linux-kernel - Re: [PATCH] PowerPC: kernel: memory access violation when rtas_data

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Date:	Wed, 24 Apr 2013 12:53:49 +0530
From:	Vasant Hegde <hegdevasant@...ux.vnet.ibm.com>
To:	Chen Gang <gang.chen@...anux.com>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] PowerPC: kernel: memory access violation when rtas_data_buf
 contents are more than 1026

On 04/24/2013 12:33 PM, Chen Gang wrote:
> On 2013年04月24日 14:28, Vasant Hegde wrote:
>> On 04/23/2013 08:42 AM, Chen Gang wrote:
>>>
>>> need set '\0' for 'local_buffer'.
>>>
>>> SPLPAR_MAXLENGTH is 1026, RTAS_DATA_BUF_SIZE is 4096. so the contents of
>>> rtas_data_buf may truncated in memcpy.
>>>
>>> if contents are really truncated.
>>>     the splpar_strlen is more than 1026. the next while loop checking will
>>>     not find the end of buffer. that will cause memory access violation.
>>>
>>
>> Per parameter length in ibm,get-system-parameter RTAS call is limited to
>> 1026 bytes (1024 bytes of data + 2 bytes  length). And 'rtas_data_buf'
>> was set to 0 (first 1026 bytes) before call RTAS call. At the worst if
>> we get junk in RTAS output length field helps to exit from the while
>> loop. So I don't think we need this patch.
>
> Is get-system-parameter return the NUL terminated string ? if so, it
> will no issue (just like your discription).
>

Length includes the length of the NULL. So (idx < splpar_strlen)
is safe. IMO existing code is proper.

-Vasant


> If it will not return NUL terminated string, please see line 326:
>
>    "while ((*local_buffer)&&  (idx<  splpar_strlen))"
>    (when idx == 1024, *local_buffer is memory access violation).
>
> Since we use the first 2 bytes as length, and also be sure of the real
> length will never more than 1024, I suggest to:
>
> ---------------------------patch begin--------------------------------
>
> diff --git a/arch/powerpc/kernel/lparcfg.c b/arch/powerpc/kernel/lparcfg.c
> index 801a757..f8bd7cf 100644
> --- a/arch/powerpc/kernel/lparcfg.c
> +++ b/arch/powerpc/kernel/lparcfg.c
> @@ -323,7 +323,7 @@ static void parse_system_parameter_string(struct seq_file *m)
>
>   		w_idx = 0;
>   		idx = 0;
> -		while ((*local_buffer)&&  (idx<  splpar_strlen)) {
> +		while (idx<  splpar_strlen) {
>   			workbuffer[w_idx++] = local_buffer[idx++];
>   			if ((local_buffer[idx] == ',')
>   			    || (local_buffer[idx] == '\0')) {
>
> ---------------------------patch end----------------------------------
>
> Thanks.
>

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