linux-ext4 - [PATCH] ext4: fix a data-race around bg_free_blocks_count

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Date: Sun, 12 May 2024 14:42:03 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: tytso@....edu,
	adilger.kernel@...ger.ca
Cc: linux-ext4@...r.kernel.org,
	kerneljasonxing@...il.com,
	Jason Xing <kernelxing@...cent.com>
Subject: [PATCH] ext4: fix a data-race around bg_free_blocks_count_lo

From: Jason Xing <kernelxing@...cent.com>

KCSAN reported a data-race issue due to two different cpus accessing
this member.

BUG: KCSAN: data-race in ext4_free_inodes_count / ext4_free_inodes_set

write to 0xffff888104a9e00e of 2 bytes by task 4435 on cpu 0:
 ext4_free_inodes_set+0x1f/0x80 fs/ext4/super.c:397
 __ext4_new_inode+0x15c8/0x2290 fs/ext4/ialloc.c:1216
 ext4_symlink+0x242/0x580 fs/ext4/namei.c:3393
 vfs_symlink+0xc2/0x1a0 fs/namei.c:4475
 do_symlinkat+0xe3/0x320 fs/namei.c:4501
 __do_sys_symlinkat fs/namei.c:4517 [inline]
 __se_sys_symlinkat fs/namei.c:4514 [inline]
 __x64_sys_symlinkat+0x62/0x70 fs/namei.c:4514
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff888104a9e00e of 2 bytes by task 4440 on cpu 1:
 ext4_free_inodes_count+0x1c/0x80 fs/ext4/super.c:341
 __ext4_new_inode+0xb71/0x2290 fs/ext4/ialloc.c:1040
 ext4_symlink+0x242/0x580 fs/ext4/namei.c:3393
 vfs_symlink+0xc2/0x1a0 fs/namei.c:4475
 do_symlinkat+0xe3/0x320 fs/namei.c:4501
 __do_sys_symlinkat fs/namei.c:4517 [inline]
 __se_sys_symlinkat fs/namei.c:4514 [inline]
 __x64_sys_symlinkat+0x62/0x70 fs/namei.c:4514
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x185c -> 0x185b

Signed-off-by: Jason Xing <kernelxing@...cent.com>
---
 fs/ext4/super.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index cf817a6a6e27..6db71cc1e5cd 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -338,7 +338,7 @@ ext4_fsblk_t ext4_inode_table(struct super_block *sb,
 __u32 ext4_free_group_clusters(struct super_block *sb,
 			       struct ext4_group_desc *bg)
 {
-	return le16_to_cpu(bg->bg_free_blocks_count_lo) |
+	return le16_to_cpu(READ_ONCE(bg->bg_free_blocks_count_lo)) |
 		(EXT4_DESC_SIZE(sb) >= EXT4_MIN_DESC_SIZE_64BIT ?
 		 (__u32)le16_to_cpu(bg->bg_free_blocks_count_hi) << 16 : 0);
 }
@@ -394,7 +394,7 @@ void ext4_inode_table_set(struct super_block *sb,
 void ext4_free_group_clusters_set(struct super_block *sb,
 				  struct ext4_group_desc *bg, __u32 count)
 {
-	bg->bg_free_blocks_count_lo = cpu_to_le16((__u16)count);
+	WRITE_ONCE(bg->bg_free_blocks_count_lo, cpu_to_le16((__u16)count));
 	if (EXT4_DESC_SIZE(sb) >= EXT4_MIN_DESC_SIZE_64BIT)
 		bg->bg_free_blocks_count_hi = cpu_to_le16(count >> 16);
 }
-- 
2.37.3