linux-kernel - Re: [PATCH 1/2] char: lp: ensure that index has not exceeded LP

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Date:   Fri, 10 Jun 2022 19:12:02 +0530
From:   Shreenidhi Shedi <yesshedi@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     arnd@...db.de, linux-kernel@...r.kernel.org,
        Shreenidhi Shedi <sshedi@...are.com>
Subject: Re: [PATCH 1/2] char: lp: ensure that index has not exceeded LP_NO

On 10/06/22 6:58 pm, Greg KH wrote:
> On Fri, Jun 03, 2022 at 06:30:39PM +0530, Shreenidhi Shedi wrote:
>> From: Shreenidhi Shedi <sshedi@...are.com>
>>
>> After finishing the loop, index value can be equal to LP_NO and lp_table
>> array is of size LP_NO, so this can end up in accessing an out of bound
>> address in lp_register function.
>>
>> Signed-off-by: Shreenidhi Shedi <sshedi@...are.com>
>> ---
>>  drivers/char/lp.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/lp.c b/drivers/char/lp.c
>> index 0e22e3b0a..d474d02b6 100644
>> --- a/drivers/char/lp.c
>> +++ b/drivers/char/lp.c
>> @@ -972,7 +972,7 @@ static void lp_attach(struct parport *port)
>>  			if (port_num[i] == -1)
>>  				break;
>>
>> -		if (!lp_register(i, port))
>> +		if (i < LP_NO && !lp_register(i, port))
>>  			lp_count++;
> 
> How can this ever be needed?  Look at the check further up for the check
> of lp_count which prevents this from every going too large.
> 
> So how can an address be accessed out of bound here?
> 
> thanks,
> 
> greg k-h

Thanks for the review. Assume lp_count is less than LP_NO now and we enter the for loop
and for some reason for loop exits after i reaching the value LP_NO and right after that 
we call lp_register() with i and I can be equal to LP_NO.

-- 
Shedi