[syzbot] [trace?] KASAN: slab-use-after-free Read in ring_buffer_iter_advance

[syzbot] [trace?] KASAN: slab-use-after-free Read in ring_buffer_iter_advance

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    11028ab62899 Merge tag 'probes-fixes-v7.2-rc5' of git://gi..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17954aea580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=e0cc44465d6bae735679
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/dbf4cb7b4116/disk-11028ab6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/77761f1559a8/vmlinux-11028ab6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/bb1ed2bbff0e/bzImage-11028ab6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e0cc44465d6bae735679@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in rb_page_size kernel/trace/ring_buffer.c:391 [inline]
BUG: KASAN: slab-use-after-free in rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
Read of size 8 at addr ffff88802d79b8b8 by task syz.4.389/6869

CPU: 0 UID: 0 PID: 6869 Comm: syz.4.389 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x13d/0x4b0 mm/kasan/report.c:482
 kasan_report+0xdf/0x1c0 mm/kasan/report.c:595
 rb_page_size kernel/trace/ring_buffer.c:391 [inline]
 rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
 ring_buffer_iter_advance+0x69/0x90 kernel/trace/ring_buffer.c:6445
 trace_find_next_entry_inc kernel/trace/trace.c:2679 [inline]
 s_next+0x270/0x410 kernel/trace/trace.c:2711
 seq_read_iter+0xac5/0x1270 fs/seq_file.c:263
 seq_read+0x344/0x4d0 fs/seq_file.c:163
 vfs_read+0x1e4/0xb40 fs/read_write.c:572
 ksys_read+0x12a/0x250 fs/read_write.c:716
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f734c99df99
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f734d820028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007f734cc25fa0 RCX: 00007f734c99df99
RDX: 0000000000001000 RSI: 0000200000000340 RDI: 0000000000000005
RBP: 00007f734ca34ec4 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f734cc26038 R14: 00007f734cc25fa0 R15: 00007ffcf813c748
 </TASK>

Allocated by task 6869:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
 __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
 kasan_kmalloc include/linux/kasan.h:263 [inline]
 __do_kmalloc_node mm/slub.c:5334 [inline]
 __kmalloc_node_noprof+0x339/0x830 mm/slub.c:5346
 _kmalloc_node_noprof include/linux/slab.h:1197 [inline]
 __rb_allocate_pages+0x399/0x10a0 kernel/trace/ring_buffer.c:2402
 ring_buffer_subbuf_order_set+0x3ef/0x18a0 kernel/trace/ring_buffer.c:7379
 buffer_subbuf_size_write+0x182/0x280 kernel/trace/trace.c:8221
 vfs_write+0x2aa/0x1050 fs/read_write.c:685
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 6871:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
 poison_slab_object mm/kasan/common.c:253 [inline]
 __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:2677 [inline]
 slab_free mm/slub.c:6377 [inline]
 kfree+0x22b/0x6c0 mm/slub.c:6692
 free_buffer_page kernel/trace/ring_buffer.c:399 [inline]
 ring_buffer_subbuf_order_set+0x116b/0x18a0 kernel/trace/ring_buffer.c:7439
 buffer_subbuf_size_write+0x182/0x280 kernel/trace/trace.c:8221
 vfs_write+0x2aa/0x1050 fs/read_write.c:685
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88802d79b880
 which belongs to the cache kmalloc-64 of size 64
The buggy address is located 56 bytes inside of
 freed 64-byte region [ffff88802d79b880, ffff88802d79b8c0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x2d79b
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000000 ffff88813fe238c0 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800200020 00000000f5000000 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0xd2cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 1, tgid 1 (swapper/0), ts 13416095140, free_ts 0
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1859
 prep_new_page mm/page_alloc.c:1867 [inline]
 get_page_from_freelist+0xf48/0x3530 mm/page_alloc.c:3946
 __alloc_frozen_pages_noprof+0x299/0x2dc0 mm/page_alloc.c:5304
 alloc_slab_page mm/slub.c:3266 [inline]
 allocate_slab mm/slub.c:3380 [inline]
 new_slab+0xa2/0x640 mm/slub.c:3426
 refill_objects+0xe3/0x410 mm/slub.c:7310
 refill_sheaf mm/slub.c:2804 [inline]
 __pcs_replace_empty_main+0x376/0x680 mm/slub.c:4675
 alloc_from_pcs mm/slub.c:4773 [inline]
 slab_alloc_node mm/slub.c:4905 [inline]
 __do_kmalloc_node mm/slub.c:5333 [inline]
 __kmalloc_noprof+0x66d/0x820 mm/slub.c:5359
 _kmalloc_noprof include/linux/slab.h:992 [inline]
 _kzalloc_noprof include/linux/slab.h:1309 [inline]
 handler_new_ref+0x1ab/0xc60 drivers/media/v4l2-core/v4l2-ctrls-core.c:1895
 v4l2_ctrl_add_handler drivers/media/v4l2-core/v4l2-ctrls-core.c:2456 [inline]
 v4l2_ctrl_add_handler+0x223/0x300 drivers/media/v4l2-core/v4l2-ctrls-core.c:2430
 vivid_create_controls+0x33ce/0x3e80 drivers/media/test-drivers/vivid/vivid-ctrls.c:2072
 vivid_create_instance drivers/media/test-drivers/vivid/vivid-core.c:1933 [inline]
 vivid_probe.cold+0x4f1e/0x921e drivers/media/test-drivers/vivid/vivid-core.c:2095
 platform_probe+0x106/0x1d0 drivers/base/platform.c:1439
 call_driver_probe drivers/base/dd.c:628 [inline]
 really_probe+0x241/0xa60 drivers/base/dd.c:706
 __driver_probe_device+0x20e/0x450 drivers/base/dd.c:868
 driver_probe_device+0x4a/0x140 drivers/base/dd.c:898
 __driver_attach+0x21f/0x5b0 drivers/base/dd.c:1292
page_owner free stack trace missing

Memory state around the buggy address:
 ffff88802d79b780: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
 ffff88802d79b800: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
>ffff88802d79b880: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
                                        ^
 ffff88802d79b900: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
 ffff88802d79b980: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
==================================================================


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Re: [syzbot] [trace?] KASAN: slab-use-after-free Read in ring_buffer_iter_advance

[Steven Rostedt]

On Mon, 03 Aug 2026 17:52:33 -0700
syzbot <syzbot+e0cc44465d6bae735679@syzkaller.appspotmail.com> wrote:

> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+e0cc44465d6bae735679@syzkaller.appspotmail.com

This is a legitimate bug.

> 
> ==================================================================
> BUG: KASAN: slab-use-after-free in rb_page_size kernel/trace/ring_buffer.c:391 [inline]
> BUG: KASAN: slab-use-after-free in rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
> Read of size 8 at addr ffff88802d79b8b8 by task syz.4.389/6869
> 
> CPU: 0 UID: 0 PID: 6869 Comm: syz.4.389 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
> Call Trace:
>  <TASK>

>  rb_page_size kernel/trace/ring_buffer.c:391 [inline]
>  rb_advance_iter+0x40e/0x460 kernel/trace/ring_buffer.c:6024
>  ring_buffer_iter_advance+0x69/0x90 kernel/trace/ring_buffer.c:6445

The iterator is updated under the raw cpu_buffer reader lock.

> 
> Freed by task 6871:
>  kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
>  kasan_save_track+0x14/0x30 mm/kasan/common.c:78
>  kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
>  poison_slab_object mm/kasan/common.c:253 [inline]
>  __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
>  kasan_slab_free include/linux/kasan.h:235 [inline]
>  slab_free_hook mm/slub.c:2677 [inline]
>  slab_free mm/slub.c:6377 [inline]
>  kfree+0x22b/0x6c0 mm/slub.c:6692
>  free_buffer_page kernel/trace/ring_buffer.c:399 [inline]
>  ring_buffer_subbuf_order_set+0x116b/0x18a0 kernel/trace/ring_buffer.c:7439

The pages are removed from the visible reader list onto a temporary link
list under the raw cpu_buffer reader lock.

After the reader lock is released, the list is then freed. No reader should
have access to any of these items as they were only visible under the
reader_lock.

The bug is that the ring buffer iterator keeps state! It can still have a
pointer to a sub-buffer that has been freed. When another read happens, it
may have a stale pointer.

The fix is to add some state to the cpu_buffer for when it frees pages so
that the next read from the iterator will reset its state.

I'll have to address this when I get some time.

Thanks,

-- Steve