linux-kernel - Re: [PATCH v1 2/6] virtio console: Harden port adding

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Date:   Fri, 20 Jan 2023 08:15:15 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, elena.reshetova@...el.com,
        kirill.shutemov@...ux.intel.com, Andi Kleen <ak@...ux.intel.com>,
        Amit Shah <amit@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v1 2/6] virtio console: Harden port adding

On Thu, Jan 19, 2023 at 10:13:18PM +0200, Alexander Shishkin wrote:
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> 
> > Then you need to copy it out once, and then only deal with the local
> > copy.  Otherwise you have an incomplete snapshot.
> 
> Ok, would you be partial to something like this:
> 
> >From 1bc9bb84004154376c2a0cf643d53257da6d1cd7 Mon Sep 17 00:00:00 2001
> From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Date: Thu, 19 Jan 2023 21:59:02 +0200
> Subject: [PATCH] virtio console: Keep a local copy of the control structure
> 
> When handling control messages, instead of peeking at the device memory
> to obtain bits of the control structure, take a snapshot of it once and
> use it instead, to prevent it from changing under us. This avoids races
> between port id validation and control event decoding, which can lead
> to, for example, a NULL dereference in port removal of a nonexistent
> port.
> 
> The control structure is small enough (8 bytes) that it can be cached
> directly on the stack.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Amit Shah <amit@...nel.org>
> ---
>  drivers/char/virtio_console.c | 29 +++++++++++++++--------------
>  1 file changed, 15 insertions(+), 14 deletions(-)

Yes, this looks much better, thanks!

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>