CVE-2021-46958: In the Linux kernel, the following vulnerability has been resolved:  btrfs: fix race between transaction aborts and fsyncs leading to use-after-free  There is a race between a task aborting a transaction during a commit, a task doing an fsync and the transaction kthread, which leads to an use-after-free of the log root tree. When this happens, it results in a stack trace like the following:    BTRFS info (device dm-0): forced readonly   BTRFS warning (device dm-0): Skipping commit of aborted transaction.   BTRFS: error (device dm-0) in cleanup_transaction:1958: errno=-5 IO failure   BTRFS warning (device dm-0): lost page write due to IO error on /dev/mapper/error-test (-5)   BTRFS warning (device dm-0): Skipping commit of aborted transaction.   BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0xa4e8 len 4096 err no 10   BTRFS error (device dm-0): error writing primary super block to device 1   BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e000 len 4096 err no 10   BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e008 len 4096 err no 10   BTRFS warning (device dm-0): direct IO failed ino 261 rw 0,0 sector 0x12e010 len 4096 err no 10   BTRFS: error (device dm-0) in write_all_supers:4110: errno=-5 IO failure (1 errors while writing supers)   BTRFS: error (device dm-0) in btrfs_sync_log:3308: errno=-5 IO failure   general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6b68: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI   CPU: 2 PID: 2458471 Comm: fsstress Not tainted 5.12.0-rc5-btrfs-next-84 #1   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014   RIP: 0010:__mutex_lock+0x139/0xa40   Code: c0 74 19 (...)   RSP: 0018:ffff9f18830d7b00 EFLAGS: 00010202   RAX: 6b6b6b6b6b6b6b68 RBX: 0000000000000001 RCX: 0000000000000002   RDX: ffffffffb9c54d13 RSI: 0000000000000000 RDI: 0000000000000000   RBP: ffff9f18830d7bc0 R08: 0000000000000000 R09: 0000000000000000   R10: ffff9f18830d7be0 R11: 0000000000000001 R12: ffff8c6cd199c040   R13: ffff8c6c95821358 R14: 00000000fffffffb R15: ffff8c6cbcf01358   FS:  00007fa9140c2b80(0000) GS:ffff8c6fac600000(0000) knlGS:0000000000000000   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033   CR2: 00007fa913d52000 CR3: 000000013d2b4003 CR4: 0000000000370ee0   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400   Call Trace:    ? __btrfs_handle_fs_error+0xde/0x146 [btrfs]    ? btrfs_sync_log+0x7c1/0xf20 [btrfs]    ? btrfs_sync_log+0x7c1/0xf20 [btrfs]    btrfs_sync_log+0x7c1/0xf20 [btrfs]    btrfs_sync_file+0x40c/0x580 [btrfs]    do_fsync+0x38/0x70    __x64_sys_fsync+0x10/0x20    do_syscall_64+0x33/0x80    entry_SYSCALL_64_after_hwframe+0x44/0xae   RIP: 0033:0x7fa9142a55c3   Code: 8b 15 09 (...)   RSP: 002b:00007fff26278d48 EFLAGS: 00000246 ORIG_RAX: 000000000000004a   RAX: ffffffffffffffda RBX: 0000563c83cb4560 RCX: 00007fa9142a55c3   RDX: 00007fff26278cb0 RSI: 00007fff26278cb0 RDI: 0000000000000005   RBP: 0000000000000005 R08: 0000000000000001 R09: 00007fff26278d5c   R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000340   R13: 00007fff26278de0 R14: 00007fff26278d96 R15: 0000563c83ca57c0   Modules linked in: btrfs dm_zero dm_snapshot dm_thin_pool (...)   ---[ end trace ee2f1b19327d791d ]---  The steps that lead to this crash are the following:  1) We are at transaction N;  2) We have two tasks with a transaction handle attached to transaction N.    Task A and Task B. Task B is doing an fsync;  3) Task B is at btrfs_sync_log(), and has saved fs_info->log_root_tree    into a local variable named 'log_root_tree' at the top of    btrfs_sync_log(). Task B is about to call write_all_supers(), but    before that...  4) Task A calls btrfs_commit_transaction(), and after it sets the    transaction state to TRANS_STATE_COMMIT_START, an error happens before    it w ---truncated---