netdev - Re: [PATCH 4/5] kprobes: Do not expose probe addresses to non-CAP

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Date:   Fri, 3 Jul 2020 08:50:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dominik Czarnota <dominik.czarnota@...ilofbits.com>,
        stable <stable@...r.kernel.org>, Jessica Yu <jeyu@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        KP Singh <kpsingh@...omium.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Will Deacon <will@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matteo Croce <mcroce@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] kprobes: Do not expose probe addresses to
 non-CAP_SYSLOG

On Thu, Jul 02, 2020 at 06:00:17PM -0700, Linus Torvalds wrote:
> If somebody is interested in looking into things like that, it might
> be a good idea to have kernel threads with that counter incremented by
> default.

With 67 kthreads on a booted system, this patch does not immediately
blow up... And it likely needs some beautification. (Note that
current_cred_*() calls current_cred() under the hood, so AFAICT, only
current_cred() needs coverage.)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 18639c069263..a624847cb0ce 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -295,7 +295,10 @@ static inline void put_cred(const struct cred *_cred)
  * since nobody else can modify it.
  */
 #define current_cred() \
-	rcu_dereference_protected(current->cred, 1)
+({							\
+	WARN_ON_ONCE(current->warn_on_current_cred);	\
+	rcu_dereference_protected(current->cred, 1);	\
+})
 
 /**
  * current_real_cred - Access the current task's objective credentials
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b62e6aaf28f0..21ab1b81aa40 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -652,6 +652,7 @@ struct task_struct {
 	/* Per task flags (PF_*), defined further below: */
 	unsigned int			flags;
 	unsigned int			ptrace;
+	unsigned int			warn_on_current_cred;
 
 #ifdef CONFIG_SMP
 	struct llist_node		wake_entry;
diff --git a/kernel/fork.c b/kernel/fork.c
index 142b23645d82..2e181b9bfd3f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2527,8 +2527,12 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 		.stack		= (unsigned long)fn,
 		.stack_size	= (unsigned long)arg,
 	};
+	pid_t pid;
 
-	return _do_fork(&args);
+	pid = _do_fork(&args);
+	if (pid == 0)
+		current->warn_on_current_cred = 1;
+	return pid;
 }
 
 #ifdef __ARCH_WANT_SYS_FORK


-- 
Kees Cook