linux-kernel - Re: [PATCH v7 1/3] random: add vgetrandom

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Message-ID: <87bkouyd90.ffs@tglx>
Date:   Fri, 25 Nov 2022 21:45:31 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-crypto@...r.kernel.org, linux-api@...r.kernel.org,
        x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v7 1/3] random: add vgetrandom_alloc() syscall

On Thu, Nov 24 2022 at 17:55, Jason A. Donenfeld wrote:
> ---
>  MAINTAINERS                             |  1 +
>  arch/x86/Kconfig                        |  1 +
>  arch/x86/entry/syscalls/syscall_64.tbl  |  1 +
>  arch/x86/include/asm/unistd.h           |  1 +
>  drivers/char/random.c                   | 59 +++++++++++++++++++++++++
>  include/uapi/asm-generic/unistd.h       |  7 ++-
>  kernel/sys_ni.c                         |  3 ++
>  lib/vdso/getrandom.h                    | 23 ++++++++++
>  scripts/checksyscalls.sh                |  4 ++
>  tools/include/uapi/asm-generic/unistd.h |  7 ++-
>  10 files changed, 105 insertions(+), 2 deletions(-)
>  create mode 100644 lib/vdso/getrandom.h

I think I asked for this before:

Please split these things properly up. Provide the syscall and then wire
it up.

> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 67745ceab0db..331e21ba961a 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -59,6 +59,7 @@ config X86
>  	#
>  	select ACPI_LEGACY_TABLES_LOOKUP	if ACPI
>  	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
> +	select ADVISE_SYSCALLS			if X86_64

Why is this x86_64 specific?

> --- a/arch/x86/include/asm/unistd.h
> +++ b/arch/x86/include/asm/unistd.h
> @@ -27,6 +27,7 @@
>  #  define __ARCH_WANT_COMPAT_SYS_PWRITEV64
>  #  define __ARCH_WANT_COMPAT_SYS_PREADV64V2
>  #  define __ARCH_WANT_COMPAT_SYS_PWRITEV64V2
> +#  define __ARCH_WANT_VGETRANDOM_ALLOC

So instead of this define, why can't you do:

config VGETRADOM_ALLOC
       bool
       select ADVISE_SYSCALLS

and then have

config GENERIC_VDSO_RANDOM_WHATEVER
       bool
       select VGETRANDOM_ALLOC

This gives a clear Kconfig dependency instead of the random
ADVISE_SYSCALLS select.

>--- a/drivers/char/random.c
> +++ b/drivers/char/random.c

> +#include "../../lib/vdso/getrandom.h"

Seriously?

include/vdso/ exists for a reason.

> +#ifdef __ARCH_WANT_VGETRANDOM_ALLOC
> +/*
> + * The vgetrandom() function in userspace requires an opaque state, which this
> + * function provides to userspace, by mapping a certain number of special pages
> + * into the calling process. It takes a hint as to the number of opaque states
> + * desired, and returns the number of opaque states actually allocated, the
> + * size of each one in bytes, and the address of the first state.

As this is a syscall which can be invoked outside of the VDSO, can you
please provide proper kernel-doc which explains the arguments, the
functionality and the return value?

> + */
> +SYSCALL_DEFINE3(vgetrandom_alloc, unsigned int __user *, num,
> +		unsigned int __user *, size_per_each, unsigned int, flags)
> +{
> +	size_t alloc_size, num_states;
> +	unsigned long pages_addr;
> +	unsigned int num_hint;
> +	int ret;
> +
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (get_user(num_hint, num))
> +		return -EFAULT;
> +
> +	num_states = clamp_t(size_t, num_hint, 1, (SIZE_MAX & PAGE_MASK) / sizeof(struct vgetrandom_state));
> +	alloc_size = PAGE_ALIGN(num_states * sizeof(struct vgetrandom_state));
> +
> +	if (put_user(alloc_size / sizeof(struct vgetrandom_state), num) ||
> +	    put_user(sizeof(struct vgetrandom_state), size_per_each))
> +		return -EFAULT;

That's a total of four sizeof(struct vgetrandom_state) usage sites.

       size_t state_size = sizeof(struct vgetrandom_state);

perhaps?

> diff --git a/lib/vdso/getrandom.h b/lib/vdso/getrandom.h
> new file mode 100644
> index 000000000000..c7f727db2aaa
> --- /dev/null
> +++ b/lib/vdso/getrandom.h

Wrong place. See above.

> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2022 Jason A. Donenfeld <Jason@...c4.com>. All Rights Reserved.
> + */
> +
> +#ifndef _VDSO_LIB_GETRANDOM_H
> +#define _VDSO_LIB_GETRANDOM_H
> +
> +#include <crypto/chacha.h>
> +
> +struct vgetrandom_state {
> +	union {
> +		struct {
> +			u8 batch[CHACHA_BLOCK_SIZE * 3 / 2];
> +			u32 key[CHACHA_KEY_SIZE / sizeof(u32)];
> +		};
> +		u8 batch_key[CHACHA_BLOCK_SIZE * 2];
> +	};
> +	unsigned long generation;
> +	u8 pos;
> +};

Thanks,

        tglx