linux-kernel - Re: [PATCH] vt_ioctl: fix potential spectre v1 in VT

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Message-ID: <e0071c1e-cc20-e000-0673-f98e6f8fc316@kernel.org>
Date:   Tue, 22 Feb 2022 07:56:13 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vt_ioctl: fix potential spectre v1 in VT_DISALLOCATE

On 22. 02. 22, 4:56, Xiaomeng Tong wrote:
> In VT_ACTIVATE an almost identical code path has been patched
> with array_index_nospec. In the VT_DISALLOCATE path, the arg is
> the user input from a system call argument and lately used as a index
> for vc_cons[index].d access, which can be reached through path like
> vt_disallocate->vc_busy or vt_disallocate->vc_deallocate.
> For consistency both code paths should have the same mitigations
> applied.
> 
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
> ---
>   drivers/tty/vt/vt_ioctl.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> index 580136986..acd53c1c9 100644
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -898,10 +898,13 @@ int vt_ioctl(struct tty_struct *tty,
>   		if (arg > MAX_NR_CONSOLES)
>   			return -ENXIO;
>   
> -		if (arg == 0)
> +		if (arg == 0) {
>   			vt_disallocate_all();

Could you add a break; here and remove the else branching completely?

> -		else
> -			return vt_disallocate(--arg);
> +		} else {
> +			--arg;
> +			arg = array_index_nospec(arg, MAX_NR_CONSOLES);
You could simply do:
   arg = array_index_nospec(arg - 1, MAX_NR_CONSOLES);
and remove
   --arg;
completely.

> +			return vt_disallocate(arg);
> +		}
>   		break;

When you do the above, this break is superfluous.

thanks,
-- 
js
suse labs