linux-kernel - Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

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Date:   Tue, 2 Aug 2022 18:46:29 +0000
From:   Nadav Amit <namit@...are.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
CC:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Peter Xu <peterx@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        zhangyi <yi.zhang@...wei.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained
 access control

On Jul 19, 2022, at 12:56 PM, Axel Rasmussen <axelrasmussen@...gle.com> wrote:

> 
> +static int new_userfaultfd(bool is_syscall, int flags)
> {
> 	struct userfaultfd_ctx *ctx;
> 	int fd;
> 
> -	if (!sysctl_unprivileged_userfaultfd &&
> -	    (flags & UFFD_USER_MODE_ONLY) == 0 &&
> -	    !capable(CAP_SYS_PTRACE)) {
> -		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");
> +	if (is_syscall && !userfaultfd_syscall_allowed(flags))
> 		return -EPERM;
> -	}
> 
> 	BUG_ON(!current->mm);
> 
> @@ -2098,8 +2105,42 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
> 	return fd;
> }
> 
> +SYSCALL_DEFINE1(userfaultfd, int, flags)
> +{
> +	return new_userfaultfd(true, flags);
> +}

Not critical, but why not to put the userfaultfd_syscall_allowed() check
here? You would be able to lose the “is_syscall”.

I also had a small comment for patch 5.

But these are minor issues, so for the series:

Acked-by: Nadav Amit <namit@...are.com>


[ Sorry again for misunderstanding the scheme you were using is similar to
KVM and therefore reasonable. ]