Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

Tycho Andersen
2018-10-30T15:54:11Z

On Tue, Oct 30, 2018 at 04:02:54PM +0100, Oleg Nesterov wrote:
> On 10/29, Tycho Andersen wrote:
> >
> > +static long seccomp_notify_recv(struct seccomp_filter *filter,
> > + void __user *buf)
> > +{
> > + struct seccomp_knotif *knotif = NULL, *cur;
> > + struct seccomp_notif unotif;
> > + ssize_t ret;
> > +
> > + memset(&unotif, 0, sizeof(unotif));
> > +
> > + ret = down_interruptible(&filter->notif->request);
> > + if (ret < 0)
> > + return ret;
> > +
> > + mutex_lock(&filter->notify_lock);
> > + list_for_each_entry(cur, &filter->notif->notifications, list) {
> > + if (cur->state == SECCOMP_NOTIFY_INIT) {
> > + knotif = cur;
> > + break;
> > + }
> > + }
> > +
> > + /*
> > + * If we didn't find a notification, it could be that the task was
> > + * interrupted by a fatal signal between the time we were woken and
> > + * when we were able to acquire the rw lock.
> > + *
> > + * This is the place where we handle the extra high semaphore count
> > + * mentioned in seccomp_do_user_notification().
> > + */
> > + if (!knotif) {
> > + ret = -ENOENT;
> > + goto out;
> > + }
> > +
> > + unotif.id = knotif->id;
> > + unotif.pid = task_pid_vnr(knotif->task);
> > + if (knotif->signaled)
> > + unotif.flags |= SECCOMP_NOTIF_FLAG_SIGNALED;
> > + unotif.data = *(knotif->data);
>
> Tycho, I forgot everything about seccomp, most probably I am wrong but let me
> ask anyway.
>
> __seccomp_filter(SECCOMP_RET_TRACE) does
>
> /*
> * Recheck the syscall, since it may have changed. This
> * intentionally uses a NULL struct seccomp_data to force
> * a reload of all registers. This does not goto skip since
> * a skip would have already been reported.
> */
> if (__seccomp_filter(this_syscall, NULL, true))
> return -1;
>
> and the next seccomp_run_filters() can return SECCOMP_RET_USER_NOTIF, right?
> seccomp_do_user_notification() doesn't check recheck_after_trace and it simply
> does n.data = sd.
>
> Doesn't this mean that "unotif.data = *(knotif->data)" can hit NULL ?
>
> seccomp_run_filters() does populate_seccomp_data() in this case, but this
> won't affect "seccomp_data *sd" passed to seccomp_do_user_notification().

Oof, yes, you're right. Seems like there are no other users of sd in
__seccomp_filter(). Seems to me like we can just do the
populate_seccomp_data() one level higher in __seccomp_filter()?

Tycho

From 9e0f75ea51a2c328567910df3122a236ebeccab0 Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tycho@tycho.ws>
Date: Tue, 30 Oct 2018 09:51:14 -0600
Subject: [PATCH] seccomp: hoist struct seccomp_data recalculation higher

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
---
kernel/seccomp.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 4c5fb6ced4cd..1525cb753ad2 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -257,7 +257,6 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
static u32 seccomp_run_filters(const struct seccomp_data *sd,
struct seccomp_filter **match)
{
- struct seccomp_data sd_local;
u32 ret = SECCOMP_RET_ALLOW;
/* Make sure cross-thread synced filter points somewhere sane. */
struct seccomp_filter *f =
@@ -267,11 +266,6 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
if (unlikely(WARN_ON(f == NULL)))
return SECCOMP_RET_KILL_PROCESS;

- if (!sd) {
- populate_seccomp_data(&sd_local);
- sd = &sd_local;
- }
-
/*
* All filters in the list are evaluated and the lowest BPF return
* value always takes priority (ignoring the DATA).
@@ -821,6 +815,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
u32 filter_ret, action;
struct seccomp_filter *match = NULL;
int data;
+ struct seccomp_data sd_local;

/*
* Make sure that any changes to mode from another thread have
@@ -828,6 +823,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
*/
rmb();

+ if (!sd) {
+ populate_seccomp_data(&sd_local);
+ sd = &sd_local;
+ }
+
filter_ret = seccomp_run_filters(sd, &match);
data = filter_ret & SECCOMP_RET_DATA;
action = filter_ret & SECCOMP_RET_ACTION_FULL;
--
2.17.1