udf: Fix data corruption on file type conversion

UDF has two types of files - files with data stored in inode (ICB in
UDF terminology) and files with data stored in external data blocks. We
convert file from in-inode format to external format in
udf_file_aio_write() when we find out data won't fit into inode any
longer. However the following race between two O_APPEND writes can happen:

CPU1					CPU2
udf_file_aio_write()			udf_file_aio_write()
  down_write(&iinfo->i_data_sem);
  checks that i_size + count1 fits within inode
    => no need to convert
  up_write(&iinfo->i_data_sem);
					  down_write(&iinfo->i_data_sem);
					  checks that i_size + count2 fits
					    within inode => no need to convert
					  up_write(&iinfo->i_data_sem);
  generic_file_aio_write()
    - extends file by count1 bytes
					  generic_file_aio_write()
					    - extends file by count2 bytes

Clearly if count1 + count2 doesn't fit into the inode, we overwrite
kernel buffers beyond inode, possibly corrupting the filesystem as well.

Fix the problem by acquiring i_mutex before checking whether write fits
into the inode and using __generic_file_aio_write() afterwards which
puts check and write into one critical section.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/udf/inode.c b/fs/udf/inode.c
index 062b792..982ce05 100644
--- a/fs/udf/inode.c
+++ b/fs/udf/inode.c
@@ -265,6 +265,7 @@ int udf_expand_file_adinicb(struct inode *inode)
 		.nr_to_write = 1,
 	};
 
+	WARN_ON_ONCE(!mutex_is_locked(&inode->i_mutex));
 	if (!iinfo->i_lenAlloc) {
 		if (UDF_QUERY_FLAG(inode->i_sb, UDF_FLAG_USE_SHORT_AD))
 			iinfo->i_alloc_type = ICBTAG_FLAG_AD_SHORT;