Re: [Kernel Bug] KASAN: slab-use-after-free Read in filemap_free_folio
From: Longxing Li
Date: Tue Jun 30 2026 - 11:25:40 EST
Dear maintainers,
I'm trying to fix this issue. The bug analysis and patch fix are as follows.
========================================================
Bug Summary
ext4_add_complete_io() queues i_rsv_conversion_work without holding an
extra inode reference. If the inode is unlinked & evicted before the
delayed work runs, ext4_do_flush_completed_IO() accesses a freed inode —
use-after-free.
========================================================
Fix
Take igrab(inode) when first queueing the work (detected via
list_empty check), and iput() after processing in
ext4_do_flush_completed_IO(). A per-inode bit (i_rsv_need_iput) tracks
whether the reference was taken.
Changes:
fs/ext4/ext4.h - add i_rsv_need_iput bit to struct ext4_inode_info
fs/ext4/super.c - init the bit to 0 in ext4_alloc_inode()
fs/ext4/page-io.c - igrab() before queue_work, iput() after flush
Full patch is as follows and is attached in the link.
Best regards,
Longxing Li
========================================================
Patch:
From: Longxing Li <coregee2000@xxxxxxxxx>
Date: Tue, 30 Jun 2026
Subject: [PATCH] ext4: fix use-after-free in ext4 delayed I/O completion
ext4_add_complete_io() queues the inode's i_rsv_conversion_work without
holding an extra reference to the inode. If the inode is unlinked and
evicted before the delayed work runs, the inode slab object is freed.
When ext4_do_flush_completed_IO() later accesses io_end->inode, it is
accessing a freed/reallocated object — a use-after-free detected by KASAN.
Fix: take an inode reference in ext4_add_complete_io() (when first queueing
the work), and drop it in ext4_do_flush_completed_IO() after processing
all io_ends. A per-inode flag tracks whether the extra reference was taken.
Reported-by: Longxing Li
Signed-off-by: Longxing Li <coregee2000@xxxxxxxxx>
---
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1158,10 +1158,12 @@
#endif
spinlock_t i_block_reservation_lock;
/* Lock protecting lists below */
spinlock_t i_completed_io_lock;
+ /* Track if ext4_add_complete_io() took an extra inode reference. */
+ unsigned short i_rsv_need_iput:1;
/*
* Completed IOs that need unwritten extents handling and have
* transaction reserved
*/
struct list_head i_rsv_conversion_list;
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1414,10 +1414,11 @@
memset(&ei->i_dquot, 0, sizeof(ei->i_dquot));
#endif
ei->jinode = NULL;
INIT_LIST_HEAD(&ei->i_rsv_conversion_list);
spin_lock_init(&ei->i_completed_io_lock);
+ ei->i_rsv_need_iput = 0;
ei->i_sync_tid = 0;
ei->i_datasync_tid = 0;
INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
ext4_fc_init_inode(&ei->vfs_inode);
spin_lock_init(&ei->i_fc_lock);
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -260,12 +260,16 @@
!io_end->handle && sbi->s_journal);
WARN_ON(!io_end->bio);
spin_lock_irqsave(&ei->i_completed_io_lock, flags);
wq = sbi->rsv_conversion_wq;
- if (list_empty(&ei->i_rsv_conversion_list))
- queue_work(wq, &ei->i_rsv_conversion_work);
+ if (list_empty(&ei->i_rsv_conversion_list)) {
+ if (igrab(io_end->inode)) {
+ ei->i_rsv_need_iput = 1;
+ queue_work(wq, &ei->i_rsv_conversion_work);
+ }
+ }
list_add_tail(&io_end->list, &ei->i_rsv_conversion_list);
spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
}
static int ext4_do_flush_completed_IO(struct inode *inode,
@@ -289,10 +293,17 @@
err = ext4_end_io_end(io_end);
if (unlikely(!ret && err))
ret = err;
}
+
+ /* Release inode reference from ext4_add_complete_io. */
+ if (ei->i_rsv_need_iput) {
+ ei->i_rsv_need_iput = 0;
+ iput(&ei->vfs_inode);
+ }
+
return ret;
}
/*
* Used to convert unwritten extents to written extents upon IO completion,
========================================================
Patch Link:
https://drive.google.com/file/d/1rPdp91vPxDnc74LxOJcJV3juvWhRxUtR/view?usp=drive_link
李龙兴 <coregee2000@xxxxxxxxx> 于2026年2月2日周一 14:20写道：
>
> Dear Linux kernel developers and maintainers,
>
> We would like to report a new kernel bug found by our tool. KASAN:
> slab-use-after-free Read in filemap_free_folio. Details are as
> follows.
>
> Kernel commit: v6.18.2
> Kernel config: see attachment
> report: see attachment
>
> We are currently analyzing the root cause and working on a
> reproducible PoC. We will provide further updates in this thread as
> soon as we have more information.
>
> Best regards,
> Longxing Li
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in filemap_free_folio+0x147/0x170
> mm/filemap.c:234
> Read of size 8 at addr ffff88805e6535a8 by task kworker/u9:28/46327
>
> CPU: 1 UID: 0 PID: 46327 Comm: kworker/u9:28 Not tainted 6.18.2 #1 PREEMPT(full)
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
> Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:94 [inline]
> dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
> print_address_description mm/kasan/report.c:378 [inline]
> print_report+0xcd/0x630 mm/kasan/report.c:482
> kasan_report+0xe0/0x110 mm/kasan/report.c:595
> filemap_free_folio+0x147/0x170 mm/filemap.c:234
> folio_unmap_invalidate+0x514/0x850 mm/truncate.c:653
> filemap_end_dropbehind+0x17f/0x1d0 mm/filemap.c:1616
> folio_end_dropbehind mm/filemap.c:1637 [inline]
> folio_end_dropbehind+0xbe/0xe0 mm/filemap.c:1624
> folio_end_writeback+0xe4/0x1f0 mm/filemap.c:1695
> ext4_finish_bio+0x78f/0xa20 fs/ext4/page-io.c:144
> ext4_release_io_end+0x119/0x3a0 fs/ext4/page-io.c:159
> ext4_end_io_end+0x13e/0x4a0 fs/ext4/page-io.c:210
> ext4_do_flush_completed_IO fs/ext4/page-io.c:290 [inline]
> ext4_end_io_rsv_work+0x205/0x380 fs/ext4/page-io.c:305
> process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
> process_scheduled_works kernel/workqueue.c:3346 [inline]
> worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
> kthread+0x3c5/0x780 kernel/kthread.c:463
> ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
>
> Allocated by task 49607:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
> kasan_save_track+0x14/0x30 mm/kasan/common.c:77
> unpoison_slab_object mm/kasan/common.c:342 [inline]
> __kasan_slab_alloc+0x89/0x90 mm/kasan/common.c:368
> kasan_slab_alloc include/linux/kasan.h:252 [inline]
> slab_post_alloc_hook mm/slub.c:4978 [inline]
> slab_alloc_node mm/slub.c:5288 [inline]
> kmem_cache_alloc_lru_noprof+0x254/0x6e0 mm/slub.c:5307
> ext4_alloc_inode+0x28/0x610 fs/ext4/super.c:1393
> alloc_inode+0x64/0x240 fs/inode.c:346
> new_inode+0x22/0x1c0 fs/inode.c:1145
> __ext4_new_inode+0x392/0x4f00 fs/ext4/ialloc.c:961
> ext4_create+0x303/0x550 fs/ext4/namei.c:2822
> lookup_open.isra.0+0x11d3/0x1580 fs/namei.c:3796
> open_last_lookups fs/namei.c:3895 [inline]
> path_openat+0x893/0x2cb0 fs/namei.c:4131
> do_filp_open+0x20b/0x470 fs/namei.c:4161
> do_sys_openat2+0x11b/0x1d0 fs/open.c:1437
> do_sys_open fs/open.c:1452 [inline]
> __do_sys_openat fs/open.c:1468 [inline]
> __se_sys_openat fs/open.c:1463 [inline]
> __x64_sys_openat+0x174/0x210 fs/open.c:1463
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 49611:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
> kasan_save_track+0x14/0x30 mm/kasan/common.c:77
> __kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:587
> kasan_save_free_info mm/kasan/kasan.h:406 [inline]
> poison_slab_object mm/kasan/common.c:252 [inline]
> __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:284
> kasan_slab_free include/linux/kasan.h:234 [inline]
> slab_free_hook mm/slub.c:2543 [inline]
> slab_free mm/slub.c:6642 [inline]
> kmem_cache_free+0x2d4/0x6c0 mm/slub.c:6752
> i_callback+0x46/0x70 fs/inode.c:325
> rcu_do_batch kernel/rcu/tree.c:2605 [inline]
> rcu_core+0x79c/0x1530 kernel/rcu/tree.c:2861
> handle_softirqs+0x219/0x8e0 kernel/softirq.c:622
> __do_softirq kernel/softirq.c:656 [inline]
> invoke_softirq kernel/softirq.c:496 [inline]
> __irq_exit_rcu+0x109/0x170 kernel/softirq.c:723
> irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
> instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1052 [inline]
> sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1052
> asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
>
> Last potentially related work creation:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
> kasan_record_aux_stack+0xa7/0xc0 mm/kasan/generic.c:559
> __call_rcu_common.constprop.0+0xa5/0xa10 kernel/rcu/tree.c:3123
> destroy_inode+0x12c/0x1b0 fs/inode.c:401
> evict+0x5b4/0x920 fs/inode.c:834
> iput_final fs/inode.c:1914 [inline]
> iput.part.0+0x6a9/0xb00 fs/inode.c:1966
> iput+0x35/0x40 fs/inode.c:1929
> do_unlinkat+0x518/0x6a0 fs/namei.c:4744
> __do_sys_unlink fs/namei.c:4783 [inline]
> __se_sys_unlink fs/namei.c:4781 [inline]
> __x64_sys_unlink+0xc5/0x110 fs/namei.c:4781
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Second to last potentially related work creation:
> kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
> kasan_record_aux_stack+0xa7/0xc0 mm/kasan/generic.c:559
> insert_work+0x36/0x230 kernel/workqueue.c:2186
> __queue_work+0x97e/0x1160 kernel/workqueue.c:2341
> queue_work_on+0x15f/0x1f0 kernel/workqueue.c:2392
> queue_work include/linux/workqueue.h:669 [inline]
> ext4_add_complete_io fs/ext4/page-io.c:266 [inline]
> ext4_put_io_end_defer fs/ext4/page-io.c:325 [inline]
> ext4_put_io_end_defer+0x398/0x460 fs/ext4/page-io.c:321
> ext4_end_bio+0x2bc/0x580 fs/ext4/page-io.c:385
> bio_endio+0x713/0x860 block/bio.c:1672
> blk_update_request+0x93e/0x15f0 block/blk-mq.c:999
> scsi_end_request+0x7c/0x9c0 drivers/scsi/scsi_lib.c:637
> scsi_io_completion+0x17d/0x14c0 drivers/scsi/scsi_lib.c:1078
> scsi_complete+0x124/0x250 drivers/scsi/scsi_lib.c:1547
> blk_complete_reqs+0xb1/0xf0 block/blk-mq.c:1236
> handle_softirqs+0x219/0x8e0 kernel/softirq.c:622
> __do_softirq kernel/softirq.c:656 [inline]
> invoke_softirq kernel/softirq.c:496 [inline]
> __irq_exit_rcu+0x109/0x170 kernel/softirq.c:723
> irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
> instr_sysvec_call_function_single arch/x86/kernel/smp.c:266 [inline]
> sysvec_call_function_single+0x57/0xc0 arch/x86/kernel/smp.c:266
> asm_sysvec_call_function_single+0x1a/0x20 arch/x86/include/asm/idtentry.h:704
>
> The buggy address belongs to the object at ffff88805e652fd0
> which belongs to the cache ext4_inode_cache of size 2320
> The buggy address is located 1496 bytes inside of
> freed 2320-byte region [ffff88805e652fd0, ffff88805e6538e0)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x5e650
> head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> memcg:ffff8880268b4801
> flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 00fff00000000040 ffff888101bc3a00 ffffea0001822400 dead000000000002
> raw: 0000000000000000 00000000000d000d 00000000f5000000 ffff8880268b4801
> head: 00fff00000000040 ffff888101bc3a00 ffffea0001822400 dead000000000002
> head: 0000000000000000 00000000000d000d 00000000f5000000 ffff8880268b4801
> head: 00fff00000000003 ffffea0001799401 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 3, migratetype Reclaimable, gfp_mask
> 0xd2050(__GFP_RECLAIMABLE|__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC),
> pid 15633, tgid 15633 (syz-executor.7), ts 460463106471, free_ts 0
> set_page_owner include/linux/page_owner.h:32 [inline]
> post_alloc_hook+0x1af/0x220 mm/page_alloc.c:1845
> prep_new_page mm/page_alloc.c:1853 [inline]
> get_page_from_freelist+0x10a3/0x3a30 mm/page_alloc.c:3879
> __alloc_frozen_pages_noprof+0x25f/0x2470 mm/page_alloc.c:5178
> alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2416
> alloc_slab_page mm/slub.c:3059 [inline]
> allocate_slab mm/slub.c:3232 [inline]
> new_slab+0x24a/0x360 mm/slub.c:3286
> ___slab_alloc+0xd79/0x1a50 mm/slub.c:4655
> __slab_alloc.constprop.0+0x63/0x110 mm/slub.c:4778
> __slab_alloc_node mm/slub.c:4854 [inline]
> slab_alloc_node mm/slub.c:5276 [inline]
> kmem_cache_alloc_lru_noprof+0x443/0x6e0 mm/slub.c:5307
> ext4_alloc_inode+0x28/0x610 fs/ext4/super.c:1393
> alloc_inode+0x64/0x240 fs/inode.c:346
> new_inode+0x22/0x1c0 fs/inode.c:1145
> __ext4_new_inode+0x392/0x4f00 fs/ext4/ialloc.c:961
> ext4_symlink+0x462/0xde0 fs/ext4/namei.c:3388
> vfs_symlink fs/namei.c:4817 [inline]
> vfs_symlink+0x403/0x680 fs/namei.c:4801
> do_symlinkat+0x261/0x310 fs/namei.c:4843
> __do_sys_symlinkat fs/namei.c:4859 [inline]
> __se_sys_symlinkat fs/namei.c:4856 [inline]
> __x64_sys_symlinkat+0x93/0xc0 fs/namei.c:4856
> page_owner free stack trace missing
>
> Memory state around the buggy address:
> ffff88805e653480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88805e653500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88805e653580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ^
> ffff88805e653600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88805e653680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
>
> https://drive.google.com/file/d/1ZjRNEOf0XFYtFl5UZ7dvpJpzmPhje8it/view?usp=drive_link
>
> https://drive.google.com/file/d/1ShZndTsP1CZuwyZmbuZ-e5nHEdPjeIVX/view?usp=drive_link
From: Longxing Li <coregee2000@xxxxxxxxx>
Date: Tue, 30 Jun 2026
Subject: [PATCH] ext4: fix use-after-free in ext4 delayed I/O completion
ext4_add_complete_io() queues the inode's i_rsv_conversion_work without
holding an extra reference to the inode. If the inode is unlinked and
evicted before the delayed work runs, the inode slab object is freed.
When ext4_do_flush_completed_IO() later accesses io_end->inode, it is
accessing a freed/reallocated object — a use-after-free detected by KASAN.
Fix: take an inode reference in ext4_add_complete_io() (when first queueing
the work), and drop it in ext4_do_flush_completed_IO() after processing
all io_ends. A per-inode flag tracks whether the extra reference was taken.
Reported-by: syzkaller
Signed-off-by: Longxing Li <coregee2000@xxxxxxxxx>
---
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1158,10 +1158,12 @@
#endif
spinlock_t i_block_reservation_lock;
/* Lock protecting lists below */
spinlock_t i_completed_io_lock;
+ /* Track if ext4_add_complete_io() took an extra inode reference. */
+ unsigned short i_rsv_need_iput:1;
/*
* Completed IOs that need unwritten extents handling and have
* transaction reserved
*/
struct list_head i_rsv_conversion_list;
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1414,10 +1414,11 @@
memset(&ei->i_dquot, 0, sizeof(ei->i_dquot));
#endif
ei->jinode = NULL;
INIT_LIST_HEAD(&ei->i_rsv_conversion_list);
spin_lock_init(&ei->i_completed_io_lock);
+ ei->i_rsv_need_iput = 0;
ei->i_sync_tid = 0;
ei->i_datasync_tid = 0;
INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
ext4_fc_init_inode(&ei->vfs_inode);
spin_lock_init(&ei->i_fc_lock);
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -260,12 +260,16 @@
!io_end->handle && sbi->s_journal);
WARN_ON(!io_end->bio);
spin_lock_irqsave(&ei->i_completed_io_lock, flags);
wq = sbi->rsv_conversion_wq;
- if (list_empty(&ei->i_rsv_conversion_list))
- queue_work(wq, &ei->i_rsv_conversion_work);
+ if (list_empty(&ei->i_rsv_conversion_list)) {
+ if (igrab(io_end->inode)) {
+ ei->i_rsv_need_iput = 1;
+ queue_work(wq, &ei->i_rsv_conversion_work);
+ }
+ }
list_add_tail(&io_end->list, &ei->i_rsv_conversion_list);
spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
}
static int ext4_do_flush_completed_IO(struct inode *inode,
@@ -289,10 +293,17 @@
err = ext4_end_io_end(io_end);
if (unlikely(!ret && err))
ret = err;
}
+
+ /* Release inode reference from ext4_add_complete_io. */
+ if (ei->i_rsv_need_iput) {
+ ei->i_rsv_need_iput = 0;
+ iput(&ei->vfs_inode);
+ }
+
return ret;
}
/*
* Used to convert unwritten extents to written extents upon IO completion,