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Date:   Fri, 20 Nov 2020 15:33:37 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Lokesh Gidra <lokeshgidra@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Daniel Colascione <dancol@...col.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kaleshsingh@...gle.com,
        calin@...gle.com, surenb@...gle.com, jeffv@...gle.com,
        kernel-team@...roid.com, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Shaohua Li <shli@...com>, Jerome Glisse <jglisse@...hat.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Nitin Gupta <nigupta@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-mm@...ck.kernel.org, Daniel Colascione <dancol@...gle.com>
Subject: Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY

On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@...gle.com> wrote:

> userfaultfd handles page faults from both user and kernel code.
> Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes
> the resulting userfaultfd object refuse to handle faults from kernel
> mode, treating these faults as if SIGBUS were always raised, causing
> the kernel code to fail with EFAULT.
> 
> A future patch adds a knob allowing administrators to give some
> processes the ability to create userfaultfd file objects only if they
> pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes
> will exploit userfaultfd's ability to delay kernel page faults to open
> timing windows for future exploits.

Can we assume that an update to the userfaultfd(2) manpage is in the
works?

> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -405,6 +405,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  
>  	if (ctx->features & UFFD_FEATURE_SIGBUS)
>  		goto out;
> +	if ((vmf->flags & FAULT_FLAG_USER) == 0 &&
> +	    ctx->flags & UFFD_USER_MODE_ONLY) {
> +		printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
> +			"sysctl knob to 1 if kernel faults must be handled "
> +			"without obtaining CAP_SYS_PTRACE capability\n");
> +		goto out;
> +	}
>  
>  	/*
>  	 * If it's already released don't get it. This avoids to loop
> @@ -1965,10 +1972,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>  	BUG_ON(!current->mm);
>  
>  	/* Check the UFFD_* constants for consistency.  */
> +	BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);

Are we sure this is true for all architectures?

>  	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
>  	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
>  
> -	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
> +	if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
>  		return -EINVAL;
>  
>  	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index e7e98bde221f..5f2d88212f7c 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -257,4 +257,13 @@ struct uffdio_writeprotect {
>  	__u64 mode;
>  };
>  
> +/*
> + * Flags for the userfaultfd(2) system call itself.
> + */
> +
> +/*
> + * Create a userfaultfd that can handle page faults only in user mode.
> + */
> +#define UFFD_USER_MODE_ONLY 1
> +
>  #endif /* _LINUX_USERFAULTFD_H */

It would be nice to define this in include/linux/userfaultfd_k.h,
alongside the other flags.  But I guess it has to be here because it's
part of the userspace API.

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