Re: [PATCH v3] ext4: prevent data-race that occur when read/write ext4_group_desc structure members

[Andreas Dilger]

On Oct 3, 2024, at 6:53 AM, Jeongjun Park <aha310510@xxxxxxxxx> wrote:
> 
> Currently, data-race like [1] occur in fs/ext4/ialloc.c
> 
> find_group_other() and find_group_orlov() read *_lo, *_hi with
> ext4_free_inodes_count without additional locking. This can cause data-race,
> but since the lock is held for most writes and free inodes value is generally
> not a problem even if it is incorrect, it is more appropriate to use
> READ_ONCE()/WRITE_ONCE() than to add locking.

Thanks for the updated patch.

Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>

> 
> [1]
> 
> ==================================================================
> BUG: KCSAN: data-race in ext4_free_inodes_count / ext4_free_inodes_set
> 
> write to 0xffff88810404300e of 2 bytes by task 6254 on cpu 1:
> ext4_free_inodes_set+0x1f/0x80 fs/ext4/super.c:405
> __ext4_new_inode+0x15ca/0x2200 fs/ext4/ialloc.c:1216
> ext4_symlink+0x242/0x5a0 fs/ext4/namei.c:3391
> vfs_symlink+0xca/0x1d0 fs/namei.c:4615
> do_symlinkat+0xe3/0x340 fs/namei.c:4641
> __do_sys_symlinkat fs/namei.c:4657 [inline]
> __se_sys_symlinkat fs/namei.c:4654 [inline]
> __x64_sys_symlinkat+0x5e/0x70 fs/namei.c:4654
> x64_sys_call+0x1dda/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:267
> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> read to 0xffff88810404300e of 2 bytes by task 6257 on cpu 0:
> ext4_free_inodes_count+0x1c/0x80 fs/ext4/super.c:349
> find_group_other fs/ext4/ialloc.c:594 [inline]
> __ext4_new_inode+0x6ec/0x2200 fs/ext4/ialloc.c:1017
> ext4_symlink+0x242/0x5a0 fs/ext4/namei.c:3391
> vfs_symlink+0xca/0x1d0 fs/namei.c:4615
> do_symlinkat+0xe3/0x340 fs/namei.c:4641
> __do_sys_symlinkat fs/namei.c:4657 [inline]
> __se_sys_symlinkat fs/namei.c:4654 [inline]
> __x64_sys_symlinkat+0x5e/0x70 fs/namei.c:4654
> x64_sys_call+0x1dda/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:267
> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> value changed: 0x185c -> 0x185b
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: ac27a0ec112a ("[PATCH] ext4: initial copy of files from ext3")
> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
> ---
> fs/ext4/super.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 16a4ce704460..8337c4999f90 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -346,9 +346,9 @@ __u32 ext4_free_group_clusters(struct super_block *sb,
> __u32 ext4_free_inodes_count(struct super_block *sb,
> 			      struct ext4_group_desc *bg)
> {
> -	return le16_to_cpu(bg->bg_free_inodes_count_lo) |
> +	return le16_to_cpu(READ_ONCE(bg->bg_free_inodes_count_lo)) |
> 		(EXT4_DESC_SIZE(sb) >= EXT4_MIN_DESC_SIZE_64BIT ?
> -		 (__u32)le16_to_cpu(bg->bg_free_inodes_count_hi) << 16 : 0);
> +		 (__u32)le16_to_cpu(READ_ONCE(bg->bg_free_inodes_count_hi)) << 16 : 0);
> }
> 
> __u32 ext4_used_dirs_count(struct super_block *sb,
> @@ -402,9 +402,9 @@ void ext4_free_group_clusters_set(struct super_block *sb,
> void ext4_free_inodes_set(struct super_block *sb,
> 			  struct ext4_group_desc *bg, __u32 count)
> {
> -	bg->bg_free_inodes_count_lo = cpu_to_le16((__u16)count);
> +	WRITE_ONCE(bg->bg_free_inodes_count_lo, cpu_to_le16((__u16)count));
> 	if (EXT4_DESC_SIZE(sb) >= EXT4_MIN_DESC_SIZE_64BIT)
> -		bg->bg_free_inodes_count_hi = cpu_to_le16(count >> 16);
> +		WRITE_ONCE(bg->bg_free_inodes_count_hi, cpu_to_le16(count >> 16));
> }
> 
> void ext4_used_dirs_set(struct super_block *sb,
> --


Cheers, Andreas

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