linux-kernel - Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer

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Date:   Fri, 24 Jul 2020 10:10:47 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     "Andres Beltran" <lkmlabelt@...il.com>
Cc:     "KY Srinivasan" <kys@...rosoft.com>,
        "Haiyang Zhang" <haiyangz@...rosoft.com>,
        "Stephen Hemminger" <sthemmin@...rosoft.com>, <wei.liu@...nel.org>,
        <linux-hyperv@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Michael Kelley" <mikelley@...rosoft.com>,
        <parri.andrea@...il.com>,
        "Saruhan Karademir" <skarade@...rosoft.com>
Subject: Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in
 hv_ringbuffer_read()

On Fri, 24 Jul 2020 09:46:06 -0700
"Andres Beltran" <lkmlabelt@...il.com> wrote:

> Assignments to buffer_actual_len and requestid happen before packetlen
> is checked to be within buflen. If this condition is true,
> hv_ringbuffer_read() returns with these variables already set to some
> value even though no data is actually read. This might create
> inconsistencies in any routine calling hv_ringbuffer_read(). Assign values
> to such pointers after the packetlen check.
> 
> Signed-off-by: Andres Beltran <lkmlabelt@...il.com>
> ---
>  drivers/hv/ring_buffer.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 356e22159e83..e277ce7372a4 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -350,12 +350,13 @@ int hv_ringbuffer_read(struct vmbus_channel
> *channel,
>  
>  	offset = raw ? 0 : (desc->offset8 << 3);
>  	packetlen = (desc->len8 << 3) - offset;
> -	*buffer_actual_len = packetlen;
> -	*requestid = desc->trans_id;
>  
>  	if (unlikely(packetlen > buflen))
>  		return -ENOBUFS;
>  
> +	*buffer_actual_len = packetlen;
> +	*requestid = desc->trans_id;
> +
>  	/* since ring is double mapped, only one copy is necessary */
>  	memcpy(buffer, (const char *)desc + offset, packetlen);
>  

What is the rationale for this change, it may break other code.

A common API model in Windows world where this originated
is to have a call where caller first
makes request and then if the requested buffer is not big enough the
caller look at the actual length and allocate a bigger buffer.

Did you audit all the users of this API to make sure they aren't doing that.