Re: [PATCH v7 3/3] x86: vdso: Wire up getrandom() vDSO implementation

Thomas Gleixner
2022-11-25T23:08:48Z

Jason!

On Thu, Nov 24 2022 at 17:55, Jason A. Donenfeld wrote:
> +++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
> +/*
> + * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
> + * of blocks of output with a nonce of 0, taking an input key and 8-byte
> + * counter. Importantly does not spill to the stack. Its arguments are:

Basic or not. This needs a Reviewed-by from someone who understands SSE2
and ChaCha20 before this can go anywhere near the x86 tree.

> +++ b/arch/x86/entry/vdso/vgetrandom.c
> @@ -0,0 +1,18 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> + */
> +#include <linux/kernel.h>

Why do you need kernel.h here?

> +#include <linux/types.h>
> +
> +#include "../../../../lib/vdso/getrandom.c"
> +
> +ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *state);
> +
> +ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *state)
> +{
> + return __cvdso_getrandom(buffer, len, flags, state);
> +}
> +
> +ssize_t getrandom(void *, size_t, unsigned int, void *)
> + __attribute__((weak, alias("__vdso_getrandom")));
> diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h
> new file mode 100644
> index 000000000000..099aca58ef20
> --- /dev/null
> +++ b/arch/x86/include/asm/vdso/getrandom.h
> @@ -0,0 +1,49 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> + */
> +#ifndef __ASM_VDSO_GETRANDOM_H
> +#define __ASM_VDSO_GETRANDOM_H
> +
> +#ifndef __ASSEMBLY__
> +
> +#include <asm/unistd.h>
> +#include <asm/vvar.h>
> +
> +static __always_inline ssize_t
> +getrandom_syscall(void *buffer, size_t len, unsigned int flags)

static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsigned int flags)

please. We expanded to 100 quite some time ago.

Some kernel-doc compliant comment for this would be appreciated as well.

> +{
> + long ret;
> +
> + asm ("syscall" : "=a" (ret) :
> + "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
> + "rcx", "r11", "memory");
> +
> + return ret;
> +}
> +
> +#define __vdso_rng_data (VVAR(_vdso_rng_data))
> +
> +static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
> +{
> + if (__vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
> + return (void *)&__vdso_rng_data +
> + ((void *)&__timens_vdso_data - (void *)&__vdso_data);
> + return &__vdso_rng_data;

So either bite the bullet and write it:

if (__vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);

return &__vdso_rng_data;

or comply to the well documented rules of the tip tree:

https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#bracket-rules

> +/*
> + * Generates a given positive number of block of ChaCha20 output with nonce=0,
> + * and does not write to any stack or memory outside of the parameters passed
> + * to it. This way, we don't need to worry about stack data leaking into forked
> + * child processes.

Please use proper kernel-doc

> + */
> +static __always_inline void __arch_chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks)
> +{
> + extern void chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks);
> + return chacha20_blocks_nostack(dst_bytes, key, counter, nblocks);

You surely have an issue with your newline key...

The above aside, can you please explain the value of this __arch_()
wrapper?

It's just voodoo for no value because it hands through the arguments
1:1. So where are you expecting that that __arch...() version of this is
any different than invoking the architecture specific version of
chacha20_blocks_nostack().

Can you spot the irony of your naming choices?

__arch_chacha20_blocks_nostack() {
return chacha20_blocks_nostack()
};

Thanks,

tglx