On 22.01.20 г. 14:23 ч., fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> There is a race between adding and removing elements to the tree mod log
> list and rbtree that can lead to use-after-free problems.
>
> Consider the following example that explains how/why the problems happens:
>
> 1) Task A has mod log element with sequence number 200. It currently is
> the only element in the mod log list;
>
> 2) Task A calls btrfs_put_tree_mod_seq() because it no longer needs to
> access the tree mod log. When it enters the function, it initializes
> 'min_seq' to (u64)-1. Then it acquires the lock 'tree_mod_seq_lock'
> before checking if there are other elements in the mod seq list.
> Since the list it empty, 'min_seq' remains set to (u64)-1. Then it
> unlocks the lock 'tree_mod_seq_lock';
>
> 3) Before task A acquires the lock 'tree_mod_log_lock', task B adds
> itself to the mod seq list through btrfs_get_tree_mod_seq() and gets a
> sequence number of 201;
>
> 4) Some other task, name it task C, modifies a btree and because there
> elements in the mod seq list, it adds a tree mod elem to the tree
> mod log rbtree. That node added to the mod log rbtree is assigned
> a sequence number of 202;
>
> 5) Task B, which is doing fiemap and resolving indirect back references,
> calls btrfs get_old_root(), with 'time_seq' == 201, which in turn
> calls tree_mod_log_search() - the search returns the mod log node
> from the rbtree with sequence number 202, created by task C;
>
> 6) Task A now acquires the lock 'tree_mod_log_lock', starts iterating
> the mod log rbtree and finds the node with sequence number 202. Since
> 202 is less than the previously computed 'min_seq', (u64)-1, it
> removes the node and frees it;
>
> 7) Task B still has a pointer to the node with sequence number 202, and
> it dereferences the pointer itself and through the call to
> __tree_mod_log_rewind(), resulting in a use-after-free problem._
>
> This issue can be triggered sporadically with the test case generic/561
> from fstests, and it happens more frequently with a higher number of
> duperemove processes. When it happens to me, it either freezes the vm or
> it produces a trace like the following before crashing:
>
> [ 1245.321140] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
> [ 1245.321200] CPU: 1 PID: 26997 Comm: pool Not tainted 5.5.0-rc6-btrfs-next-52 #1
> [ 1245.321235] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
> [ 1245.321287] RIP: 0010:rb_next+0x16/0x50
> [ 1245.321307] Code: ....
> [ 1245.321372] RSP: 0018:ffffa151c4d039b0 EFLAGS: 00010202
> [ 1245.321388] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8ae221363c80 RCX: 6b6b6b6b6b6b6b6b
> [ 1245.321409] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff8ae221363c80
> [ 1245.321439] RBP: ffff8ae20fcc4688 R08: 0000000000000002 R09: 0000000000000000
> [ 1245.321475] R10: ffff8ae20b120910 R11: 00000000243f8bb1 R12: 0000000000000038
> [ 1245.321506] R13: ffff8ae221363c80 R14: 000000000000075f R15: ffff8ae223f762b8
> [ 1245.321539] FS: 00007fdee1ec7700(0000) GS:ffff8ae236c80000(0000) knlGS:0000000000000000
> [ 1245.321591] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1245.321614] CR2: 00007fded4030c48 CR3: 000000021da16003 CR4: 00000000003606e0
> [ 1245.321642] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 1245.321668] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 1245.321706] Call Trace:
> [ 1245.321798] __tree_mod_log_rewind+0xbf/0x280 [btrfs]
> [ 1245.321841] btrfs_search_old_slot+0x105/0xd00 [btrfs]
> [ 1245.321877] resolve_indirect_refs+0x1eb/0xc60 [btrfs]
> [ 1245.321912] find_parent_nodes+0x3dc/0x11b0 [btrfs]
> [ 1245.321947] btrfs_check_shared+0x115/0x1c0 [btrfs]
> [ 1245.321980] ? extent_fiemap+0x59d/0x6d0 [btrfs]
> [ 1245.322029] extent_fiemap+0x59d/0x6d0 [btrfs]
> [ 1245.322066] do_vfs_ioctl+0x45a/0x750
> [ 1245.322081] ksys_ioctl+0x70/0x80
> [ 1245.322092] ? trace_hardirqs_off_thunk+0x1a/0x1c
> [ 1245.322113] __x64_sys_ioctl+0x16/0x20
> [ 1245.322126] do_syscall_64+0x5c/0x280
> [ 1245.322139] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> [ 1245.322155] RIP: 0033:0x7fdee3942dd7
> [ 1245.322177] Code: ....
> [ 1245.322258] RSP: 002b:00007fdee1ec6c88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [ 1245.322294] RAX: ffffffffffffffda RBX: 00007fded40210d8 RCX: 00007fdee3942dd7
> [ 1245.322314] RDX: 00007fded40210d8 RSI: 00000000c020660b RDI: 0000000000000004
> [ 1245.322337] RBP: 0000562aa89e7510 R08: 0000000000000000 R09: 00007fdee1ec6d44
> [ 1245.322369] R10: 0000000000000073 R11: 0000000000000246 R12: 00007fdee1ec6d48
> [ 1245.322390] R13: 00007fdee1ec6d40 R14: 00007fded40210d0 R15: 00007fdee1ec6d50
> [ 1245.322423] Modules linked in: ....
> [ 1245.323443] ---[ end trace 01de1e9ec5dff3cd ]---
>
> Fix this by ensuring that btrfs_put_tree_mod_seq() computes the minimum
> sequence number and iterates the rbtree while holding the lock
> 'tree_mod_log_lock' in write mode. Also get rid of the 'tree_mod_seq_lock'
> lock, since it is now redundant.
>
> Fixes: bd989ba359f2ac ("Btrfs: add tree modification log functions")
> Fixes: 097b8a7c9e48e2 ("Btrfs: join tree mod log code with the code holding back delayed refs")
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
