linux-kernel - Re: [PATCH] IB/core: Add mitigation for Spectre V1

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Message-ID: <95f5cf70-1a1d-f48c-efac-f389360f585e@embeddedor.com>
Date:   Tue, 30 Jul 2019 18:52:12 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Tony Luck <tony.luck@...el.com>, Doug Ledford <dledford@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
        Parav Pandit <parav@...lanox.com>,
        Ira Weiny <ira.weiny@...el.com>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/core: Add mitigation for Spectre V1



On 7/30/19 3:24 PM, Tony Luck wrote:
> Some processors may mispredict an array bounds check and
> speculatively access memory that they should not. With
> a user supplied array index we like to play things safe
> by masking the value with the array size before it is
> used as an index.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
> 
> [I don't have h/w, so just compile tested]
> 
>  drivers/infiniband/core/user_mad.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
> index 9f8a48016b41..fdce254e4f65 100644
> --- a/drivers/infiniband/core/user_mad.c
> +++ b/drivers/infiniband/core/user_mad.c
> @@ -49,6 +49,7 @@
>  #include <linux/sched.h>
>  #include <linux/semaphore.h>
>  #include <linux/slab.h>
> +#include <linux/nospec.h>
>  
>  #include <linux/uaccess.h>
>  
> @@ -888,6 +889,7 @@ static int ib_umad_unreg_agent(struct ib_umad_file *file, u32 __user *arg)
>  	mutex_lock(&file->port->file_mutex);
>  	mutex_lock(&file->mutex);
>  
> +	id = array_index_nospec(id, IB_UMAD_MAX_AGENTS);

This is wrong. This prevents the below condition id >= IB_UMAD_MAX_AGENTS
from ever being true. And I don't think this is what you want.

>  	if (id >= IB_UMAD_MAX_AGENTS || !__get_agent(file, id)) {
>  		ret = -EINVAL;
>  		goto out;
> 

--
Gustavo