Re: [PATCH v3] ext4: publish jinode after initialization

[Jan Kara]

On Wed 25-02-26 16:26:16, Li Chen wrote:
> ext4_inode_attach_jinode() publishes ei->jinode to concurrent users.
> It used to set ei->jinode before jbd2_journal_init_jbd_inode(),
> allowing a reader to observe a non-NULL jinode with i_vfs_inode
> still unset.
> 
> The fast commit flush path can then pass this jinode to
> jbd2_wait_inode_data(), which dereferences i_vfs_inode->i_mapping and
> may crash.
> 
> Below is the crash I observe:
> ```
> BUG: unable to handle page fault for address: 000000010beb47f4
> PGD 110e51067 P4D 110e51067 PUD 0
> Oops: Oops: 0000 [#1] SMP NOPTI
> CPU: 1 UID: 0 PID: 4850 Comm: fc_fsync_bench_ Not tainted 6.18.0-00764-g795a690c06a5 #1 PREEMPT(voluntary)
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-2-2 04/01/2014
> RIP: 0010:xas_find_marked+0x3d/0x2e0
> Code: e0 03 48 83 f8 02 0f 84 f0 01 00 00 48 8b 47 08 48 89 c3 48 39 c6 0f 82 fd 01 00 00 48 85 c9 74 3d 48 83 f9 03 77 63 4c 8b 0f <49> 8b 71 08 48 c7 47 18 00 00 00 00 48 89 f1 83 e1 03 48 83 f9 02
> RSP: 0018:ffffbbee806e7bf0 EFLAGS: 00010246
> RAX: 000000000010beb4 RBX: 000000000010beb4 RCX: 0000000000000003
> RDX: 0000000000000001 RSI: 0000002000300000 RDI: ffffbbee806e7c10
> RBP: 0000000000000001 R08: 0000002000300000 R09: 000000010beb47ec
> R10: ffff9ea494590090 R11: 0000000000000000 R12: 0000002000300000
> R13: ffffbbee806e7c90 R14: ffff9ea494513788 R15: ffffbbee806e7c88
> FS: 00007fc2f9e3e6c0(0000) GS:ffff9ea6b1444000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000010beb47f4 CR3: 0000000119ac5000 CR4: 0000000000750ef0
> PKRU: 55555554
> Call Trace:
> <TASK>
> filemap_get_folios_tag+0x87/0x2a0
> __filemap_fdatawait_range+0x5f/0xd0
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? __schedule+0x3e7/0x10c0
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? preempt_count_sub+0x5f/0x80
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? cap_safe_nice+0x37/0x70
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? preempt_count_sub+0x5f/0x80
> ? srso_alias_return_thunk+0x5/0xfbef5
> filemap_fdatawait_range_keep_errors+0x12/0x40
> ext4_fc_commit+0x697/0x8b0
> ? ext4_file_write_iter+0x64b/0x950
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? preempt_count_sub+0x5f/0x80
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? vfs_write+0x356/0x480
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? preempt_count_sub+0x5f/0x80
> ext4_sync_file+0xf7/0x370
> do_fsync+0x3b/0x80
> ? syscall_trace_enter+0x108/0x1d0
> __x64_sys_fdatasync+0x16/0x20
> do_syscall_64+0x62/0x2c0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> ...
> ```
> 
> Fix this by initializing the jbd2_inode first.
> Use smp_wmb() and WRITE_ONCE() to publish ei->jinode after
> initialization. Readers use READ_ONCE() to fetch the pointer.
> 
> Fixes: a361293f5fede ("jbd2: Fix oops in jbd2_journal_file_inode()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Li Chen <me@linux.beauty>

Thanks! Looks good to me now. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
> Changes since v2 (as suggested by Jan Kara:
> - Drop READ_ONCE() changes in ext4_jbd2_inode_add_*() and ext4_clear_inode().
> - Inline READ_ONCE(ei->jinode) in ext4_fc_flush_data().
> 
>  fs/ext4/fast_commit.c |  4 ++--
>  fs/ext4/inode.c       | 15 +++++++++++----
>  2 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index f575751f1cae..6e949c21842d 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -975,13 +975,13 @@ static int ext4_fc_flush_data(journal_t *journal)
>  	int ret = 0;
>  
>  	list_for_each_entry(ei, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
> -		ret = jbd2_submit_inode_data(journal, ei->jinode);
> +		ret = jbd2_submit_inode_data(journal, READ_ONCE(ei->jinode));
>  		if (ret)
>  			return ret;
>  	}
>  
>  	list_for_each_entry(ei, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
> -		ret = jbd2_wait_inode_data(journal, ei->jinode);
> +		ret = jbd2_wait_inode_data(journal, READ_ONCE(ei->jinode));
>  		if (ret)
>  			return ret;
>  	}
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index da96db5f2345..d99296d7315f 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -128,6 +128,8 @@ void ext4_inode_csum_set(struct inode *inode, struct ext4_inode *raw,
>  static inline int ext4_begin_ordered_truncate(struct inode *inode,
>  					      loff_t new_size)
>  {
> +	struct jbd2_inode *jinode = READ_ONCE(EXT4_I(inode)->jinode);
> +
>  	trace_ext4_begin_ordered_truncate(inode, new_size);
>  	/*
>  	 * If jinode is zero, then we never opened the file for
> @@ -135,10 +137,10 @@ static inline int ext4_begin_ordered_truncate(struct inode *inode,
>  	 * jbd2_journal_begin_ordered_truncate() since there's no
>  	 * outstanding writes we need to flush.
>  	 */
> -	if (!EXT4_I(inode)->jinode)
> +	if (!jinode)
>  		return 0;
>  	return jbd2_journal_begin_ordered_truncate(EXT4_JOURNAL(inode),
> -						   EXT4_I(inode)->jinode,
> +						   jinode,
>  						   new_size);
>  }
>  
> @@ -4478,8 +4480,13 @@ int ext4_inode_attach_jinode(struct inode *inode)
>  			spin_unlock(&inode->i_lock);
>  			return -ENOMEM;
>  		}
> -		ei->jinode = jinode;
> -		jbd2_journal_init_jbd_inode(ei->jinode, inode);
> +		jbd2_journal_init_jbd_inode(jinode, inode);
> +		/*
> +		 * Publish ->jinode only after it is fully initialized so that
> +		 * readers never observe a partially initialized jbd2_inode.
> +		 */
> +		smp_wmb();
> +		WRITE_ONCE(ei->jinode, jinode);
>  		jinode = NULL;
>  	}
>  	spin_unlock(&inode->i_lock);
> -- 
> 2.52.0
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR