linux-kernel - Re: [PATCH] seccomp: Check flags on seccomp

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Message-ID: <20191226115245.usf7z5dkui7ndp4w@wittgenstein>
Date:   Thu, 26 Dec 2019 12:52:45 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        tycho@...ho.ws, jannh@...gle.com, keescook@...omium.org
Subject: Re: [PATCH] seccomp: Check flags on seccomp_notif is unset

On Wed, Dec 25, 2019 at 09:45:33PM +0000, Sargun Dhillon wrote:
> This patch is a small change in enforcement of the uapi for
> SECCOMP_IOCTL_NOTIF_RECV ioctl. Specificaly, the datastructure which is
> passed (seccomp_notif), has a flags member. Previously that could be
> set to a nonsense value, and we would ignore it. This ensures that
> no flags are set.
> 
> Signed-off-by: Sargun Dhillon <sargun@...gun.me>
> Cc: Kees Cook <keescook@...omium.org>

I'm fine with this since we soon want to make use of the flag argument
when we add a flag to get a pidfd from the seccomp notifier on receive.
The major users I could identify already pass in seccomp_notif with all
fields set to 0. If we really break users we can always revert; this
seems very unlikely to me though.

One more question below, otherwise:

Reviewed-by: Christian Brauner <christian.brauner@...ntu.com>

> ---
>  kernel/seccomp.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 12d2227e5786..455925557490 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1026,6 +1026,13 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
>  	struct seccomp_notif unotif;
>  	ssize_t ret;
>  
> +	if (copy_from_user(&unotif, buf, sizeof(unotif)))
> +		return -EFAULT;
> +
> +	/* flags is reserved right now, make sure it's unset */
> +	if (unotif.flags)
> +		return -EINVAL;
> +

Might it make sense to use

	err = copy_struct_from_user(&unotif, sizeof(unotif), buf, sizeof(unotif));
	if (err)
		return err;

This way we check that the whole struct is 0 and report an error as soon
as one of the members is non-zero. That's more drastic but it'd ensure
that other fields can be used in the future for whatever purposes.
It would also let us get rid of the memset() below. 

>  	memset(&unotif, 0, sizeof(unotif));
>  
>  	ret = down_interruptible(&filter->notif->request);
> -- 
> 2.20.1
>