[PATCH 2/2] splice: fix race beween splice_write vs fcntl

[Dmitry Monakhov]

file->f_flags & O_APPEND is checked twice
-> do_splice_direct or do_splice: return EINVAL if O_APPEND enabled
   -> generic_write_checks: seek to end in case of O_APPEND
This is obviously whong and result in unpredictable behaviour if raced with
fcntl. It is reasonable to recheck append flag after kiocb was constructed
(where ->ki_flags is stable), for that reason we should use special
analog of vfs_write_iter() which asserts non-append behaviour.

Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
---
 fs/splice.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 41cbb16..d49615d 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -922,6 +922,27 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 
 	return ret;
 }
+ssize_t splice_iter_write(struct file *file, struct iov_iter *iter, loff_t *ppos)
+{
+	struct kiocb kiocb;
+	ssize_t ret;
+
+	if (!file->f_op->write_iter)
+		return -EINVAL;
+
+	init_sync_kiocb(&kiocb, file);
+	if (kiocb.ki_flags & IOCB_APPEND)
+		return -EINVAL;
+
+	kiocb.ki_pos = *ppos;
+	iter->type |= WRITE;
+	ret = file->f_op->write_iter(&kiocb, iter);
+	BUG_ON(ret == -EIOCBQUEUED);
+	if (ret > 0)
+		*ppos = kiocb.ki_pos;
+	return ret;
+}
+
 
 /**
  * iter_file_splice_write - splice data from a pipe to a file
@@ -1005,7 +1026,7 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 		iov_iter_bvec(&from, ITER_BVEC | WRITE, array, n,
 			      sd.total_len - left);
-		ret = vfs_iter_write(out, &from, &sd.pos);
+		ret = splice_iter_write(out, &from, &sd.pos);
 		if (ret <= 0)
 			break;
 
-- 
1.7.1

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