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

commit 66a733ea6b611aecf0119514d2dddab5f9d6c01e upstream.

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seccomp.filter to have been replaced by SECCOMP_FILTER_FLAG_TSYNC.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>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 15a1795..efd384f 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -457,14 +457,19 @@ static long seccomp_attach_filter(unsigned int flags,
        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)
@@ -475,10 +480,8 @@ 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 && atomic_dec_and_test(&orig->usage)) {
                struct seccomp_filter *freeme = orig;
@@ -487,6 +490,12 @@ 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);
+}
+
 /**
  * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
  * @syscall: syscall number to send to userland
@@ -927,13 +936,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
        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: