linux-kernel - Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT

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Message-ID: <20191105155953.3i7fvzm76aublkbi@yavin.dot.cyphar.com>
Date:   Wed, 6 Nov 2019 02:59:53 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Tim Murray <timmurray@...gle.com>, linux-api@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for
 UFFD_FEATURE_EVENT_FORK

On 2019-11-05, Mike Rapoport <rppt@...ux.ibm.com> wrote:
> Current implementation of UFFD_FEATURE_EVENT_FORK modifies the file
> descriptor table from the read() implementation of uffd, which may have
> security implications for unprivileged use of the userfaultfd.
> 
> Limit availability of UFFD_FEATURE_EVENT_FORK only for callers that have
> CAP_SYS_PTRACE.
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>  fs/userfaultfd.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index f9fd18670e22..d99d166fd892 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1834,13 +1834,12 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
>  	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
>  		goto out;
>  	features = uffdio_api.features;
> -	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
> -		memset(&uffdio_api, 0, sizeof(uffdio_api));
> -		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
> -			goto out;
> -		ret = -EINVAL;
> -		goto out;
> -	}
> +	ret = -EINVAL;
> +	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES))
> +		goto err_out;
> +	ret = -EPERM;
> +	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
> +		goto err_out;
>  	/* report all available features and ioctls to userland */
>  	uffdio_api.features = UFFD_API_FEATURES;
>  	uffdio_api.ioctls = UFFD_API_IOCTLS;
> @@ -1853,6 +1852,11 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
>  	ret = 0;
>  out:
>  	return ret;
> +err_out:
> +	memset(&uffdio_api, 0, sizeof(uffdio_api));
> +	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))

Wouldn't it be simpler to do clear_user()?

> +		ret = -EFAULT;
> +	goto out;
>  }
>  
>  static long userfaultfd_ioctl(struct file *file, unsigned cmd,

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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