Messages in this thread | | | Date | Sat, 26 Dec 2015 15:51:01 -0600 | From | "Serge E. Hallyn" <> | Subject | Re: [PATCH] ptrace: being capable wrt a process requires mapped uids/gids |
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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 <jann@thejh.net>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>
> --- > 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; > + } > + > 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 majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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