[PATCH] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()

[Oleg Nesterov <oleg@redhat.com>]

As Chris explains, get_seccomp_filter() and put_seccomp_filter() can
use the different filters, once we drop ->siglock task->seccomp.filter
can be replaced by SECCOMP_FILTER_FLAG_TSYNC.

Fixes: f8e529ed941b ("seccomp, ptrace: add support for dumping seccomp filters")
Reported-by: Chris Salls <chrissalls5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/seccomp.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 98b59b5..897f153 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -476,10 +476,8 @@ static inline void seccomp_filter_free(struct seccomp_filter *filter)
 	}
 }
 
-/* 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 && refcount_dec_and_test(&orig->usage)) {
 		struct seccomp_filter *freeme = orig;
@@ -488,6 +486,12 @@ void put_seccomp_filter(struct task_struct *tsk)
 	}
 }
 
+/* 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);
+}
+
 static void seccomp_init_siginfo(siginfo_t *info, int syscall, int reason)
 {
 	memset(info, 0, sizeof(*info));
@@ -908,13 +912,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
 	if (!data)
 		goto out;
 
-	get_seccomp_filter(task);
+	refcount_inc(&filter->usage);
 	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:
-- 
2.5.0