Quoting Eric W. Biederman (ebied...@xmission.com):
>
> 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.
>
> Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
Acked-by: Serge Hallyn <serge.hal...@canonical.com>
> ---
> kernel/cred.c | 27 ++++++++++++++++++++++++++-
> 1 files changed, 26 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 48cea3d..709d521 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -455,6 +455,31 @@ error_put:
> 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;
> --
> 1.7.5.4
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