linux-kernel - Re: introduce get_nth

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Message-ID: <20170920161425.GA29312@redhat.com>
Date:   Wed, 20 Sep 2017 18:14:25 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     Chris Salls <chrissalls5@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, security@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: introduce get_nth_filter()

On 09/20, Oleg Nesterov wrote:
>
> On 09/20, Tycho Andersen wrote:
> >
> > Thanks for cleaning this up, I'll be happy to test whatever final
> > patch we come up with.
>
> Well, I just noticed you sent another "[PATCH] ptrace, seccomp: add support
> for retrieving seccomp flags" today...
>
> So if we need get_nth() helper please consider the UNTESTED change below
> (on top of this fix). If you agree with this code, feel free to incorporate
> it into your patch.

and probably we should shift the CAP_SYS_ADMIN/SECCOMP_MODE_DISABLED into
get_nth() too, see v2 below.

Perhaps it makes sense to add a comment to explain that spin_lock_irq(siglock)
is only correct because the caller is the tracer, and thus the TASK_TRACED
"task" can't exit. Otherwise we would need lock_task_sighand().

Oleg.


--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -855,48 +855,53 @@ 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;
+	struct seccomp_filter *orig, *filter;
+	unsigned long count;
 
 	if (!capable(CAP_SYS_ADMIN) ||
 	    current->seccomp.mode != SECCOMP_MODE_DISABLED) {
-		return -EACCES;
+		return ERR_PTR(-EACCES);
 	}
 
+	if (task->seccomp.mode != SECCOMP_MODE_FILTER)
+		return ERR_PTR(-EINVAL);
+
 	spin_lock_irq(&task->sighand->siglock);
-	if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
-		ret = -EINVAL;
-		goto out;
-	}
+	get_seccomp_filter(task);
+	orig = task->seccomp.filter;
+	spin_unlock_irq(&task->sighand->siglock);
 
-	filter = task->seccomp.filter;
-	while (filter) {
-		filter = filter->prev;
+	count = 0;
+	for (filter = orig; filter; filter = filter->prev)
 		count++;
-	}
 
-	if (filter_off >= count) {
-		ret = -ENOENT;
+	filter = ERR_PTR(-ENOENT);
+	if (filter_off >= count)
 		goto out;
-	}
-	count -= filter_off;
 
-	filter = task->seccomp.filter;
-	while (filter && count > 1) {
-		filter = filter->prev;
+	count -= filter_off;
+	for (filter = orig; 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;
-		goto out;
-	}
+	refcount_inc(&filter->usage);
+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;
+
+	filter = get_nth_filter(task, filter_off);
+	if (IS_ERR(filter))
+		return PTR_ERR(filter);
 
 	fprog = filter->prog->orig_prog;
 	if (!fprog) {
@@ -912,17 +917,10 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
 	if (!data)
 		goto out;
 
-	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(filter);
-	return ret;
-
 out:
-	spin_unlock_irq(&task->sighand->siglock);
+	__put_seccomp_filter(filter);
 	return ret;
 }
 #endif