[PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Tycho Andersen]

As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
because they both return positive values, one in the case of success and
one in the case of error. So, let's disallow both of these flags together.

While this is technically a userspace break, all the users I know of are
still waiting on me to land this feature in libseccomp, so I think it'll be
safe. Also, at present my use case doesn't require TSYNC at all, so this
isn't a big deal to disallow. If someone wanted to support this, a path
forward would be to add a new flag like
TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
the use cases are so different I don't see it really happening.

Finally, it's worth noting that this does actually fix a UAF issue: at the end
of seccomp_set_mode_filter(), we have:

        if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
                if (ret < 0) {
                        listener_f->private_data = NULL;
                        fput(listener_f);
                        put_unused_fd(listener);
                } else {
                        fd_install(listener, listener_f);
                        ret = listener;
                }
        }
out_free:
        seccomp_filter_free(prepared);

But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
the filter out from underneath it, causing a UAF when the task closes it or
dies. This patch also switches the condition to be simply if (ret), so that
if someone does add the flag mentioned above, they won't have to remember
to fix this too.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
CC: stable@vger.kernel.org # v5.0+
---
 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.
+	 */
+	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

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Christian Brauner]

On Wed, Mar 06, 2019 at 01:14:13PM -0700, Tycho Andersen wrote:
> As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> because they both return positive values, one in the case of success and
> one in the case of error. So, let's disallow both of these flags together.
> 
> While this is technically a userspace break, all the users I know of are
> still waiting on me to land this feature in libseccomp, so I think it'll be
> safe. Also, at present my use case doesn't require TSYNC at all, so this
> isn't a big deal to disallow. If someone wanted to support this, a path
> forward would be to add a new flag like
> TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> the use cases are so different I don't see it really happening.
> 
> Finally, it's worth noting that this does actually fix a UAF issue: at the end
> of seccomp_set_mode_filter(), we have:
> 
>         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
>                 if (ret < 0) {
>                         listener_f->private_data = NULL;
>                         fput(listener_f);
>                         put_unused_fd(listener);
>                 } else {
>                         fd_install(listener, listener_f);
>                         ret = listener;
>                 }
>         }
> out_free:
>         seccomp_filter_free(prepared);
> 
> But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> the filter out from underneath it, causing a UAF when the task closes it or
> dies. This patch also switches the condition to be simply if (ret), so that
> if someone does add the flag mentioned above, they won't have to remember
> to fix this too.
> 
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> CC: stable@vger.kernel.org # v5.0+
> ---
>  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.
> +	 */
> +	if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
> +	    (flags && SECCOMP_FILTER_FLAG_NEW_LISTENER))
> +		return -EINVAL;

May license a manpage entry that this makes it potentially unsafe to use
with multiple threads. But I don't see a use-case for this right now so
it looks sane to me. :)

(Though one simple question below.)

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

> +
>  	/* 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) {

Why that change but keep checking if (ret < 0) further up?

>  			listener_f->private_data = NULL;
>  			fput(listener_f);
>  			put_unused_fd(listener);
> -- 
> 2.19.1
> 

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Tycho Andersen]

On Wed, Mar 06, 2019 at 09:39:35PM +0100, Christian Brauner wrote:
> > +
> >  	/* 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) {
> 
> Why that change but keep checking if (ret < 0) further up?

Not sure what you mean here. The only other place I see that we check
something is < 0 in that function is the return value of
get_unused_fd_flags(), which looks right to me?

Tycho

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Christian Brauner]

On Wed, Mar 6, 2019 at 9:46 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> On Wed, Mar 06, 2019 at 09:39:35PM +0100, Christian Brauner wrote:
> > > +
> > >     /* 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) {
> >
> > Why that change but keep checking if (ret < 0) further up?
>
> Not sure what you mean here. The only other place I see that we check
> something is < 0 in that function is the return value of
> get_unused_fd_flags(), which looks right to me?

The change just seemed it had nothing to do with the rest of the patch.
Just making sure this didn't happen on accident and would cause regressions.

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Tycho Andersen]

On Wed, Mar 06, 2019 at 10:02:25PM +0100, Christian Brauner wrote:
> On Wed, Mar 6, 2019 at 9:46 PM Tycho Andersen <tycho@tycho.ws> wrote:
> >
> > On Wed, Mar 06, 2019 at 09:39:35PM +0100, Christian Brauner wrote:
> > > > +
> > > >     /* 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) {
> > >
> > > Why that change but keep checking if (ret < 0) further up?
> >
> > Not sure what you mean here. The only other place I see that we check
> > something is < 0 in that function is the return value of
> > get_unused_fd_flags(), which looks right to me?
> 
> The change just seemed it had nothing to do with the rest of the patch.
> Just making sure this didn't happen on accident and would cause regressions.

No, not on accident :). See the second half of the patch notes.

I can split it out into two separate patches if that makes more sense.
In fact this hunk alone fixes the UAF, but you still get non-sensical
return results even if it doesn't do anything terrible, hence the
first hunk.

Cheers,

Tycho

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Kees Cook]

On Wed, Mar 6, 2019 at 12:14 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> because they both return positive values, one in the case of success and
> one in the case of error. So, let's disallow both of these flags together.
>
> While this is technically a userspace break, all the users I know of are
> still waiting on me to land this feature in libseccomp, so I think it'll be
> safe. Also, at present my use case doesn't require TSYNC at all, so this
> isn't a big deal to disallow. If someone wanted to support this, a path
> forward would be to add a new flag like
> TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> the use cases are so different I don't see it really happening.
>
> Finally, it's worth noting that this does actually fix a UAF issue: at the end
> of seccomp_set_mode_filter(), we have:
>
>         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
>                 if (ret < 0) {
>                         listener_f->private_data = NULL;
>                         fput(listener_f);
>                         put_unused_fd(listener);
>                 } else {
>                         fd_install(listener, listener_f);
>                         ret = listener;
>                 }
>         }
> out_free:
>         seccomp_filter_free(prepared);
>
> But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> the filter out from underneath it, causing a UAF when the task closes it or
> dies. This patch also switches the condition to be simply if (ret), so that
> if someone does add the flag mentioned above, they won't have to remember
> to fix this too.
>
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> CC: stable@vger.kernel.org # v5.0+

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>

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

-- 
Kees Cook

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[James Morris]

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>

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Kees Cook]

On Tue, Apr 23, 2019 at 3:09 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Mar 6, 2019 at 12:14 PM Tycho Andersen <tycho@tycho.ws> wrote:
> >
> > As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> > because they both return positive values, one in the case of success and
> > one in the case of error. So, let's disallow both of these flags together.
> >
> > While this is technically a userspace break, all the users I know of are
> > still waiting on me to land this feature in libseccomp, so I think it'll be
> > safe. Also, at present my use case doesn't require TSYNC at all, so this
> > isn't a big deal to disallow. If someone wanted to support this, a path
> > forward would be to add a new flag like
> > TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> > the use cases are so different I don't see it really happening.
> >
> > Finally, it's worth noting that this does actually fix a UAF issue: at the end
> > of seccomp_set_mode_filter(), we have:
> >
> >         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> >                 if (ret < 0) {
> >                         listener_f->private_data = NULL;
> >                         fput(listener_f);
> >                         put_unused_fd(listener);
> >                 } else {
> >                         fd_install(listener, listener_f);
> >                         ret = listener;
> >                 }
> >         }
> > out_free:
> >         seccomp_filter_free(prepared);
> >
> > But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> > the filter out from underneath it, causing a UAF when the task closes it or
> > dies. This patch also switches the condition to be simply if (ret), so that
> > if someone does add the flag mentioned above, they won't have to remember
> > to fix this too.
> >
> > Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> > Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> > CC: stable@vger.kernel.org # v5.0+
>
> 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>
>
> 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))

also a typo: && should be &

> > +               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
>
> --
> Kees Cook



-- 
Kees Cook

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Tycho Andersen]

On Tue, Apr 23, 2019 at 04:31:45PM -0700, Kees Cook wrote:
> On Tue, Apr 23, 2019 at 3:09 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Wed, Mar 6, 2019 at 12:14 PM Tycho Andersen <tycho@tycho.ws> wrote:
> > >
> > > As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> > > because they both return positive values, one in the case of success and
> > > one in the case of error. So, let's disallow both of these flags together.
> > >
> > > While this is technically a userspace break, all the users I know of are
> > > still waiting on me to land this feature in libseccomp, so I think it'll be
> > > safe. Also, at present my use case doesn't require TSYNC at all, so this
> > > isn't a big deal to disallow. If someone wanted to support this, a path
> > > forward would be to add a new flag like
> > > TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> > > the use cases are so different I don't see it really happening.
> > >
> > > Finally, it's worth noting that this does actually fix a UAF issue: at the end
> > > of seccomp_set_mode_filter(), we have:
> > >
> > >         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> > >                 if (ret < 0) {
> > >                         listener_f->private_data = NULL;
> > >                         fput(listener_f);
> > >                         put_unused_fd(listener);
> > >                 } else {
> > >                         fd_install(listener, listener_f);
> > >                         ret = listener;
> > >                 }
> > >         }
> > > out_free:
> > >         seccomp_filter_free(prepared);
> > >
> > > But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> > > the filter out from underneath it, causing a UAF when the task closes it or
> > > dies. This patch also switches the condition to be simply if (ret), so that
> > > if someone does add the flag mentioned above, they won't have to remember
> > > to fix this too.
> > >
> > > Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> > > Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> > > CC: stable@vger.kernel.org # v5.0+
> >
> > 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>
> >
> > 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))
> 
> also a typo: && should be &

Oh, yes. Do you want me to send another version?

Tycho

Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Kees Cook]

On Tue, Apr 23, 2019 at 4:34 PM Tycho Andersen <tycho@tycho.ws> wrote:
>
> On Tue, Apr 23, 2019 at 04:31:45PM -0700, Kees Cook wrote:
> > On Tue, Apr 23, 2019 at 3:09 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > On Wed, Mar 6, 2019 at 12:14 PM Tycho Andersen <tycho@tycho.ws> wrote:
> > > >
> > > > As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> > > > because they both return positive values, one in the case of success and
> > > > one in the case of error. So, let's disallow both of these flags together.
> > > >
> > > > While this is technically a userspace break, all the users I know of are
> > > > still waiting on me to land this feature in libseccomp, so I think it'll be
> > > > safe. Also, at present my use case doesn't require TSYNC at all, so this
> > > > isn't a big deal to disallow. If someone wanted to support this, a path
> > > > forward would be to add a new flag like
> > > > TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> > > > the use cases are so different I don't see it really happening.
> > > >
> > > > Finally, it's worth noting that this does actually fix a UAF issue: at the end
> > > > of seccomp_set_mode_filter(), we have:
> > > >
> > > >         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> > > >                 if (ret < 0) {
> > > >                         listener_f->private_data = NULL;
> > > >                         fput(listener_f);
> > > >                         put_unused_fd(listener);
> > > >                 } else {
> > > >                         fd_install(listener, listener_f);
> > > >                         ret = listener;
> > > >                 }
> > > >         }
> > > > out_free:
> > > >         seccomp_filter_free(prepared);
> > > >
> > > > But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> > > > the filter out from underneath it, causing a UAF when the task closes it or
> > > > dies. This patch also switches the condition to be simply if (ret), so that
> > > > if someone does add the flag mentioned above, they won't have to remember
> > > > to fix this too.
> > > >
> > > > Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> > > > Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> > > > CC: stable@vger.kernel.org # v5.0+
> > >
> > > 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>
> > >
> > > 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))
> >
> > also a typo: && should be &
>
> Oh, yes. Do you want me to send another version?

Nah, I fixed it up. :)


-- 
Kees Cook