seccomp: introduce writer locking

Normally, task_struct.seccomp.filter is only ever read or modified by
the task that owns it (current). This property aids in fast access
during system call filtering as read access is lockless.

Updating the pointer from another task, however, opens up race
conditions. To allow cross-thread filter pointer updates, writes to the
seccomp fields are now protected by the sighand spinlock (which is shared
by all threads in the thread group). Read access remains lockless because
pointer updates themselves are atomic.  However, writes (or cloning)
often entail additional checking (like maximum instruction counts)
which require locking to perform safely.

In the case of cloning threads, the child is invisible to the system
until it enters the task list. To make sure a child can't be cloned from
a thread and left in a prior state, seccomp duplication is additionally
moved under the sighand lock. Then parent and child are certain have
the same seccomp state when they exit the lock.

Based on patches by Will Drewry and David Drysdale.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>

1 files changed, 15 insertions, 1 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 58125160417c..d5543e787e4e 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -199,6 +199,8 @@ static u32 seccomp_run_filters(int syscall)
 
 static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
 {
+        BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
         if (current->seccomp.mode && current->seccomp.mode != seccomp_mode)
                 return false;
 
@@ -207,6 +209,8 @@ static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
 
 static inline void seccomp_assign_mode(unsigned long seccomp_mode)
 {
+        BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
         current->seccomp.mode = seccomp_mode;
         set_tsk_thread_flag(current, TIF_SECCOMP);
 }
@@ -332,6 +336,8 @@ out:
  * @flags:  flags to change filter behavior
  * @filter: seccomp filter to add to the current process
  *
+ * Caller must be holding current->sighand->siglock lock.
+ *
  * Returns 0 on success, -ve on error.
  */
 static long seccomp_attach_filter(unsigned int flags,
@@ -340,6 +346,8 @@ static long seccomp_attach_filter(unsigned int flags,
         unsigned long total_insns;
         struct seccomp_filter *walker;
 
+        BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
         /* Validate resulting filter length. */
         total_insns = filter->prog->len;
         for (walker = current->seccomp.filter; walker; walker = walker->prev)
@@ -529,6 +537,8 @@ static long seccomp_set_mode_strict(void)
         const unsigned long seccomp_mode = SECCOMP_MODE_STRICT;
         long ret = -EINVAL;
 
+        spin_lock_irq(&current->sighand->siglock);
+
         if (!seccomp_may_assign_mode(seccomp_mode))
                 goto out;
 
@@ -539,6 +549,7 @@ static long seccomp_set_mode_strict(void)
         ret = 0;
 
 out:
+        spin_unlock_irq(&current->sighand->siglock);
 
         return ret;
 }
@@ -566,13 +577,15 @@ static long seccomp_set_mode_filter(unsigned int flags,
 
         /* Validate flags. */
         if (flags != 0)
-                goto out;
+                return -EINVAL;
 
         /* Prepare the new filter before holding any locks. */
         prepared = seccomp_prepare_user_filter(filter);
         if (IS_ERR(prepared))
                 return PTR_ERR(prepared);
 
+        spin_lock_irq(&current->sighand->siglock);
+
         if (!seccomp_may_assign_mode(seccomp_mode))
                 goto out;
 
@@ -584,6 +597,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
 
         seccomp_assign_mode(seccomp_mode);
 out:
+        spin_unlock_irq(&current->sighand->siglock);
         seccomp_filter_free(prepared);
         return ret;
 }