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, 12 insertions, 2 deletions

diff --git a/fs/udf/file.c b/fs/udf/file.c
index c02a27a19c6d..1037637957c7 100644
--- a/fs/udf/file.c
+++ b/fs/udf/file.c
@@ -144,6 +144,7 @@ static ssize_t udf_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
         size_t count = iocb->ki_nbytes;
         struct udf_inode_info *iinfo = UDF_I(inode);
 
+        mutex_lock(&inode->i_mutex);
         down_write(&iinfo->i_data_sem);
         if (iinfo->i_alloc_type == ICBTAG_FLAG_AD_IN_ICB) {
                 if (file->f_flags & O_APPEND)
@@ -156,6 +157,7 @@ static ssize_t udf_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
                                                 pos + count)) {
                         err = udf_expand_file_adinicb(inode);
                         if (err) {
+                                mutex_unlock(&inode->i_mutex);
                                 udf_debug("udf_expand_adinicb: err=%d\n", err);
                                 return err;
                         }
@@ -169,9 +171,17 @@ static ssize_t udf_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
         } else
                 up_write(&iinfo->i_data_sem);
 
-        retval = generic_file_aio_write(iocb, iov, nr_segs, ppos);
-        if (retval > 0)
+        retval = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
+        mutex_unlock(&inode->i_mutex);
+
+        if (retval > 0) {
+                ssize_t err;
+
                 mark_inode_dirty(inode);
+                err = generic_write_sync(file, iocb->ki_pos - retval, retval);
+                if (err < 0)
+                        retval = err;
+        }
 
         return retval;
 }