--- /dev/null
+From dbcc7d57bffc0c8cac9dac11bec548597d59a6a5 Mon Sep 17 00:00:00 2001
+From: Filipe Manana <fdmanana@suse.com>
+Date: Thu, 11 Mar 2021 14:31:05 +0000
+Subject: btrfs: fix race when cloning extent buffer during rewind of an old root
+
+From: Filipe Manana <fdmanana@suse.com>
+
+commit dbcc7d57bffc0c8cac9dac11bec548597d59a6a5 upstream.
+
+While resolving backreferences, as part of a logical ino ioctl call or
+fiemap, we can end up hitting a BUG_ON() when replaying tree mod log
+operations of a root, triggering a stack trace like the following:
+
+ ------------[ cut here ]------------
+ kernel BUG at fs/btrfs/ctree.c:1210!
+ invalid opcode: 0000 [#1] SMP KASAN PTI
+ CPU: 1 PID: 19054 Comm: crawl_335 Tainted: G W 5.11.0-2d11c0084b02-misc-next+ #89
+ Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+ RIP: 0010:__tree_mod_log_rewind+0x3b1/0x3c0
+ Code: 05 48 8d 74 10 (...)
+ RSP: 0018:ffffc90001eb70b8 EFLAGS: 00010297
+ RAX: 0000000000000000 RBX: ffff88812344e400 RCX: ffffffffb28933b6
+ RDX: 0000000000000007 RSI: dffffc0000000000 RDI: ffff88812344e42c
+ RBP: ffffc90001eb7108 R08: 1ffff11020b60a20 R09: ffffed1020b60a20
+ R10: ffff888105b050f9 R11: ffffed1020b60a1f R12: 00000000000000ee
+ R13: ffff8880195520c0 R14: ffff8881bc958500 R15: ffff88812344e42c
+ FS: 00007fd1955e8700(0000) GS:ffff8881f5600000(0000) knlGS:0000000000000000
+ CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
+ CR2: 00007efdb7928718 CR3: 000000010103a006 CR4: 0000000000170ee0
+ Call Trace:
+ btrfs_search_old_slot+0x265/0x10d0
+ ? lock_acquired+0xbb/0x600
+ ? btrfs_search_slot+0x1090/0x1090
+ ? free_extent_buffer.part.61+0xd7/0x140
+ ? free_extent_buffer+0x13/0x20
+ resolve_indirect_refs+0x3e9/0xfc0
+ ? lock_downgrade+0x3d0/0x3d0
+ ? __kasan_check_read+0x11/0x20
+ ? add_prelim_ref.part.11+0x150/0x150
+ ? lock_downgrade+0x3d0/0x3d0
+ ? __kasan_check_read+0x11/0x20
+ ? lock_acquired+0xbb/0x600
+ ? __kasan_check_write+0x14/0x20
+ ? do_raw_spin_unlock+0xa8/0x140
+ ? rb_insert_color+0x30/0x360
+ ? prelim_ref_insert+0x12d/0x430
+ find_parent_nodes+0x5c3/0x1830
+ ? resolve_indirect_refs+0xfc0/0xfc0
+ ? lock_release+0xc8/0x620
+ ? fs_reclaim_acquire+0x67/0xf0
+ ? lock_acquire+0xc7/0x510
+ ? lock_downgrade+0x3d0/0x3d0
+ ? lockdep_hardirqs_on_prepare+0x160/0x210
+ ? lock_release+0xc8/0x620
+ ? fs_reclaim_acquire+0x67/0xf0
+ ? lock_acquire+0xc7/0x510
+ ? poison_range+0x38/0x40
+ ? unpoison_range+0x14/0x40
+ ? trace_hardirqs_on+0x55/0x120
+ btrfs_find_all_roots_safe+0x142/0x1e0
+ ? find_parent_nodes+0x1830/0x1830
+ ? btrfs_inode_flags_to_xflags+0x50/0x50
+ iterate_extent_inodes+0x20e/0x580
+ ? tree_backref_for_extent+0x230/0x230
+ ? lock_downgrade+0x3d0/0x3d0
+ ? read_extent_buffer+0xdd/0x110
+ ? lock_downgrade+0x3d0/0x3d0
+ ? __kasan_check_read+0x11/0x20
+ ? lock_acquired+0xbb/0x600
+ ? __kasan_check_write+0x14/0x20
+ ? _raw_spin_unlock+0x22/0x30
+ ? __kasan_check_write+0x14/0x20
+ iterate_inodes_from_logical+0x129/0x170
+ ? iterate_inodes_from_logical+0x129/0x170
+ ? btrfs_inode_flags_to_xflags+0x50/0x50
+ ? iterate_extent_inodes+0x580/0x580
+ ? __vmalloc_node+0x92/0xb0
+ ? init_data_container+0x34/0xb0
+ ? init_data_container+0x34/0xb0
+ ? kvmalloc_node+0x60/0x80
+ btrfs_ioctl_logical_to_ino+0x158/0x230
+ btrfs_ioctl+0x205e/0x4040
+ ? __might_sleep+0x71/0xe0
+ ? btrfs_ioctl_get_supported_features+0x30/0x30
+ ? getrusage+0x4b6/0x9c0
+ ? __kasan_check_read+0x11/0x20
+ ? lock_release+0xc8/0x620
+ ? __might_fault+0x64/0xd0
+ ? lock_acquire+0xc7/0x510
+ ? lock_downgrade+0x3d0/0x3d0
+ ? lockdep_hardirqs_on_prepare+0x210/0x210
+ ? lockdep_hardirqs_on_prepare+0x210/0x210
+ ? __kasan_check_read+0x11/0x20
+ ? do_vfs_ioctl+0xfc/0x9d0
+ ? ioctl_file_clone+0xe0/0xe0
+ ? lock_downgrade+0x3d0/0x3d0
+ ? lockdep_hardirqs_on_prepare+0x210/0x210
+ ? __kasan_check_read+0x11/0x20
+ ? lock_release+0xc8/0x620
+ ? __task_pid_nr_ns+0xd3/0x250
+ ? lock_acquire+0xc7/0x510
+ ? __fget_files+0x160/0x230
+ ? __fget_light+0xf2/0x110
+ __x64_sys_ioctl+0xc3/0x100
+ do_syscall_64+0x37/0x80
+ entry_SYSCALL_64_after_hwframe+0x44/0xa9
+ RIP: 0033:0x7fd1976e2427
+ Code: 00 00 90 48 8b 05 (...)
+ RSP: 002b:00007fd1955e5cf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
+ RAX: ffffffffffffffda RBX: 00007fd1955e5f40 RCX: 00007fd1976e2427
+ RDX: 00007fd1955e5f48 RSI: 00000000c038943b RDI: 0000000000000004
+ RBP: 0000000001000000 R08: 0000000000000000 R09: 00007fd1955e6120
+ R10: 0000557835366b00 R11: 0000000000000246 R12: 0000000000000004
+ R13: 00007fd1955e5f48 R14: 00007fd1955e5f40 R15: 00007fd1955e5ef8
+ Modules linked in:
+ ---[ end trace ec8931a1c36e57be ]---
+
+ (gdb) l *(__tree_mod_log_rewind+0x3b1)
+ 0xffffffff81893521 is in __tree_mod_log_rewind (fs/btrfs/ctree.c:1210).
+ 1205 * the modification. as we're going backwards, we do the
+ 1206 * opposite of each operation here.
+ 1207 */
+ 1208 switch (tm->op) {
+ 1209 case MOD_LOG_KEY_REMOVE_WHILE_FREEING:
+ 1210 BUG_ON(tm->slot < n);
+ 1211 fallthrough;
+ 1212 case MOD_LOG_KEY_REMOVE_WHILE_MOVING:
+ 1213 case MOD_LOG_KEY_REMOVE:
+ 1214 btrfs_set_node_key(eb, &tm->key, tm->slot);
+
+Here's what happens to hit that BUG_ON():
+
+1) We have one tree mod log user (through fiemap or the logical ino ioctl),
+ with a sequence number of 1, so we have fs_info->tree_mod_seq == 1;
+
+2) Another task is at ctree.c:balance_level() and we have eb X currently as
+ the root of the tree, and we promote its single child, eb Y, as the new
+ root.
+
+ Then, at ctree.c:balance_level(), we call:
+
+ tree_mod_log_insert_root(eb X, eb Y, 1);
+
+3) At tree_mod_log_insert_root() we create tree mod log elements for each
+ slot of eb X, of operation type MOD_LOG_KEY_REMOVE_WHILE_FREEING each
+ with a ->logical pointing to ebX->start. These are placed in an array
+ named tm_list.
+ Lets assume there are N elements (N pointers in eb X);
+
+4) Then, still at tree_mod_log_insert_root(), we create a tree mod log
+ element of operation type MOD_LOG_ROOT_REPLACE, ->logical set to
+ ebY->start, ->old_root.logical set to ebX->start, ->old_root.level set
+ to the level of eb X and ->generation set to the generation of eb X;
+
+5) Then tree_mod_log_insert_root() calls tree_mod_log_free_eb() with
+ tm_list as argument. After that, tree_mod_log_free_eb() calls
+ __tree_mod_log_insert() for each member of tm_list in reverse order,
+ from highest slot in eb X, slot N - 1, to slot 0 of eb X;
+
+6) __tree_mod_log_insert() sets the sequence number of each given tree mod
+ log operation - it increments fs_info->tree_mod_seq and sets
+ fs_info->tree_mod_seq as the sequence number of the given tree mod log
+ operation.
+
+ This means that for the tm_list created at tree_mod_log_insert_root(),
+ the element corresponding to slot 0 of eb X has the highest sequence
+ number (1 + N), and the element corresponding to the last slot has the
+ lowest sequence number (2);
+
+7) Then, after inserting tm_list's elements into the tree mod log rbtree,
+ the MOD_LOG_ROOT_REPLACE element is inserted, which gets the highest
+ sequence number, which is N + 2;
+
+8) Back to ctree.c:balance_level(), we free eb X by calling
+ btrfs_free_tree_block() on it. Because eb X was created in the current
+ transaction, has no other references and writeback did not happen for
+ it, we add it back to the free space cache/tree;
+
+9) Later some other task T allocates the metadata extent from eb X, since
+ it is marked as free space in the space cache/tree, and uses it as a
+ node for some other btree;
+
+10) The tree mod log user task calls btrfs_search_old_slot(), which calls
+ get_old_root(), and finally that calls __tree_mod_log_oldest_root()
+ with time_seq == 1 and eb_root == eb Y;
+
+11) First iteration of the while loop finds the tree mod log element with
+ sequence number N + 2, for the logical address of eb Y and of type
+ MOD_LOG_ROOT_REPLACE;
+
+12) Because the operation type is MOD_LOG_ROOT_REPLACE, we don't break out
+ of the loop, and set root_logical to point to tm->old_root.logical
+ which corresponds to the logical address of eb X;
+
+13) On the next iteration of the while loop, the call to
+ tree_mod_log_search_oldest() returns the smallest tree mod log element
+ for the logical address of eb X, which has a sequence number of 2, an
+ operation type of MOD_LOG_KEY_REMOVE_WHILE_FREEING and corresponds to
+ the old slot N - 1 of eb X (eb X had N items in it before being freed);
+
+14) We then break out of the while loop and return the tree mod log operation
+ of type MOD_LOG_ROOT_REPLACE (eb Y), and not the one for slot N - 1 of
+ eb X, to get_old_root();
+
+15) At get_old_root(), we process the MOD_LOG_ROOT_REPLACE operation
+ and set "logical" to the logical address of eb X, which was the old
+ root. We then call tree_mod_log_search() passing it the logical
+ address of eb X and time_seq == 1;
+
+16) Then before calling tree_mod_log_search(), task T adds a key to eb X,
+ which results in adding a tree mod log operation of type
+ MOD_LOG_KEY_ADD to the tree mod log - this is done at
+ ctree.c:insert_ptr() - but after adding the tree mod log operation
+ and before updating the number of items in eb X from 0 to 1...
+
+17) The task at get_old_root() calls tree_mod_log_search() and gets the
+ tree mod log operation of type MOD_LOG_KEY_ADD just added by task T.
+ Then it enters the following if branch:
+
+ if (old_root && tm && tm->op != MOD_LOG_KEY_REMOVE_WHILE_FREEING) {
+ (...)
+ } (...)
+
+ Calls read_tree_block() for eb X, which gets a reference on eb X but
+ does not lock it - task T has it locked.
+ Then it clones eb X while it has nritems set to 0 in its header, before
+ task T sets nritems to 1 in eb X's header. From hereupon we use the
+ clone of eb X which no other task has access to;
+
+18) Then we call __tree_mod_log_rewind(), passing it the MOD_LOG_KEY_ADD
+ mod log operation we just got from tree_mod_log_search() in the
+ previous step and the cloned version of eb X;
+
+19) At __tree_mod_log_rewind(), we set the local variable "n" to the number
+ of items set in eb X's clone, which is 0. Then we enter the while loop,
+ and in its first iteration we process the MOD_LOG_KEY_ADD operation,
+ which just decrements "n" from 0 to (u32)-1, since "n" is declared with
+ a type of u32. At the end of this iteration we call rb_next() to find the
+ next tree mod log operation for eb X, that gives us the mod log operation
+ of type MOD_LOG_KEY_REMOVE_WHILE_FREEING, for slot 0, with a sequence
+ number of N + 1 (steps 3 to 6);
+
+20) Then we go back to the top of the while loop and trigger the following
+ BUG_ON():
+
+ (...)
+ switch (tm->op) {
+ case MOD_LOG_KEY_REMOVE_WHILE_FREEING:
+ BUG_ON(tm->slot < n);
+ fallthrough;
+ (...)
+
+ Because "n" has a value of (u32)-1 (4294967295) and tm->slot is 0.
+
+Fix this by taking a read lock on the extent buffer before cloning it at
+ctree.c:get_old_root(). This should be done regardless of the extent
+buffer having been freed and reused, as a concurrent task might be
+modifying it (while holding a write lock on it).
+
+Reported-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
+Link: https://lore.kernel.org/linux-btrfs/20210227155037.GN28049@hungrycats.org/
+Fixes: 834328a8493079 ("Btrfs: tree mod log's old roots could still be part of the tree")
+CC: stable@vger.kernel.org # 4.4+
+Signed-off-by: Filipe Manana <fdmanana@suse.com>
+Signed-off-by: David Sterba <dsterba@suse.com>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ fs/btrfs/ctree.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+--- a/fs/btrfs/ctree.c
++++ b/fs/btrfs/ctree.c
+@@ -1414,7 +1414,9 @@ get_old_root(struct btrfs_root *root, u6
+ "failed to read tree block %llu from get_old_root",
+ logical);
+ } else {
++ btrfs_tree_read_lock(old);
+ eb = btrfs_clone_extent_buffer(old);
++ btrfs_tree_read_unlock(old);
+ free_extent_buffer(old);
+ }
+ } else if (old_root) {
projects / thirdparty / kernel / stable-queue.git / commitdiff
