f2fs: fix null-ptr-deref in f2fs_get_dnode_of_data

There is issue as follows when test f2fs atomic write:
F2FS-fs (loop0): Can't find valid F2FS filesystem in 2th superblock
F2FS-fs (loop0): invalid crc_offset: 0
F2FS-fs (loop0): f2fs_check_nid_range: out-of-range nid=1, run fsck to fix.
F2FS-fs (loop0): f2fs_check_nid_range: out-of-range nid=2, run fsck to fix.
==================================================================
BUG: KASAN: null-ptr-deref in f2fs_get_dnode_of_data+0xac/0x16d0
Read of size 8 at addr 0000000000000028 by task rep/1990

CPU: 4 PID: 1990 Comm: rep Not tainted 5.19.0-rc6-next-20220715 #266
Call Trace:
 <TASK>
 dump_stack_lvl+0x6e/0x91
 print_report.cold+0x49a/0x6bb
 kasan_report+0xa8/0x130
 f2fs_get_dnode_of_data+0xac/0x16d0
 f2fs_do_write_data_page+0x2a5/0x1030
 move_data_page+0x3c5/0xdf0
 do_garbage_collect+0x2015/0x36c0
 f2fs_gc+0x554/0x1d30
 f2fs_balance_fs+0x7f5/0xda0
 f2fs_write_single_data_page+0xb66/0xdc0
 f2fs_write_cache_pages+0x716/0x1420
 f2fs_write_data_pages+0x84f/0x9a0
 do_writepages+0x130/0x3a0
 filemap_fdatawrite_wbc+0x87/0xa0
 file_write_and_wait_range+0x157/0x1c0
 f2fs_do_sync_file+0x206/0x12d0
 f2fs_sync_file+0x99/0xc0
 vfs_fsync_range+0x75/0x140
 f2fs_file_write_iter+0xd7b/0x1850
 vfs_write+0x645/0x780
 ksys_write+0xf1/0x1e0
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

As 3db1de0e commit changed atomic write way which new a cow_inode for
atomic write file, and also mark cow_inode as FI_ATOMIC_FILE.
When f2fs_do_write_data_page write cow_inode will use cow_inode's cow_inode
which is NULL. Then will trigger null-ptr-deref.
To solve above issue, introduce FI_COW_FILE flag for COW inode.

Fiexes: 3db1de0e("f2fs: change the current atomic write way")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -757,6 +757,7 @@ enum {
 	FI_ENABLE_COMPRESS,	/* enable compression in "user" compression mode */
 	FI_COMPRESS_RELEASED,	/* compressed blocks were released */
 	FI_ALIGNED_WRITE,	/* enable aligned write */
+	FI_COW_FILE,		/* indicate COW file */
 	FI_MAX,			/* max flag, never be used */
 };
 
@@ -3236,6 +3237,11 @@ static inline bool f2fs_is_atomic_file(struct inode *inode)
 	return is_inode_flag_set(inode, FI_ATOMIC_FILE);
 }
 
+static inline bool f2fs_is_cow_file(struct inode *inode)
+{
+	return is_inode_flag_set(inode, FI_COW_FILE);
+}
+
 static inline bool f2fs_is_first_block_written(struct inode *inode)
 {
 	return is_inode_flag_set(inode, FI_FIRST_BLOCK_WRITTEN);

fs/f2fs/file.c

@@ -2061,7 +2061,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
 	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
 
 	set_inode_flag(inode, FI_ATOMIC_FILE);
-	set_inode_flag(fi->cow_inode, FI_ATOMIC_FILE);
+	set_inode_flag(fi->cow_inode, FI_COW_FILE);
 	clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
 	f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
 

fs/f2fs/segment.c

@@ -193,7 +193,7 @@ void f2fs_abort_atomic_write(struct inode *inode, bool clean)
 	if (f2fs_is_atomic_file(inode)) {
 		if (clean)
 			truncate_inode_pages_final(inode->i_mapping);
-		clear_inode_flag(fi->cow_inode, FI_ATOMIC_FILE);
+		clear_inode_flag(fi->cow_inode, FI_COW_FILE);
 		iput(fi->cow_inode);
 		fi->cow_inode = NULL;
 		release_atomic_write_cnt(inode);
@@ -3172,7 +3172,7 @@ static int __get_segment_type_6(struct f2fs_io_info *fio)
 			return CURSEG_COLD_DATA;
 		if (file_is_hot(inode) ||
 				is_inode_flag_set(inode, FI_HOT_DATA) ||
-				f2fs_is_atomic_file(inode))
+				f2fs_is_cow_file(inode))
 			return CURSEG_HOT_DATA;
 		return f2fs_rw_hint_to_seg_type(inode->i_write_hint);
 	} else {