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From:   Jeff Moyer <jmoyer@redhat.com>
To:     Matteo Rizzo <matteorizzo@google.com>
Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        io-uring@vger.kernel.org, jordyzomer@google.com, evn@google.com,
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        asml.silence@gmail.com, akpm@linux-foundation.org,
        keescook@chromium.org, rostedt@goodmis.org,
        dave.hansen@linux.intel.com, ribalda@chromium.org,
        chenhuacai@kernel.org, steve@sk2.org, gpiccoli@igalia.com,
        ldufour@linux.ibm.com, bhe@redhat.com, oleksandr@natalenko.name
Subject: Re: [PATCH v2 1/1] Add a new sysctl to disable io_uring system-wide
References: <20230629132711.1712536-1-matteorizzo@google.com>
        <20230629132711.1712536-2-matteorizzo@google.com>
Date:   Thu, 29 Jun 2023 12:17:20 -0400
In-Reply-To: <20230629132711.1712536-2-matteorizzo@google.com> (Matteo Rizzo's
        message of "Thu, 29 Jun 2023 13:27:11 +0000")
Message-ID: <x49mt0i5jlb.fsf@segfault.boston.devel.redhat.com>
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Matteo Rizzo <matteorizzo@google.com> writes:

> Introduce a new sysctl (io_uring_disabled) which can be either 0, 1,
> or 2. When 0 (the default), all processes are allowed to create io_uring
> instances, which is the current behavior. When 1, all calls to
> io_uring_setup fail with -EPERM unless the calling process has
> CAP_SYS_ADMIN. When 2, calls to io_uring_setup fail with -EPERM
> regardless of privilege.
>
> Signed-off-by: Matteo Rizzo <matteorizzo@google.com>

This looks good to me.  You may also consider updating the
io_uring_setup(2) man page (part of liburing) to reflect this new
meaning for -EPERM.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>

> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 19 +++++++++++++
>  io_uring/io_uring.c                         | 30 +++++++++++++++++++++
>  2 files changed, 49 insertions(+)
>
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 3800fab1619b..ee65f7aeb0cf 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -450,6 +450,25 @@ this allows system administrators to override the
>  ``IA64_THREAD_UAC_NOPRINT`` ``prctl`` and avoid logs being flooded.
>  
>  
> +io_uring_disabled
> +=================
> +
> +Prevents all processes from creating new io_uring instances. Enabling this
> +shrinks the kernel's attack surface.
> +
> += ==================================================================
> +0 All processes can create io_uring instances as normal. This is the
> +  default setting.
> +1 io_uring creation is disabled for unprivileged processes.
> +  io_uring_setup fails with -EPERM unless the calling process is
> +  privileged (CAP_SYS_ADMIN). Existing io_uring instances can
> +  still be used.
> +2 io_uring creation is disabled for all processes. io_uring_setup
> +  always fails with -EPERM. Existing io_uring instances can still be
> +  used.
> += ==================================================================
> +
> +
>  kexec_load_disabled
>  ===================
>  
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 1b53a2ab0a27..2343ae518546 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -153,6 +153,22 @@ static __cold void io_fallback_tw(struct io_uring_task *tctx);
>  
>  struct kmem_cache *req_cachep;
>  
> +static int __read_mostly sysctl_io_uring_disabled;
> +#ifdef CONFIG_SYSCTL
> +static struct ctl_table kernel_io_uring_disabled_table[] = {
> +	{
> +		.procname	= "io_uring_disabled",
> +		.data		= &sysctl_io_uring_disabled,
> +		.maxlen		= sizeof(sysctl_io_uring_disabled),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_TWO,
> +	},
> +	{},
> +};
> +#endif
> +
>  struct sock *io_uring_get_socket(struct file *file)
>  {
>  #if defined(CONFIG_UNIX)
> @@ -4000,9 +4016,18 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
>  	return io_uring_create(entries, &p, params);
>  }
>  
> +static inline bool io_uring_allowed(void)
> +{
> +	return sysctl_io_uring_disabled == 0 ||
> +		(sysctl_io_uring_disabled == 1 && capable(CAP_SYS_ADMIN));
> +}
> +
>  SYSCALL_DEFINE2(io_uring_setup, u32, entries,
>  		struct io_uring_params __user *, params)
>  {
> +	if (!io_uring_allowed())
> +		return -EPERM;
> +
>  	return io_uring_setup(entries, params);
>  }
>  
> @@ -4577,6 +4602,11 @@ static int __init io_uring_init(void)
>  
>  	req_cachep = KMEM_CACHE(io_kiocb, SLAB_HWCACHE_ALIGN | SLAB_PANIC |
>  				SLAB_ACCOUNT | SLAB_TYPESAFE_BY_RCU);
> +
> +#ifdef CONFIG_SYSCTL
> +	register_sysctl_init("kernel", kernel_io_uring_disabled_table);
> +#endif
> +
>  	return 0;
>  };
>  __initcall(io_uring_init);