() by final clearance")
Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Reported-by: syzbot+e985d3026c4fd041578e@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6729f475.050a0220.701a.0019.GAE@xxxxxxxxxx
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c6c7bb3ea71b..216fbbfbedcf 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1048,6 +1048,7 @@ __always_inline bool free_pages_prepare(struct page *page,
bool skip_kasan_poison = should_skip_kasan_poison(page);
bool init = want_init_on_free();
bool compound = PageCompound(page);
+ struct folio *folio = page_folio(page);
VM_BUG_ON_PAGE(PageTail(page), page);
@@ -1057,6 +1058,20 @@ __always_inline bool free_pages_prepare(struct page *page,
if (memcg_kmem_online() && PageMemcgKmem(page))
__memcg_kmem_uncharge_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 */
reset_page_owner(page, order);
diff --git a/mm/swap.c b/mm/swap.c
index b8e3259ea2c4..59f30a981c6f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -78,20 +78,6 @@ static void __page_cache_release(struct folio *folio, struct lruvec **lruvecp,
lruvec_del_folio(*lruvecp, folio);
__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))) {
- 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);
- }
}
/*
