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([138.199.21.245]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-829a48b2c41sm12433318b3a.54.2026.03.10.00.54.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Mar 2026 00:54:58 -0700 (PDT) From: ZhengYuan Huang To: dsterba@suse.com, clm@fb.com, wqu@suse.com Cc: linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com, r33s3n6@gmail.com, zzzccc427@gmail.com, ZhengYuan Huang , stable@vger.kernel.org Subject: [PATCH] btrfs: reloc: unlink orphan reloc roots before dropping them Date: Tue, 10 Mar 2026 15:54:47 +0800 Message-ID: <20260310075447.2088205-1-gality369@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- 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: __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 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