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

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Date:   Fri, 3 Feb 2017 20:34:16 -0800
From:   Matt Mullins <mmullins@...x.us>
To:     Yisheng Xie <xieyisheng1@...wei.com>
Cc:     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>,
        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()

On Sat, Feb 04, 2017 at 11:47:38AM +0800, Yisheng Xie wrote:
> 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.

My original report was
https://lkml.kernel.org/r/20161016002151.GA18235@hydra.tuxags.com.

The virtio ring APIs use scatterlists to keep track of the buffers, and
scatterlist requires addresses to be in the kernel direct-mapped address range.
This is not the case for vmalloc()ed addresses, such as the original on-stack
"bytes" array when VMAP_STACK=y.