From: Oleg Nesterov <oleg@redhat.com>
Date: Wed, 27 Sep 2017 09:25:30 -0600
Subject: [PATCH] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()
Patch-mainline: v4.14-rc3
Git-commit: 66a733ea6b611aecf0119514d2dddab5f9d6c01e
References: bsc#1051510

As Chris explains, get_seccomp_filter() and put_seccomp_filter() can end
up using different filters. Once we drop ->siglock it is possible for
task->seccomp.filter to have been replaced by SECCOMP_FILTER_FLAG_TSYNC.

Fixes: f8e529ed941b ("seccomp, ptrace: add support for dumping seccomp filters")
Reported-by: Chris Salls <chrissalls5@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[tycho: add __get_seccomp_filter vs. open coding refcount_inc()]
Signed-off-by: Tycho Andersen <tycho@docker.com>
[kees: tweak commit log]
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Takashi Iwai <tiwai@suse.de>
---
 kernel/seccomp.c |   23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -458,14 +458,19 @@ static long seccomp_attach_filter(unsign
 	return 0;
 }
 
+void __get_seccomp_filter(struct seccomp_filter *filter)
+{
+	/* Reference count is bounded by the number of total processes. */
+	atomic_inc(&filter->usage);
+}
+
 /* get_seccomp_filter - increments the reference count of the filter on @tsk */
 void get_seccomp_filter(struct task_struct *tsk)
 {
 	struct seccomp_filter *orig = tsk->seccomp.filter;
 	if (!orig)
 		return;
-	/* Reference count is bounded by the number of total processes. */
-	atomic_inc(&orig->usage);
+	__get_seccomp_filter(orig);
 }
 
 static inline void seccomp_filter_free(struct seccomp_filter *filter)
@@ -476,10 +481,8 @@ static inline void seccomp_filter_free(s
 	}
 }
 
-/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
-void put_seccomp_filter(struct task_struct *tsk)
+static void __put_seccomp_filter(struct seccomp_filter *orig)
 {
-	struct seccomp_filter *orig = tsk->seccomp.filter;
 	/* Clean up single-reference branches iteratively. */
 	while (orig && atomic_dec_and_test(&orig->usage)) {
 		struct seccomp_filter *freeme = orig;
@@ -499,6 +502,12 @@ static void seccomp_init_siginfo(siginfo
 	info->si_syscall = syscall;
 }
 
+/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
+void put_seccomp_filter(struct task_struct *tsk)
+{
+	__put_seccomp_filter(tsk->seccomp.filter);
+}
+
 /**
  * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
  * @syscall: syscall number to send to userland
@@ -908,13 +917,13 @@ long seccomp_get_filter(struct task_stru
 	if (!data)
 		goto out;
 
-	get_seccomp_filter(task);
+	__get_seccomp_filter(filter);
 	spin_unlock_irq(&task->sighand->siglock);
 
 	if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
 		ret = -EFAULT;
 
-	put_seccomp_filter(task);
+	__put_seccomp_filter(filter);
 	return ret;
 
 out: