Commit f2ca3796 authored Oct 20, 2015 by Dirk Steinmetz Committed by Eric W. Biederman Oct 27, 2015

namei: permit linking with CAP_FOWNER in userns



Attempting to hardlink to an unsafe file (e.g. a setuid binary) from
within an unprivileged user namespace fails, even if CAP_FOWNER is held
within the namespace. This may cause various failures, such as a gentoo
installation within a lxc container failing to build and install specific
packages.

This change permits hardlinking of files owned by mapped uids, if
CAP_FOWNER is held for that namespace. Furthermore, it improves consistency
by using the existing inode_owner_or_capable(), which is aware of
namespaced capabilities as of 23adbe12 ("fs,userns: Change
inode_capable to capable_wrt_inode_uidgid").

Signed-off-by: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>

This is hitting us in Ubuntu during some dpkg upgrades in containers.
When upgrading a file dpkg creates a hard link to the old file to back
it up before overwriting it. When packages upgrade suid files owned by a
non-root user the link isn't permitted, and the package upgrade fails.
This patch fixes our problem.

Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

parent 6ff33f39

fs/namei.c

+2 −5

Original line number	Diff line number	Diff line
		@@ -955,26 +955,23 @@ static bool safe_hardlink_source(struct inode *inode)
		* - sysctl_protected_hardlinks enabled
		* - fsuid does not match inode
		* - hardlink source is unsafe (see safe_hardlink_source() above)
		* - not CAP_FOWNER
		* - not CAP_FOWNER in a namespace with the inode owner uid mapped
		*
		* Returns 0 if successful, -ve on error.
		*/
		static int may_linkat(struct path *link)
		{
		const struct cred *cred;
		struct inode *inode;

		if (!sysctl_protected_hardlinks)
		return 0;

		cred = current_cred();
		inode = link->dentry->d_inode;

		/* Source inode owner (or CAP_FOWNER) can hardlink all they like,
		* otherwise, it must be a safe source.
		*/
		if (uid_eq(cred->fsuid, inode->i_uid) \|\| safe_hardlink_source(inode) \|\|
		capable(CAP_FOWNER))
		if (inode_owner_or_capable(inode) \|\| safe_hardlink_source(inode))
		return 0;

		audit_log_link_denied("linkat", link);