[PATCH V2] fat: Prevent the race of read/write the FAT32 entry

[Edward Adam Davis <eadavis@qq.com>]

syzbot reports data-race in fat32_ent_get/fat32_ent_put. 

	CPU0(Task A)			CPU1(Task B)
	====				====
	vfs_write
	new_sync_write
	generic_file_write_iter
	fat_write_begin
	block_write_begin		vfs_statfs
	fat_get_block			statfs_by_dentry
	fat_add_cluster			fat_statfs
	fat_ent_write			fat_count_free_clusters
	fat32_ent_put			fat32_ent_get

Task A's write operation on CPU0 and Task B's read operation on CPU1 occur
simultaneously, generating an race condition.

Add READ/WRITE_ONCE to solve the race condition that occurs when accessing
FAT32 entry.

Reported-by: syzbot+d3c29ed63db6ddf8406e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d3c29ed63db6ddf8406e
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
V1 -> V2: using READ/WRITE_ONCE to fix

 fs/fat/fatent.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index 1db348f8f887..a9eecd090d91 100644
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -146,8 +146,8 @@ static int fat16_ent_get(struct fat_entry *fatent)
 
 static int fat32_ent_get(struct fat_entry *fatent)
 {
-	int next = le32_to_cpu(*fatent->u.ent32_p) & 0x0fffffff;
-	WARN_ON((unsigned long)fatent->u.ent32_p & (4 - 1));
+	int next = le32_to_cpu(READ_ONCE(*fatent->u.ent32_p)) & 0x0fffffff;
+	WARN_ON((unsigned long)READ_ONCE(fatent->u.ent32_p) & (4 - 1));
 	if (next >= BAD_FAT32)
 		next = FAT_ENT_EOF;
 	return next;
@@ -187,8 +187,8 @@ static void fat16_ent_put(struct fat_entry *fatent, int new)
 static void fat32_ent_put(struct fat_entry *fatent, int new)
 {
 	WARN_ON(new & 0xf0000000);
-	new |= le32_to_cpu(*fatent->u.ent32_p) & ~0x0fffffff;
-	*fatent->u.ent32_p = cpu_to_le32(new);
+	new |= le32_to_cpu(READ_ONCE(*fatent->u.ent32_p)) & ~0x0fffffff;
+	WRITE_ONCE(*fatent->u.ent32_p, cpu_to_le32(new));
 	mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode);
 }
 
-- 
2.43.0