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SubjectRe: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags
On Tue, 23 Apr 2019, Kees Cook wrote:

> Thanks! Sorry I missed this. James, can you take this for Linus's
> fixes for v5.1? (Or should I send a pull request to you?)
>
> Acked-by: Kees Cook <keescook@chromium.org>

These are standalone for v5.1 fixes currently so you can send them
directly to Linus.

>
> Let's also add:
>
> Reported-by: syzbot+b562969adb2e04af3442@syzkaller.appspotmail.com
>
> > ---
> > kernel/seccomp.c | 17 +++++++++++++++--
> > 1 file changed, 15 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > index d0d355ded2f4..79bada51091b 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -500,7 +500,10 @@ seccomp_prepare_user_filter(const char __user *user_filter)
> > *
> > * Caller must be holding current->sighand->siglock lock.
> > *
> > - * Returns 0 on success, -ve on error.
> > + * Returns 0 on success, -ve on error, or
> > + * - in TSYNC mode: the pid of a thread which was either not in the correct
> > + * seccomp mode or did not have an ancestral seccomp filter
> > + * - in NEW_LISTENER mode: the fd of the new listener
> > */
> > static long seccomp_attach_filter(unsigned int flags,
> > struct seccomp_filter *filter)
> > @@ -1256,6 +1259,16 @@ static long seccomp_set_mode_filter(unsigned int flags,
> > if (flags & ~SECCOMP_FILTER_FLAG_MASK)
> > return -EINVAL;
> >
> > + /*
> > + * In the successful case, NEW_LISTENER returns the new listener fd.
> > + * But in the failure case, TSYNC returns the thread that died. If you
> > + * combine these two flags, there's no way to tell whether something
> > + * succeded or failed. So, let's disallow this combination.
>
> also a tiny typo: succeeded
>
> > + */
> > + if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
> > + (flags && SECCOMP_FILTER_FLAG_NEW_LISTENER))
> > + return -EINVAL;
> > +
> > /* Prepare the new filter before holding any locks. */
> > prepared = seccomp_prepare_user_filter(filter);
> > if (IS_ERR(prepared))
> > @@ -1302,7 +1315,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
> > mutex_unlock(&current->signal->cred_guard_mutex);
> > out_put_fd:
> > if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> > - if (ret < 0) {
> > + if (ret) {
> > listener_f->private_data = NULL;
> > fput(listener_f);
> > put_unused_fd(listener);
> > --
> > 2.19.1
> >
>
> -Kees
>
>

--
James Morris
<jmorris@namei.org>

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