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userns: Add a more complete capability subset test to commit_creds 

When unsharing a user namespace we reduce our credentials to just what
can be done in that user namespace.  This is a subset of the credentials
we previously had.  Teach commit_creds to recognize this is a subset
of the credentials we have had before and don't clear the dumpability flag.

This allows an unprivileged  program to do:
unshare(CLONE_NEWUSER);
fd = open("/proc/self/uid_map", O_RDWR);

Where previously opening the uid_map writable would fail because
the the task had been made non-dumpable.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2012-12-14 08:50:54 -08:00
parent 5e4a08476b
commit aa6d054e5c
1 changed files with 26 additions and 1 deletions
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27
kernel/cred.c
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@ -455,6 +455,31 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
return ret;
}
static bool cred_cap_issubset(const struct cred *set, const struct cred *subset)
{
const struct user_namespace *set_ns = set->user_ns;
const struct user_namespace *subset_ns = subset->user_ns;
/* If the two credentials are in the same user namespace see if
* the capabilities of subset are a subset of set.
*/
if (set_ns == subset_ns)
return cap_issubset(subset->cap_permitted, set->cap_permitted);
/* The credentials are in a different user namespaces
* therefore one is a subset of the other only if a set is an
* ancestor of subset and set->euid is owner of subset or one
* of subsets ancestors.
*/
for (;subset_ns != &init_user_ns; subset_ns = subset_ns->parent) {
if ((set_ns == subset_ns->parent) &&
uid_eq(subset_ns->owner, set->euid))
return true;
}
return false;
}
/**
* commit_creds - Install new credentials upon the current task
* @new: The credentials to be assigned
@ -493,7 +518,7 @@ int commit_creds(struct cred *new)
!gid_eq(old->egid, new->egid) ||
!uid_eq(old->fsuid, new->fsuid) ||
!gid_eq(old->fsgid, new->fsgid) ||
!cap_issubset(new->cap_permitted, old->cap_permitted)) {
!cred_cap_issubset(old, new)) {
if (task->mm)
set_dumpable(task->mm, suid_dumpable);
task->pdeath_signal = 0;