From: Andrew Morton <akpm@linux-foundation.org>
To: Lokesh Gidra <lokeshgidra@google.com>
Cc: Kees Cook <keescook@chromium.org>,
Jonathan Corbet <corbet@lwn.net>, Peter Xu <peterx@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Stephen Smalley <stephen.smalley.work@gmail.com>,
Eric Biggers <ebiggers@kernel.org>,
Daniel Colascione <dancol@dancol.org>,
"Joel Fernandes (Google)" <joel@joelfernandes.org>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-doc@vger.kernel.org, kaleshsingh@google.com,
calin@google.com, surenb@google.com, jeffv@google.com,
kernel-team@android.com, Mike Rapoport <rppt@linux.vnet.ibm.com>,
Shaohua Li <shli@fb.com>, Jerome Glisse <jglisse@redhat.com>,
Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@techsingularity.net>,
Nitin Gupta <nigupta@nvidia.com>,
Vlastimil Babka <vbabka@suse.cz>,
Iurii Zaikin <yzaikin@google.com>,
Luis Chamberlain <mcgrof@kernel.org>,
linux-mm@kvack.kernel.org, Daniel Colascione <dancol@google.com>
Subject: Re: [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY
Date: Fri, 20 Nov 2020 15:33:37 -0800 [thread overview]
Message-ID: <20201120153337.431dc36c1975507bb1e44596@linux-foundation.org> (raw)
In-Reply-To: <20201120030411.2690816-2-lokeshgidra@google.com>
On Thu, 19 Nov 2020 19:04:10 -0800 Lokesh Gidra <lokeshgidra@google.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.
next prev parent reply other threads:[~2020-11-20 23:34 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-20 3:04 [PATCH v6 0/2] Control over userfaultfd kernel-fault handling Lokesh Gidra
2020-11-20 3:04 ` [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY Lokesh Gidra
2020-11-20 3:09 ` Lokesh Gidra
2020-11-20 23:33 ` Andrew Morton [this message]
2020-11-23 19:17 ` Lokesh Gidra
2020-11-23 20:11 ` Andrew Morton
2020-11-20 3:04 ` [PATCH v6 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob Lokesh Gidra
2020-11-20 3:10 ` Lokesh Gidra
2020-11-20 3:08 ` [PATCH v6 0/2] Control over userfaultfd kernel-fault handling Lokesh Gidra
-- strict thread matches above, loose matches on Subject: below --
2020-10-26 21:00 Lokesh Gidra
2020-10-26 21:00 ` [PATCH v6 1/2] Add UFFD_USER_MODE_ONLY Lokesh Gidra
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