On Sat, Dec 26, 2015 at 03:52:31AM +0100, Jann Horn wrote:
> ptrace_has_cap() checks whether the current process should be
> treated as having a certain capability for ptrace checks
> against another process. Until now, this was equivalent to
> has_ns_capability(current, target_ns, CAP_SYS_PTRACE).
>
> However, if a root-owned process wants to enter a user
> namespace for some reason without knowing who owns it and
> therefore can't change to the namespace owner's uid and gid
> before entering, as soon as it has entered the namespace,
> the namespace owner can attach to it via ptrace and thereby
> gain access to its uid and gid.
>
> While it is possible for the entering process to switch to
> the uid of a claimed namespace owner before entering,
> causing the attempt to enter to fail if the claimed uid is
> wrong, this doesn't solve the problem of determining an
> appropriate gid.
>
> With this change, the entering process can first enter the
> namespace and then safely inspect the namespace's
> properties, e.g. through /proc/self/{uid_map,gid_map},
> assuming that the namespace owner doesn't have access to
> uid 0.
>
> Changed in v2: The caller needs to be capable in the
> namespace into which tcred's uids/gids can be mapped.
>
> Signed-off-by: Jann Horn <j...@thejh.net>
> ---
> kernel/ptrace.c | 33 ++++++++++++++++++++++++++++-----
> 1 file changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index b760bae..260a08d 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -20,6 +20,7 @@
> #include <linux/uio.h>
> #include <linux/audit.h>
> #include <linux/pid_namespace.h>
> +#include <linux/user_namespace.h>
> #include <linux/syscalls.h>
> #include <linux/uaccess.h>
> #include <linux/regset.h>
> @@ -207,12 +208,34 @@ static int ptrace_check_attach(struct task_struct
> *child, bool ignore_state)
> return ret;
> }
>
> -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> +static bool ptrace_has_cap(const struct cred *tcred, unsigned int mode)
> {
> + struct user_namespace *tns = tcred->user_ns;
> +
> + /* When a root-owned process enters a user namespace created by a
> + * malicious user, the user shouldn't be able to execute code under
> + * uid 0 by attaching to the root-owned process via ptrace.
> + * Therefore, similar to the capable_wrt_inode_uidgid() check,
> + * verify that all the uids and gids of the target process are
> + * mapped into a namespace below the current one in which the caller
> + * is capable.
> + * No fsuid/fsgid check because __ptrace_may_access doesn't do it
> + * either.
> + */
> + while (
> + !kuid_has_mapping(tns, tcred->euid) ||
> + !kuid_has_mapping(tns, tcred->suid) ||
> + !kuid_has_mapping(tns, tcred->uid) ||
> + !kgid_has_mapping(tns, tcred->egid) ||
> + !kgid_has_mapping(tns, tcred->sgid) ||
> + !kgid_has_mapping(tns, tcred->gid)) {
> + tns = tns->parent;
Sorry, i can't quite remember - is there a way for a task in init_user_ns to
have
INVALID_UID | INVALID_GID ? I.e. any point in breaking here if tns ==
&init_user_n?
> + }
> +
> if (mode & PTRACE_MODE_NOAUDIT)
> - return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> + return has_ns_capability_noaudit(current, tns, CAP_SYS_PTRACE);
> else
> - return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> + return has_ns_capability(current, tns, CAP_SYS_PTRACE);
> }
>
> /* Returns 0 on success, -errno on denial. */
> @@ -241,7 +264,7 @@ static int __ptrace_may_access(struct task_struct *task,
> unsigned int mode)
> gid_eq(cred->gid, tcred->sgid) &&
> gid_eq(cred->gid, tcred->gid))
> goto ok;
> - if (ptrace_has_cap(tcred->user_ns, mode))
> + if (ptrace_has_cap(tcred, mode))
> goto ok;
> rcu_read_unlock();
> return -EPERM;
> @@ -252,7 +275,7 @@ ok:
> dumpable = get_dumpable(task->mm);
> rcu_read_lock();
> if (dumpable != SUID_DUMP_USER &&
> - !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
> + !ptrace_has_cap(__task_cred(task), mode)) {
> rcu_read_unlock();
> return -EPERM;
> }
> --
> 2.1.4
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
