f2fs: fix to avoid NULL pointer dereference f2fs_write_end_io()

butt3rflyh4ck reports a bug as below:

When a thread always calls F2FS_IOC_RESIZE_FS to resize fs, if resize fs is
failed, f2fs kernel thread would invoke callback function to update f2fs io
info, it would call  f2fs_write_end_io and may trigger null-ptr-deref in
NODE_MAPPING.

general protection fault, probably for non-canonical address
KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
RIP: 0010:NODE_MAPPING fs/f2fs/f2fs.h:1972 [inline]
RIP: 0010:f2fs_write_end_io+0x727/0x1050 fs/f2fs/data.c:370
 <TASK>
 bio_endio+0x5af/0x6c0 block/bio.c:1608
 req_bio_endio block/blk-mq.c:761 [inline]
 blk_update_request+0x5cc/0x1690 block/blk-mq.c:906
 blk_mq_end_request+0x59/0x4c0 block/blk-mq.c:1023
 lo_complete_rq+0x1c6/0x280 drivers/block/loop.c:370
 blk_complete_reqs+0xad/0xe0 block/blk-mq.c:1101
 __do_softirq+0x1d4/0x8ef kernel/softirq.c:571
 run_ksoftirqd kernel/softirq.c:939 [inline]
 run_ksoftirqd+0x31/0x60 kernel/softirq.c:931
 smpboot_thread_fn+0x659/0x9e0 kernel/smpboot.c:164
 kthread+0x33e/0x440 kernel/kthread.c:379
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308

The root cause is below race case can cause leaving dirty metadata
in f2fs after filesystem is remount as ro:

Thread A				Thread B
- f2fs_ioc_resize_fs
 - f2fs_readonly   --- return false
 - f2fs_resize_fs
					- f2fs_remount
					 - write_checkpoint
					 - set f2fs as ro
  - free_segment_range
   - update meta_inode's data

Then, if f2fs_put_super()  fails to write_checkpoint due to readonly
status, and meta_inode's dirty data will be writebacked after node_inode
is put, finally, f2fs_write_end_io will access NULL pointer on
sbi->node_inode.

Thread A				IRQ context
- f2fs_put_super
 - write_checkpoint fails
 - iput(node_inode)
 - node_inode = NULL
 - iput(meta_inode)
  - write_inode_now
   - f2fs_write_meta_page
					- f2fs_write_end_io
					 - NODE_MAPPING(sbi)
					 : access NULL pointer on node_inode

Fixes: b4b1006 ("f2fs: refactor resize_fs to avoid meta updates in progress")
Reported-by: butt3rflyh4ck <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Tested-by: butt3rflyh4ck <[email protected]>
Signed-off-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

fs/f2fs/f2fs.h

@@ -3879,7 +3879,7 @@ void f2fs_stop_gc_thread(struct f2fs_sb_info *sbi);
 block_t f2fs_start_bidx_of_node(unsigned int node_ofs, struct inode *inode);
 int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control);
 void f2fs_build_gc_manager(struct f2fs_sb_info *sbi);
-int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count);
+int f2fs_resize_fs(struct file *filp, __u64 block_count);
 int __init f2fs_create_garbage_collection_cache(void);
 void f2fs_destroy_garbage_collection_cache(void);
 /* victim selection function for cleaning and SSR */

fs/f2fs/file.c

@@ -3385,7 +3385,7 @@ static int f2fs_ioc_resize_fs(struct file *filp, unsigned long arg)
 			   sizeof(block_count)))
 		return -EFAULT;
 
-	return f2fs_resize_fs(sbi, block_count);
+	return f2fs_resize_fs(filp, block_count);
 }
 
 static int f2fs_ioc_enable_verity(struct file *filp, unsigned long arg)

fs/f2fs/gc.c

@@ -11,6 +11,7 @@
 #include <linux/init.h>
 #include <linux/f2fs_fs.h>
 #include <linux/kthread.h>
+#include <linux/mount.h>
 #include <linux/delay.h>
 #include <linux/freezer.h>
 #include <linux/sched/signal.h>
@@ -2104,8 +2105,9 @@ static void update_fs_metadata(struct f2fs_sb_info *sbi, int secs)
 	}
 }
 
-int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
+int f2fs_resize_fs(struct file *filp, __u64 block_count)
 {
+	struct f2fs_sb_info *sbi = F2FS_I_SB(file_inode(filp));
 	__u64 old_block_count, shrunk_blocks;
 	struct cp_control cpc = { CP_RESIZE, 0, 0, 0 };
 	unsigned int secs;
@@ -2143,12 +2145,18 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
 		return -EINVAL;
 	}
 
+	err = mnt_want_write_file(filp);
+	if (err)
+		return err;
+
 	shrunk_blocks = old_block_count - block_count;
 	secs = div_u64(shrunk_blocks, BLKS_PER_SEC(sbi));
 
 	/* stop other GC */
-	if (!f2fs_down_write_trylock(&sbi->gc_lock))
-		return -EAGAIN;
+	if (!f2fs_down_write_trylock(&sbi->gc_lock)) {
+		err = -EAGAIN;
+		goto out_drop_write;
+	}
 
 	/* stop CP to protect MAIN_SEC in free_segment_range */
 	f2fs_lock_op(sbi);
@@ -2168,10 +2176,18 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
 out_unlock:
 	f2fs_unlock_op(sbi);
 	f2fs_up_write(&sbi->gc_lock);
+out_drop_write:
+	mnt_drop_write_file(filp);
 	if (err)
 		return err;
 
 	freeze_super(sbi->sb);
+
+	if (f2fs_readonly(sbi->sb)) {
+		thaw_super(sbi->sb);
+		return -EROFS;
+	}
+
 	f2fs_down_write(&sbi->gc_lock);
 	f2fs_down_write(&sbi->cp_global_sem);