[PATCH] btrfs: reloc: unlink orphan reloc roots before dropping them

[ZhengYuan Huang <gality369@gmail.com>]

clean_dirty_subvols() walks rc->dirty_subvol_roots during relocation
recovery at mount time. That list can contain both normal subvolume
roots and orphan relocation roots.

For normal subvolume roots, clean_dirty_subvols() first removes
root->reloc_dirty_list from rc->dirty_subvol_roots and then drops the
associated relocation tree. But for orphan relocation roots it directly
calls btrfs_drop_snapshot(root, false, true) without unlinking
root->reloc_dirty_list first.

This leaves a freed btrfs_root still linked in rc->dirty_subvol_roots.
Later list_del_init() on a neighboring entry writes through that stale
list node, triggering a slab-use-after-free in clean_dirty_subvols().

Fix this by removing orphan relocation roots from
rc->dirty_subvol_roots before calling btrfs_drop_snapshot().

The bug is reproducible on 7.0.0-rc2-next-20260309 with our dynamic
metadata fuzzing tool that corrupts btrfs metadata at runtime. After
the fix, orphan relocation roots are removed from the dirty_subvol_roots
list before they can be freed, so the recovery path no longer writes
into freed memory.

Fixes: 30d40577e322 ("btrfs: reloc: Also queue orphan reloc tree for cleanup to avoid BUG_ON()")
Cc: stable@vger.kernel.org # 5.1+
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
---
Root cause
==========
clean_dirty_subvols() walks rc->dirty_subvol_roots, which can contain
both normal subvolume roots and orphan relocation roots.

For normal roots, it first removes root->reloc_dirty_list from the list
before dropping the related relocation tree. But for orphan relocation
roots it calls btrfs_drop_snapshot(root, false, true) directly, without
unlinking root->reloc_dirty_list first.

btrfs_drop_snapshot() can free the last reference to root via
btrfs_put_root(), leaving a freed btrfs_root still linked in
rc->dirty_subvol_roots. Later list_del_init() on a neighboring entry
writes through that stale list node and triggers the slab-use-after-free.

Reproduction (v6.18, x86_64, KASAN)
===================================
The PoC is relatively large, so it is provided separately through google drive:
https://drive.google.com/drive/folders/1-e2ynmoQ4JzWs4F-jMtndh5pe3Vce2Zm

To reproduce the issue:
  1. Build the ublk helper program from the ublk codebase, which is
	 used to provide the runtime corruption capability:
	  g++ -std=c++20 -fcoroutines -O2 -o standalone_replay \
      standalone_replay_btrfs.cpp targets/ublksrv_tgt.cpp \
      -I. -Iinclude -Itargets/include \
      -L./lib/.libs -lublksrv -luring -lpthread
  2. Attach the crafted image through ublk:
      ./standalone_replay add -t loop -f /path/to/image
  3. Mount the image:
	  mount -o loop /path/to/image /mnt
This reliably reproduces the bug.

Fix
===
Remove orphan relocation roots from rc->dirty_subvol_roots before
calling btrfs_drop_snapshot() on them.

That restores the normal list lifetime rule:
  unlink from external containers first,
  then allow the final put/free to happen.

This is a minimal fix. Since both branches now call
list_del_init(&root->reloc_dirty_list), it may be possible to move the
unlink before the if/else and simplify the flow. I left that out here to
avoid changing more than needed, but I can respin the patch that way if
preferred.

KASAN reports
=============
BUG: KASAN: slab-use-after-free in __list_del include/linux/list.h:204 [inline]
BUG: KASAN: slab-use-after-free in __list_del_entry include/linux/list.h:226 [inline]
BUG: KASAN: slab-use-after-free in list_del_init include/linux/list.h:295 [inline]
BUG: KASAN: slab-use-after-free in clean_dirty_subvols+0x2dc/0x350 fs/btrfs/relocation.c:1472
Write of size 8 at addr ffff888014172838 by task syz-executor/265

CPU: 1 UID: 0 PID: 265 Comm: syz-executor Not tainted 6.18.0+ #14 PREEMPT(voluntary) 
Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xbe/0x130 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xd1/0x650 mm/kasan/report.c:482
 kasan_report+0xfb/0x140 mm/kasan/report.c:595
 __asan_report_store8_noabort+0x17/0x30 mm/kasan/report_generic.c:386
 __list_del include/linux/list.h:204 [inline]
 __list_del_entry include/linux/list.h:226 [inline]
 list_del_init include/linux/list.h:295 [inline]
 clean_dirty_subvols+0x2dc/0x350 fs/btrfs/relocation.c:1472
 btrfs_recover_relocation+0xe64/0x11d0 fs/btrfs/relocation.c:4207
 btrfs_start_pre_rw_mount+0xa4d/0x1810 fs/btrfs/disk-io.c:3130
 open_ctree+0x5824/0x5fe0 fs/btrfs/disk-io.c:3640
 btrfs_fill_super fs/btrfs/super.c:987 [inline]
 btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]
 btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline]
 btrfs_get_tree+0x111c/0x2190 fs/btrfs/super.c:2128
 vfs_get_tree+0x9a/0x370 fs/super.c:1758
 fc_mount fs/namespace.c:1199 [inline]
 do_new_mount_fc fs/namespace.c:3642 [inline]
 do_new_mount fs/namespace.c:3718 [inline]
 path_mount+0x5b8/0x1ea0 fs/namespace.c:4028
 do_mount fs/namespace.c:4041 [inline]
 __do_sys_mount fs/namespace.c:4229 [inline]
 __se_sys_mount fs/namespace.c:4206 [inline]
 __x64_sys_mount+0x282/0x320 fs/namespace.c:4206
 x64_sys_call+0x1a7d/0x26a0 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x93/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7dcca37a8fde
Code: 0f 1f 40 00 48 c7 c2 b0 ff ff ff f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc521c2e08 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007ffc521c31b0 RCX: 00007dcca37a8fde
RDX: 00007ffc521c35d0 RSI: 00007ffc521c31b0 RDI: 00007dcca3865d4f
RBP: 00007dcca3865d4f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000400 R11: 0000000000000246 R12: 00007ffc521c35d0
R13: 00007dcca3865dde R14: 00000000ffffffff R15: 585858582e7a7973
 </TASK>

Allocated by task 265:
 kasan_save_stack+0x39/0x70 mm/kasan/common.c:56
 kasan_save_track+0x14/0x40 mm/kasan/common.c:77
 kasan_save_alloc_info+0x37/0x60 mm/kasan/generic.c:573
 poison_kmalloc_redzone mm/kasan/common.c:400 [inline]
 __kasan_kmalloc+0xc3/0xd0 mm/kasan/common.c:417
 kasan_kmalloc include/linux/kasan.h:262 [inline]
 __kmalloc_cache_noprof+0x25f/0x800 mm/slub.c:5771
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 btrfs_alloc_root+0xb6/0xd20 fs/btrfs/disk-io.c:643
 read_tree_root_path+0x15d/0xaa0 fs/btrfs/disk-io.c:1025
 btrfs_read_tree_root+0x42/0x70 fs/btrfs/disk-io.c:1086
 btrfs_recover_relocation+0x298/0x11d0 fs/btrfs/relocation.c:4104
 btrfs_start_pre_rw_mount+0xa4d/0x1810 fs/btrfs/disk-io.c:3130
 open_ctree+0x5824/0x5fe0 fs/btrfs/disk-io.c:3640
 btrfs_fill_super fs/btrfs/super.c:987 [inline]
 btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]
 btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline]
 btrfs_get_tree+0x111c/0x2190 fs/btrfs/super.c:2128
 vfs_get_tree+0x9a/0x370 fs/super.c:1758
 fc_mount fs/namespace.c:1199 [inline]
 do_new_mount_fc fs/namespace.c:3642 [inline]
 do_new_mount fs/namespace.c:3718 [inline]
 path_mount+0x5b8/0x1ea0 fs/namespace.c:4028
 do_mount fs/namespace.c:4041 [inline]
 __do_sys_mount fs/namespace.c:4229 [inline]
 __se_sys_mount fs/namespace.c:4206 [inline]
 __x64_sys_mount+0x282/0x320 fs/namespace.c:4206
 x64_sys_call+0x1a7d/0x26a0 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x93/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Freed by task 265:
 kasan_save_stack+0x39/0x70 mm/kasan/common.c:56
 kasan_save_track+0x14/0x40 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+0x6f/0xa0 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]
 kfree+0x2bf/0x6b0 mm/slub.c:6849
 btrfs_put_root fs/btrfs/disk-io.c:1851 [inline]
 btrfs_put_root+0x1f7/0x2f0 fs/btrfs/disk-io.c:1832
 btrfs_drop_snapshot+0x1e78/0x2a60 fs/btrfs/extent-tree.c:6286
 clean_dirty_subvols+0x23e/0x350 fs/btrfs/relocation.c:1496
 btrfs_recover_relocation+0xe64/0x11d0 fs/btrfs/relocation.c:4207
 btrfs_start_pre_rw_mount+0xa4d/0x1810 fs/btrfs/disk-io.c:3130
 open_ctree+0x5824/0x5fe0 fs/btrfs/disk-io.c:3640
 btrfs_fill_super fs/btrfs/super.c:987 [inline]
 btrfs_get_tree_super fs/btrfs/super.c:1951 [inline]
 btrfs_get_tree_subvol fs/btrfs/super.c:2094 [inline]
 btrfs_get_tree+0x111c/0x2190 fs/btrfs/super.c:2128
 vfs_get_tree+0x9a/0x370 fs/super.c:1758
 fc_mount fs/namespace.c:1199 [inline]
 do_new_mount_fc fs/namespace.c:3642 [inline]
 do_new_mount fs/namespace.c:3718 [inline]
 path_mount+0x5b8/0x1ea0 fs/namespace.c:4028
 do_mount fs/namespace.c:4041 [inline]
 __do_sys_mount fs/namespace.c:4229 [inline]
 __se_sys_mount fs/namespace.c:4206 [inline]
 __x64_sys_mount+0x282/0x320 fs/namespace.c:4206
 x64_sys_call+0x1a7d/0x26a0 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x93/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

The buggy address belongs to the object at ffff888014172000
 which belongs to the cache kmalloc-rnd-06-4k of size 4096
The buggy address is located 2104 bytes inside of
 freed 4096-byte region [ffff888014172000, ffff888014173000)
---
 fs/btrfs/relocation.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 0765e06d00b8..2bff2db7afbd 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1492,7 +1492,8 @@ static int clean_dirty_subvols(struct reloc_control *rc)
 			}
 			btrfs_put_root(root);
 		} else {
-			/* Orphan reloc tree, just clean it up */
+			/* Orphan reloc tree, unlink it first and clean it up */
+			list_del_init(&root->reloc_dirty_list);
 			ret2 = btrfs_drop_snapshot(root, false, true);
 			if (ret2 < 0) {
 				btrfs_put_root(root);
-- 
2.43.0