linux-kernel - Re: [PATCH resend 4.9] hw_random: Don't use a stack buffer in add_early

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Date:   Sat, 4 Feb 2017 11:47:38 +0800
From:   Yisheng Xie <xieyisheng1@...wei.com>
To:     Andy Lutomirski <luto@...nel.org>, <linux-crypto@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Matt Mackall <mpm@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Rusty Russell <rusty@...tcorp.com.au>
CC:     Jens Axboe <axboe@...com>, Matt Mullins <mmullins@...x.us>,
        Xishi Qiu <qiuxishi@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH resend 4.9] hw_random: Don't use a stack buffer in
 add_early_randomness()

Hi Andy,

On 2016/10/18 1:06, Andy Lutomirski wrote:
> hw_random carefully avoids using a stack buffer except in
> add_early_randomness().  This causes a crash in virtio_rng if
> CONFIG_VMAP_STACK=y.
I try to understand this patch, but I do not know why it will cause
a crash in virtio_rng with CONFIG_VMAP_STACK=y?
Could you please give me more info. about it.

Really thanks for that!
Yisheng Xie.

> 
> Reported-by: Matt Mullins <mmullins@...x.us>
> Tested-by: Matt Mullins <mmullins@...x.us>
> Fixes: d3cc7996473a ("hwrng: fetch randomness only after device init")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
> 
> This fixes a crash in 4.9-rc1.
> 
> resending because I typoed the git send-email command.  I stealthily added
> Matt's Tested-by, too.
> 
>  drivers/char/hw_random/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 9203f2d130c0..340f96e44642 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -84,14 +84,14 @@ static size_t rng_buffer_size(void)
>  
>  static void add_early_randomness(struct hwrng *rng)
>  {
> -	unsigned char bytes[16];
>  	int bytes_read;
> +	size_t size = min_t(size_t, 16, rng_buffer_size());
>  
>  	mutex_lock(&reading_mutex);
> -	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> +	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
>  	mutex_unlock(&reading_mutex);
>  	if (bytes_read > 0)
> -		add_device_randomness(bytes, bytes_read);
> +		add_device_randomness(rng_buffer, bytes_read);
>  }
>  
>  static inline void cleanup_rng(struct kref *kref)
>