[syzbot] [fuse?] KASAN: use-after-free Write in fuse_copy_do

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c025f6cf4209 Add linux-next specific files for 20260303
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=135770ba580000
kernel config: https://syzkaller.appspot.com/x/.config?x=54b410fabe2a4318
dashboard link: https://syzkaller.appspot.com/bug?extid=23299dfcac137a96834a
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1671b006580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1166bb5a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e2497bde352d/disk-c025f6cf.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/36827cb45429/vmlinux-c025f6cf.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7ebf1ed6bb73/bzImage-c025f6cf.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+23299d...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in fuse_copy_do+0x193/0x380 fs/fuse/dev.c:-1
Write of size 2 at addr ffff888070528fff by task syz.0.17/6005

CPU: 0 UID: 0 PID: 6005 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xba/0x230 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
__asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106
fuse_copy_do+0x193/0x380 fs/fuse/dev.c:-1
fuse_copy_folio+0xefc/0x1b00 fs/fuse/dev.c:1166
fuse_notify_store fs/fuse/dev.c:1821 [inline]
fuse_notify fs/fuse/dev.c:2109 [inline]
fuse_dev_do_write+0x2b9d/0x4060 fs/fuse/dev.c:2205
fuse_dev_write+0x177/0x220 fs/fuse/dev.c:2289
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x61d/0xb90 fs/read_write.c:688
ksys_write+0x150/0x270 fs/read_write.c:740
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe8c659c799
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:00007fe8c7476028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fe8c6815fa0 RCX: 00007fe8c659c799
RDX: 000000000000002a RSI: 0000200000000080 RDI: 0000000000000003
RBP: 00007fe8c6632bd9 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe8c6816038 R14: 00007fe8c6815fa0 R15: 00007ffd95eaba38
</TASK>

The buggy address belongs to the physical page:
page: refcount:3 mapcount:0 mapping:ffff88805c1c4f20 index:0x7 pfn:0x70528
memcg:ffff88802ce3a880
aops:empty_aops ino:1 dentry name(?):"/"
flags: 0xfff00000000005(locked|referenced|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000005 0000000000000000 dead000000000122 ffff88805c1c4f20
raw: 0000000000000007 0000000000000000 00000003ffffffff ffff88802ce3a880
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 6005, tgid 6004 (syz.0.17), ts 103948279019, free_ts 103868818778
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x231/0x280 mm/page_alloc.c:1892
prep_new_page mm/page_alloc.c:1900 [inline]
get_page_from_freelist+0x23a1/0x2440 mm/page_alloc.c:3965
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5253
alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2484
alloc_frozen_pages_noprof mm/mempolicy.c:2555 [inline]
alloc_pages_noprof+0xa8/0x190 mm/mempolicy.c:2575
folio_alloc_noprof+0x1e/0x30 mm/mempolicy.c:2585
filemap_alloc_folio_noprof+0x111/0x470 mm/filemap.c:1013
__filemap_get_folio_mpol+0x3fc/0xb00 mm/filemap.c:2011
__filemap_get_folio include/linux/pagemap.h:774 [inline]
filemap_grab_folio include/linux/pagemap.h:854 [inline]
fuse_notify_store fs/fuse/dev.c:1813 [inline]
fuse_notify fs/fuse/dev.c:2109 [inline]
fuse_dev_do_write+0x298b/0x4060 fs/fuse/dev.c:2205
fuse_dev_write+0x177/0x220 fs/fuse/dev.c:2289
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x61d/0xb90 fs/read_write.c:688
ksys_write+0x150/0x270 fs/read_write.c:740
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 6002 tgid 6002 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1436 [inline]
free_unref_folios+0xd71/0x1530 mm/page_alloc.c:3043
folios_put_refs+0x9ff/0xb40 mm/swap.c:1008
free_pages_and_swap_cache+0x2e7/0x5b0 mm/swap_state.c:401
__tlb_batch_free_encoded_pages mm/mmu_gather.c:138 [inline]
tlb_batch_pages_flush mm/mmu_gather.c:151 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:417 [inline]
tlb_flush_mmu+0x6d3/0xa30 mm/mmu_gather.c:424
tlb_finish_mmu+0xf9/0x230 mm/mmu_gather.c:549
exit_mmap+0x498/0xa10 mm/mmap.c:1315
__mmput+0x118/0x430 kernel/fork.c:1179
exit_mm+0x18e/0x250 kernel/exit.c:581
do_exit+0x8b9/0x2580 kernel/exit.c:962
do_group_exit+0x21b/0x2d0 kernel/exit.c:1116
__do_sys_exit_group kernel/exit.c:1127 [inline]
__se_sys_exit_group kernel/exit.c:1125 [inline]
__x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1125
x64_sys_call+0x221a/0x2240 arch/x86/include/generated/asm/syscalls_64.h:232
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
ffff888070528f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff888070528f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888070529000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff888070529080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888070529100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


---
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[Joanne Koong]

I think this is a fallout from the misordered lines detected in
https://lore.kernel.org/all/202603031522...@intel.com/T/#u

#syz test

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index da2e0f348890..2c16b94357d5 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1815,8 +1815,8 @@ static int fuse_notify_store(struct fuse_conn
*fc, unsigned int size,
if (IS_ERR(folio))
goto out_iput;

- nr_bytes = min(num, folio_size(folio) - folio_offset);
folio_offset = offset_in_folio(folio, pos);
+ nr_bytes = min(num, folio_size(folio) - folio_offset);

err = fuse_copy_folio(cs, &folio, folio_offset, nr_bytes, 0);
if (!folio_test_uptodate(folio) && !err && folio_offset == 0 &&

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file fs/fuse/dev.c
patch: **** unexpected end of file in patch



Tested on:

commit: c025f6cf Add linux-next specific files for 20260303
git tree: linux-next

kernel config: https://syzkaller.appspot.com/x/.config?x=54b410fabe2a4318
dashboard link: https://syzkaller.appspot.com/bug?extid=23299dfcac137a96834a
compiler:

patch: https://syzkaller.appspot.com/x/patch.diff?x=1620f0ba580000