[PATCH 4.19] ext4: fix race condition between ext4_ioctl_setflags and ext4_fiemap

[Baokun Li]

This patch and problem analysis is based on v4.19 LTS.
The d3b6f23f7167("ext4: move ext4_fiemap to use iomap framework") patch
is incorporated in v5.7-rc1. This patch avoids this problem by switching
to iomap in ext4_fiemap.

Hulk Robot reported a BUG on stable 4.19.252:
==================================================================
kernel BUG at fs/ext4/extents_status.c:762!
invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 7 PID: 2845 Comm: syz-executor Not tainted 4.19.252 #46
RIP: 0010:ext4_es_cache_extent+0x30e/0x370
[...]
Call Trace:
 ext4_cache_extents+0x238/0x2f0
 ext4_find_extent+0x785/0xa40
 ext4_fiemap+0x36d/0xe90
 do_vfs_ioctl+0x6af/0x1200
[...]
==================================================================

Above issue may happen as follows:
-------------------------------------
           cpu1		    cpu2
_____________________|_____________________
do_vfs_ioctl
 ext4_ioctl
  ext4_ioctl_setflags
   ext4_ind_migrate
                        do_vfs_ioctl
                         ioctl_fiemap
                          ext4_fiemap
                           ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)
                           ext4_fill_fiemap_extents
    down_write(&EXT4_I(inode)->i_data_sem);
    ext4_ext_check_inode
    ext4_clear_inode_flag(inode, EXT4_INODE_EXTENTS)
    memset(ei->i_data, 0, sizeof(ei->i_data))
    up_write(&EXT4_I(inode)->i_data_sem);
                            down_read(&EXT4_I(inode)->i_data_sem);
                            ext4_find_extent
                             ext4_cache_extents
                              ext4_es_cache_extent
                               BUG_ON(end < lblk)

We can easily reproduce this problem with the syzkaller testcase:
```
02:37:07 executing program 3:
r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file0\x00', 0x26e1, 0x0)
ioctl$FS_IOC_FSSETXATTR(r0, 0x40086602, &(0x7f0000000080)={0x17e})
mkdirat(0xffffffffffffff9c, &(0x7f00000000c0)='./file1\x00', 0x1ff)
r1 = openat(0xffffffffffffff9c, &(0x7f0000000100)='./file1\x00', 0x0, 0x0)
ioctl$FS_IOC_FIEMAP(r1, 0xc020660b, &(0x7f0000000180)={0x0, 0x1, 0x0, 0xef3, 0x6, []}) (async, rerun: 32)
ioctl$FS_IOC_FSSETXATTR(r1, 0x40086602, &(0x7f0000000140)={0x17e}) (rerun: 32)
```

To solve this issue, we use __generic_block_fiemap() instead of
generic_block_fiemap() and add inode_lock_shared to avoid race condition.

Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 fs/ext4/extents.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 6c492fca60c4..38aaf48e94cb 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5198,13 +5198,18 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 			return error;
 	}
 
+	inode_lock_shared(inode);
 	/* fallback to generic here if not in extents fmt */
-	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
-		return generic_block_fiemap(inode, fieinfo, start, len,
+	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))) {
+		error = __generic_block_fiemap(inode, fieinfo, start, len,
 			ext4_get_block);
+		goto out_unlock;
+	}
 
-	if (fiemap_check_flags(fieinfo, EXT4_FIEMAP_FLAGS))
-		return -EBADR;
+	if (fiemap_check_flags(fieinfo, EXT4_FIEMAP_FLAGS)) {
+		error = -EBADR;
+		goto out_unlock;
+	}
 
 	if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
 		error = ext4_xattr_fiemap(inode, fieinfo);
@@ -5225,6 +5230,8 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 		error = ext4_fill_fiemap_extents(inode, start_blk,
 						 len_blks, fieinfo);
 	}
+out_unlock:
+	inode_unlock_shared(inode);
 	return error;
 }
 
-- 
2.31.1