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namei: permit linking with CAP_FOWNER in userns

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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 23adbe12ef ("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>
This commit is contained in:
Dirk Steinmetz 2015-10-20 16:09:19 +02:00 committed by Eric W. Biederman
parent 6ff33f3902
commit f2ca379642
1 changed files with 2 additions and 5 deletions
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7
fs/namei.c
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@ -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);