From: Filipe Manana fdmanana@suse.com

commit cd9253c23aedd61eb5ff11f37a36247cd46faf86 upstream.

If we have 2 threads that are using the same file descriptor and one of them is doing direct IO writes while the other is doing fsync, we have a race where we can end up either:

1) Attempt a fsync without holding the inode's lock, triggering an assertion failures when assertions are enabled;

2) Do an invalid memory access from the fsync task because the file private points to memory allocated on stack by the direct IO task and it may be used by the fsync task after the stack was destroyed.

The race happens like this:

1) A user space program opens a file descriptor with O_DIRECT;

2) The program spawns 2 threads using libpthread for example;

3) One of the threads uses the file descriptor to do direct IO writes, while the other calls fsync using the same file descriptor.

4) Call task A the thread doing direct IO writes and task B the thread doing fsyncs;

5) Task A does a direct IO write, and at btrfs_direct_write() sets the file's private to an on stack allocated private with the member 'fsync_skip_inode_lock' set to true;

6) Task B enters btrfs_sync_file() and sees that there's a private structure associated to the file which has 'fsync_skip_inode_lock' set to true, so it skips locking the inode's vfs lock;

7) Task A completes the direct IO write, and resets the file's private to NULL since it had no prior private and our private was stack allocated. Then it unlocks the inode's vfs lock;

8) Task B enters btrfs_get_ordered_extents_for_logging(), then the assertion that checks the inode's vfs lock is held fails, since task B never locked it and task A has already unlocked it.

The stack trace produced is the following:

Aug 21 11:46:43 kerberos kernel: assertion failed: inode_is_locked(&inode->vfs_inode), in fs/btrfs/ordered-data.c:983 Aug 21 11:46:43 kerberos kernel: ------------[ cut here ]------------ Aug 21 11:46:43 kerberos kernel: kernel BUG at fs/btrfs/ordered-data.c:983! Aug 21 11:46:43 kerberos kernel: Oops: invalid opcode: 0000 [#1] PREEMPT SMP PTI Aug 21 11:46:43 kerberos kernel: CPU: 9 PID: 5072 Comm: worker Tainted: G U OE 6.10.5-1-default #1 openSUSE Tumbleweed 69f48d427608e1c09e60ea24c6c55e2ca1b049e8 Aug 21 11:46:43 kerberos kernel: Hardware name: Acer Predator PH315-52/Covini_CFS, BIOS V1.12 07/28/2020 Aug 21 11:46:43 kerberos kernel: RIP: 0010:btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs] Aug 21 11:46:43 kerberos kernel: Code: 50 d6 86 c0 e8 (...) Aug 21 11:46:43 kerberos kernel: RSP: 0018:ffff9e4a03dcfc78 EFLAGS: 00010246 Aug 21 11:46:43 kerberos kernel: RAX: 0000000000000054 RBX: ffff9078a9868e98 RCX: 0000000000000000 Aug 21 11:46:43 kerberos kernel: RDX: 0000000000000000 RSI: ffff907dce4a7800 RDI: ffff907dce4a7800 Aug 21 11:46:43 kerberos kernel: RBP: ffff907805518800 R08: 0000000000000000 R09: ffff9e4a03dcfb38 Aug 21 11:46:43 kerberos kernel: R10: ffff9e4a03dcfb30 R11: 0000000000000003 R12: ffff907684ae7800 Aug 21 11:46:43 kerberos kernel: R13: 0000000000000001 R14: ffff90774646b600 R15: 0000000000000000 Aug 21 11:46:43 kerberos kernel: FS: 00007f04b96006c0(0000) GS:ffff907dce480000(0000) knlGS:0000000000000000 Aug 21 11:46:43 kerberos kernel: CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 Aug 21 11:46:43 kerberos kernel: CR2: 00007f32acbfc000 CR3: 00000001fd4fa005 CR4: 00000000003726f0 Aug 21 11:46:43 kerberos kernel: Call Trace: Aug 21 11:46:43 kerberos kernel: <TASK> Aug 21 11:46:43 kerberos kernel: ? __die_body.cold+0x14/0x24 Aug 21 11:46:43 kerberos kernel: ? die+0x2e/0x50 Aug 21 11:46:43 kerberos kernel: ? do_trap+0xca/0x110 Aug 21 11:46:43 kerberos kernel: ? do_error_trap+0x6a/0x90 Aug 21 11:46:43 kerberos kernel: ? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a] Aug 21 11:46:43 kerberos kernel: ? exc_invalid_op+0x50/0x70 Aug 21 11:46:43 kerberos kernel: ? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a] Aug 21 11:46:43 kerberos kernel: ? asm_exc_invalid_op+0x1a/0x20 Aug 21 11:46:43 kerberos kernel: ? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a] Aug 21 11:46:43 kerberos kernel: ? btrfs_get_ordered_extents_for_logging.cold+0x1f/0x42 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a] Aug 21 11:46:43 kerberos kernel: btrfs_sync_file+0x21a/0x4d0 [btrfs bb26272d49b4cdc847cf3f7faadd459b62caee9a] Aug 21 11:46:43 kerberos kernel: ? __seccomp_filter+0x31d/0x4f0 Aug 21 11:46:43 kerberos kernel: __x64_sys_fdatasync+0x4f/0x90 Aug 21 11:46:43 kerberos kernel: do_syscall_64+0x82/0x160 Aug 21 11:46:43 kerberos kernel: ? do_futex+0xcb/0x190 Aug 21 11:46:43 kerberos kernel: ? __x64_sys_futex+0x10e/0x1d0 Aug 21 11:46:43 kerberos kernel: ? switch_fpu_return+0x4f/0xd0 Aug 21 11:46:43 kerberos kernel: ? syscall_exit_to_user_mode+0x72/0x220 Aug 21 11:46:43 kerberos kernel: ? do_syscall_64+0x8e/0x160 Aug 21 11:46:43 kerberos kernel: ? syscall_exit_to_user_mode+0x72/0x220 Aug 21 11:46:43 kerberos kernel: ? do_syscall_64+0x8e/0x160 Aug 21 11:46:43 kerberos kernel: ? syscall_exit_to_user_mode+0x72/0x220 Aug 21 11:46:43 kerberos kernel: ? do_syscall_64+0x8e/0x160 Aug 21 11:46:43 kerberos kernel: ? syscall_exit_to_user_mode+0x72/0x220 Aug 21 11:46:43 kerberos kernel: ? do_syscall_64+0x8e/0x160 Aug 21 11:46:43 kerberos kernel: entry_SYSCALL_64_after_hwframe+0x76/0x7e

Another problem here is if task B grabs the private pointer and then uses it after task A has finished, since the private was allocated in the stack of trask A, it results in some invalid memory access with a hard to predict result.

This issue, triggering the assertion, was observed with QEMU workloads by two users in the Link tags below.

Fix this by not relying on a file's private to pass information to fsync that it should skip locking the inode and instead pass this information through a special value stored in current->journal_info. This is safe because in the relevant section of the direct IO write path we are not holding a transaction handle, so current->journal_info is NULL.

The following C program triggers the issue:

$ cat repro.c /* Get the O_DIRECT definition. */ #ifndef _GNU_SOURCE #define _GNU_SOURCE #endif

#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <stdint.h> #include <fcntl.h> #include <errno.h> #include <string.h> #include <pthread.h>

static int fd;

static ssize_t do_write(int fd, const void *buf, size_t count, off_t offset) { while (count > 0) { ssize_t ret;

ret = pwrite(fd, buf, count, offset); if (ret < 0) { if (errno == EINTR) continue; return ret; } count -= ret; buf += ret; } return 0; }

static void *fsync_loop(void *arg) { while (1) { int ret;

ret = fsync(fd); if (ret != 0) { perror("Fsync failed"); exit(6); } } }

int main(int argc, char *argv[]) { long pagesize; void *write_buf; pthread_t fsyncer; int ret;

if (argc != 2) { fprintf(stderr, "Use: %s <file path>\n", argv[0]); return 1; }

fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC | O_DIRECT, 0666); if (fd == -1) { perror("Failed to open/create file"); return 1; }

pagesize = sysconf(_SC_PAGE_SIZE); if (pagesize == -1) { perror("Failed to get page size"); return 2; }

ret = posix_memalign(&write_buf, pagesize, pagesize); if (ret) { perror("Failed to allocate buffer"); return 3; }

ret = pthread_create(&fsyncer, NULL, fsync_loop, NULL); if (ret != 0) { fprintf(stderr, "Failed to create writer thread: %d\n", ret); return 4; }

while (1) { ret = do_write(fd, write_buf, pagesize, 0); if (ret != 0) { perror("Write failed"); exit(5); } }

return 0; }

$ mkfs.btrfs -f /dev/sdi $ mount /dev/sdi /mnt/sdi $ timeout 10 ./repro /mnt/sdi/foo

Usually the race is triggered within less than 1 second. A test case for fstests will follow soon.

Reported-by: Paulo Dias paulo.miguel.dias@gmail.com Link: https://bugzilla.kernel.org/show_bug.cgi?id=219187 Reported-by: Andreas Jahn jahn-andi@web.de Link: https://bugzilla.kernel.org/show_bug.cgi?id=219199 Reported-by: syzbot+4704b3cc972bd76024f1@syzkaller.appspotmail.com Link: https://lore.kernel.org/linux-btrfs/00000000000044ff540620d7dee2@google.com/ Fixes: 939b656bc8ab ("btrfs: fix corruption after buffer fault in during direct IO append write") CC: stable@vger.kernel.org # 5.15+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Filipe Manana fdmanana@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com --- fs/btrfs/ctree.h | 1 - fs/btrfs/file.c | 25 ++++++++++--------------- fs/btrfs/transaction.h | 6 ++++++ 3 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h index f19c6aa3ea4b..17ebcf19b444 100644 --- a/fs/btrfs/ctree.h +++ b/fs/btrfs/ctree.h @@ -1383,7 +1383,6 @@ struct btrfs_drop_extents_args { struct btrfs_file_private { void *filldir_buf; u64 last_index; - bool fsync_skip_inode_lock; };

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index c44dfb4370d7..44160d4ad53e 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -1992,13 +1992,6 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) if (IS_ERR_OR_NULL(dio)) { err = PTR_ERR_OR_ZERO(dio); } else { - struct btrfs_file_private stack_private = { 0 }; - struct btrfs_file_private *private; - const bool have_private = (file->private_data != NULL); - - if (!have_private) - file->private_data = &stack_private; - /* * If we have a synchoronous write, we must make sure the fsync * triggered by the iomap_dio_complete() call below doesn't @@ -2007,13 +2000,10 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) * partial writes due to the input buffer (or parts of it) not * being already faulted in. */ - private = file->private_data; - private->fsync_skip_inode_lock = true; + ASSERT(current->journal_info == NULL); + current->journal_info = BTRFS_TRANS_DIO_WRITE_STUB; err = iomap_dio_complete(dio); - private->fsync_skip_inode_lock = false; - - if (!have_private) - file->private_data = NULL; + current->journal_info = NULL; }

/* No increment (+=) because iomap returns a cumulative value. */ @@ -2195,7 +2185,6 @@ static inline bool skip_inode_logging(const struct btrfs_log_ctx *ctx) */ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync) { - struct btrfs_file_private *private = file->private_data; struct dentry *dentry = file_dentry(file); struct inode *inode = d_inode(dentry); struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb); @@ -2205,7 +2194,13 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync) int ret = 0, err; u64 len; bool full_sync; - const bool skip_ilock = (private ? private->fsync_skip_inode_lock : false); + bool skip_ilock = false; + + if (current->journal_info == BTRFS_TRANS_DIO_WRITE_STUB) { + skip_ilock = true; + current->journal_info = NULL; + lockdep_assert_held(&inode->i_rwsem); + }

trace_btrfs_sync_file(file, datasync);

diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h index 0ded32bbd001..a06bc6ad4764 100644 --- a/fs/btrfs/transaction.h +++ b/fs/btrfs/transaction.h @@ -11,6 +11,12 @@ #include "delayed-ref.h" #include "ctree.h"

+/* + * Signal that a direct IO write is in progress, to avoid deadlock for sync + * direct IO writes when fsync is called during the direct IO write path. + */ +#define BTRFS_TRANS_DIO_WRITE_STUB ((void *) 1) + enum btrfs_trans_state { TRANS_STATE_RUNNING, TRANS_STATE_COMMIT_START,