From: Oleg Nesterov <oleg@redhat.com>
To: Jann Horn <jann@thejh.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
Roland McGrath <roland@hack.frob.com>,
John Johansen <john.johansen@canonical.com>,
James Morris <james.l.morris@oracle.com>,
"Serge E. Hallyn" <serge@hallyn.com>,
Paul Moore <aul@paul-moore.com>,
Stephen Smalley <sds@tycho.nsa.gov>,
Eric Paris <eparis@parisplace.org>,
Casey Schaufler <casey@schaufler-ca.com>,
Kees Cook <keescook@chromium.org>,
Andrew Morton <akpm@linux-foundation.org>,
Janis Danisevskis <jdanis@google.com>,
Seth Forshee <seth.forshee@canonical.com>,
"Eric . Biederman" <ebiederm@xmission.com>,
Thomas Gleixner <tglx@linutronix.de>,
Benjamin LaHaise <bcrl@kvack.org>,
Ben Hutchings <ben@decadent.org.uk>,
Andy Lutomirski <luto@amacapital.net>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel@vger.kernel.org,
linux-security-module@vger.kernel.org, security@kernel.org
Subject: Re: [PATCH v2 4/8] futex: don't leak robust_list pointer
Date: Fri, 30 Sep 2016 16:52:57 +0200 [thread overview]
Message-ID: <20160930145256.GB12862@redhat.com> (raw)
In-Reply-To: <1474663238-22134-5-git-send-email-jann@thejh.net>
On 09/23, Jann Horn wrote:
>
> This prevents an attacker from determining the robust_list or
> compat_robust_list userspace pointer of a process created by executing
> a setuid binary. Such an attack could be performed by racing
> get_robust_list() with a setuid execution. The impact of this issue is that
> an attacker could theoretically bypass ASLR when attacking setuid binaries.
Well. I am not sure this actually needs a fix, but I won't argue.
I can't really understand what this patch actually fixes,
> @@ -3007,31 +3007,43 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
> if (!futex_cmpxchg_enabled)
> return -ENOSYS;
>
> - rcu_read_lock();
> -
> - ret = -ESRCH;
> - if (!pid)
> + if (!pid) {
> p = current;
> - else {
> + get_task_struct(p);
> + } else {
> + rcu_read_lock();
> p = find_task_by_vpid(pid);
> + /* pin the task to permit dropping the RCU read lock before
> + * acquiring the mutex
> + */
> + if (p)
> + get_task_struct(p);
> + rcu_read_unlock();
> if (!p)
> - goto err_unlock;
> + return -ESRCH;
> }
>
> + ret = mutex_lock_killable(&p->signal->cred_guard_light);
> + if (ret)
> + goto err_put;
> +
> ret = -EPERM;
> if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
> goto err_unlock;
>
> head = p->robust_list;
> - rcu_read_unlock();
OK, suppose it races with setuid exec, and mutex_lock_killable() +
ptrace_may_access() comes after flush_old_exec() but before
install_exec_creds(), in this case ptrace_may_access() can wrongly
succeed.
In theory, it is possible that the execing thread can complete exec,
return to user-mode and call sys_set_robust_list() before we read
head = p->robust_list. Yes, this is unlikely, but unless I am totally
confused the race you are trying to fix is equally unlikely?
perhaps we can make a much simpler change to prevent this, see below.
We can rely on fact that both ptrace_may_access() and exec_mmap()
takes the same task_lock(). Sure, this can "leak" robust_list too,
a set-uid binary can exec and/or lower its credentials after we
read p->robust_list, but personally I think we do not care.
Or I missed something else?
Oleg.
--- x/kernel/futex.c
+++ x/kernel/futex.c
@@ -3019,10 +3019,10 @@ SYSCALL_DEFINE3(get_robust_list, int, pi
}
ret = -EPERM;
+ head = p->robust_list;
if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock;
- head = p->robust_list;
rcu_read_unlock();
if (put_user(sizeof(*head), len_ptr))
next prev parent reply other threads:[~2016-09-30 14:54 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-23 20:40 [PATCH v2 0/8] Various fixes related to ptrace_may_access() Jann Horn
2016-09-23 20:40 ` [PATCH v2 1/8] exec: introduce cred_guard_light Jann Horn
2016-09-30 15:35 ` Oleg Nesterov
2016-09-30 18:27 ` Eric W. Biederman
2016-10-03 16:02 ` Oleg Nesterov
2016-10-30 21:12 ` Jann Horn
2016-09-23 20:40 ` [PATCH v2 2/8] exec: turn self_exec_id into self_privunit Jann Horn
2016-09-23 21:04 ` Andy Lutomirski
2016-09-23 21:33 ` Jann Horn
2016-09-30 13:20 ` Oleg Nesterov
2016-09-30 13:44 ` Oleg Nesterov
2016-09-30 18:30 ` Kees Cook
2016-09-30 18:59 ` Jann Horn
2016-09-30 19:05 ` Kees Cook
2016-10-03 16:37 ` Oleg Nesterov
2016-09-23 20:40 ` [PATCH v2 3/8] proc: use open()-time creds for ptrace checks Jann Horn
2016-09-23 20:40 ` [PATCH v2 4/8] futex: don't leak robust_list pointer Jann Horn
2016-09-30 14:52 ` Oleg Nesterov [this message]
2016-10-30 17:16 ` Jann Horn
2016-11-02 21:39 ` Jann Horn
2016-11-02 22:47 ` Jann Horn
2016-09-23 20:40 ` [PATCH v2 5/8] proc: lock properly in ptrace_may_access callers Jann Horn
2016-09-23 20:40 ` [PATCH v2 6/8] ptrace: warn on ptrace_may_access without proper locking Jann Horn
2016-09-23 20:40 ` [PATCH v2 7/8] fs/proc: fix attr access check Jann Horn
2016-09-23 20:40 ` [PATCH v2 8/8] Documentation: add security/ptrace_checks.txt Jann Horn
2016-10-02 3:16 ` Krister Johansen
2016-10-30 19:09 ` Jann Horn
2016-10-31 4:14 ` Eric W. Biederman
2016-10-31 13:39 ` Jann Horn
2016-11-03 20:43 ` Krister Johansen
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