Re: [PATCH v7 1/3] random: add vgetrandom_alloc() syscall

Thomas Gleixner
2022-11-25T20:45:39Z

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@zx2c4.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