On Mon, Oct 21, 2024 at 12:49:28PM -0700, Hugh Dickins wrote: > On Mon, 21 Oct 2024, Roman Gushchin wrote: > > > 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.0.15 pfn:1137bb > > page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff8881137bb870 pfn:0x1137bb > > flags: 0x400000000080000(mlocked|node=0|zone=1) > > raw: 0400000000080000 0000000000000000 dead000000000122 0000000000000000 > > raw: ffff8881137bb870 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 3005, tgid > > 3004 (syz.0.15), ts 61546 608067, free_ts 61390082085 > > 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+0x3008/0x31f0 mm/page_alloc.c:3457 > > __alloc_pages_noprof+0x292/0x7b0 mm/page_alloc.c:4733 > > alloc_pages_mpol_noprof+0x3e8/0x630 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:5500 [inline] > > kvm_dev_ioctl+0x13bb/0x2320 virt/kvm/kvm_main.c:5542 > > vfs_ioctl fs/ioctl.c:51 [inline] > > __do_sys_ioctl fs/ioctl.c:907 [inline] > > __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893 > > do_syscall_x64 arch/x86/entry/common.c:52 [inline] > > do_syscall_64+0x69/0x110 arch/x86/entry/common.c:83 > > entry_SYSCALL_64_after_hwframe+0x76/0x7e > > page last free pid 951 tgid 951 stack trace: > > reset_page_owner include/linux/page_owner.h:25 [inline] > > free_pages_prepare mm/page_alloc.c:1108 [inline] > > free_unref_page+0xcb1/0xf00 mm/page_alloc.c:2638 > > vfree+0x181/0x2e0 mm/vmalloc.c:3361 > > delayed_vfree_work+0x56/0x80 mm/vmalloc.c:3282 > > process_one_work kernel/workqueue.c:3229 [inline] > > process_scheduled_works+0xa5c/0x17a0 kernel/workqueue.c:3310 > > worker_thread+0xa2b/0xf70 kernel/workqueue.c:3391 > > kthread+0x2df/0x370 kernel/kthread.c:389 > > ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147 > > ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244 > > > > A reproducer is available here: > > https://syzkaller.appspot.com/x/repro.c?x=1437939f980000 > > > > The problem was originally introduced by > > commit b109b87050df ("mm/munlock: replace clear_page_mlock() by final > > clearance"): it was handling 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). > > > > Closes: https://syzkaller.appspot.com/x/report.txt?x=169a47d0580000 > > Fixes: b109b87050df ("mm/munlock: replace clear_page_mlock() by final clearance") > > Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx> > > Cc: <stable@xxxxxxxxxxxxxxx> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > > Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> > > Thanks Roman - I'd been preparing a similar patch, so agree that this is > the right fix. Thank you! > I don't think there's any need to change your text, but > let me remind us that any "Bad page" report stops that page from being > allocated again (because it's in an undefined, potentially dangerous > state): so does amount to a small memory leak even if otherwise harmless. It looks like I need to post v3 as soon as I get a publicly available syzkaller report, so I'll add this to the commit log. Thanks!
