Commit 5de4480a authored Sep 10, 2018 by Mark Fasheh Committed by Al Viro Oct 17, 2018

vfs: allow dedupe of user owned read-only files

The permission check in vfs_dedupe_file_range_one() is too coarse - We only
allow dedupe of the destination file if the user is root, or they have the
file open for write.

This effectively limits a non-root user from deduping their own read-only
files. In addition, the write file descriptor that the user is forced to
hold open can prevent execution of files. As file data during a dedupe
does not change, the behavior is unexpected and this has caused a number of
issue reports. For an example, see:

https://github.com/markfasheh/duperemove/issues/129



So change the check so we allow dedupe on the target if:

- the root or admin is asking for it
- the process has write access
- the owner of the file is asking for the dedupe
- the process could get write access

That way users can open read-only and still get dedupe.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent 995f608e

fs/read_write.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -1964,6 +1964,20 @@ out_error:
		}
		EXPORT_SYMBOL(vfs_dedupe_file_range_compare);

		/* Check whether we are allowed to dedupe the destination file */
		static bool allow_file_dedupe(struct file *file)
		{
		if (capable(CAP_SYS_ADMIN))
		return true;
		if (file->f_mode & FMODE_WRITE)
		return true;
		if (uid_eq(current_fsuid(), file_inode(file)->i_uid))
		return true;
		if (!inode_permission(file_inode(file), MAY_WRITE))
		return true;
		return false;
		}

		int vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
		struct file *dst_file, loff_t dst_pos, u64 len)
		{
		@@ -1978,7 +1992,7 @@ int vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
		goto out_drop_write;

		ret = -EINVAL;
		if (!(capable(CAP_SYS_ADMIN) \|\| (dst_file->f_mode & FMODE_WRITE)))
		if (!allow_file_dedupe(dst_file))
		goto out_drop_write;

		ret = -EXDEV;

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