Hoist out the nth filter resolving logic that ptrace uses into a new
function. We'll use this in the next patch to implement the new
PTRACE_SECCOMP_GET_FILTER_FLAGS command. This is based on an older patch
that I had sent a while ago; it significantly revamps the get_nth_filter
logic based on previous suggestions from Oleg.

Signed-off-by: Tycho Andersen <ty...@tycho.ws>
CC: Kees Cook <keesc...@chromium.org>
CC: Andy Lutomirski <l...@amacapital.net>
CC: Oleg Nesterov <o...@redhat.com>
CC: Eric W. Biederman <ebied...@xmission.com>
CC: "Serge E. Hallyn" <se...@hallyn.com>
CC: Christian Brauner <christian.brau...@ubuntu.com>
CC: Tyler Hicks <tyhi...@canonical.com>
CC: Akihiro Suda <suda.akih...@lab.ntt.co.jp>
---
 kernel/seccomp.c | 77 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 32 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 9541eb379e74..800db3f2866f 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -1179,49 +1179,68 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char 
__user *filter)
 }
 
 #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
-long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
-                       void __user *data)
+static struct seccomp_filter *get_nth_filter(struct task_struct *task,
+                                            unsigned long filter_off)
 {
-       struct seccomp_filter *filter;
-       struct sock_fprog_kern *fprog;
-       long ret;
-       unsigned long count = 0;
-
-       if (!capable(CAP_SYS_ADMIN) ||
-           current->seccomp.mode != SECCOMP_MODE_DISABLED) {
-               return -EACCES;
-       }
+       struct seccomp_filter *orig, *filter;
+       unsigned long count;
 
+       /*
+        * Note: this is only correct because the caller should be the (ptrace)
+        * tracer of the task, otherwise lock_task_sighand is needed.
+        */
        spin_lock_irq(&task->sighand->siglock);
+
        if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
-               ret = -EINVAL;
-               goto out;
+               spin_unlock_irq(&task->sighand->siglock);
+               return ERR_PTR(-EINVAL);
        }
 
-       filter = task->seccomp.filter;
-       while (filter) {
-               filter = filter->prev;
+       orig = task->seccomp.filter;
+       __get_seccomp_filter(orig);
+       spin_unlock_irq(&task->sighand->siglock);
+
+       count = 0;
+       for (filter = orig; filter; filter = filter->prev)
                count++;
-       }
 
        if (filter_off >= count) {
-               ret = -ENOENT;
+               filter = ERR_PTR(-ENOENT);
                goto out;
        }
-       count -= filter_off;
 
-       filter = task->seccomp.filter;
-       while (filter && count > 1) {
-               filter = filter->prev;
+       count -= filter_off;
+       for (filter = orig; filter && count > 1; filter = filter->prev)
                count--;
-       }
 
        if (WARN_ON(count != 1 || !filter)) {
-               /* The filter tree shouldn't shrink while we're using it. */
-               ret = -ENOENT;
+               filter = ERR_PTR(-ENOENT);
                goto out;
        }
 
+       __get_seccomp_filter(filter);
+
+out:
+       __put_seccomp_filter(orig);
+       return filter;
+}
+
+long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
+                       void __user *data)
+{
+       struct seccomp_filter *filter;
+       struct sock_fprog_kern *fprog;
+       long ret;
+
+       if (!capable(CAP_SYS_ADMIN) ||
+           current->seccomp.mode != SECCOMP_MODE_DISABLED) {
+               return -EACCES;
+       }
+
+       filter = get_nth_filter(task, filter_off);
+       if (IS_ERR(filter))
+               return PTR_ERR(filter);
+
        fprog = filter->prog->orig_prog;
        if (!fprog) {
                /* This must be a new non-cBPF filter, since we save
@@ -1236,17 +1255,11 @@ long seccomp_get_filter(struct task_struct *task, 
unsigned long filter_off,
        if (!data)
                goto out;
 
-       __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(filter);
-       return ret;
-
 out:
-       spin_unlock_irq(&task->sighand->siglock);
+       __put_seccomp_filter(filter);
        return ret;
 }
 #endif
-- 
2.14.1

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