linux-kernel - Re: [PATCH][next] cpumask: allocate enough space for string and trailing '\0' char

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Message-ID: <e0458a3f-7635-bc80-9496-731bdfceed0d@windriver.com>
Date:   Mon, 9 Nov 2020 23:57:15 -0500
From:   Paul Gortmaker <paul.gortmaker@...driver.com>
To:     Qian Cai <cai@...hat.com>, Colin King <colin.king@...onical.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux Next Mailing List <linux-next@...r.kernel.org>
Subject: Re: [PATCH][next] cpumask: allocate enough space for string and
 trailing '\0' char

On 2020-11-09 8:07 p.m., Qian Cai wrote:
> On Mon, 2020-11-09 at 13:04 +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> Currently the allocation of cpulist is based on the length of buf but does
>> not include the addition end of string '\0' terminator. Static analysis is
>> reporting this as a potential out-of-bounds access on cpulist. Fix this by
>> allocating enough space for the additional '\0' terminator.
>>
>> Addresses-Coverity: ("Out-of-bounds access")
>> Fixes: 65987e67f7ff ("cpumask: add "last" alias for cpu list specifications")
> 
> Yeah, this bad commit also introduced KASAN errors everywhere and then will
> disable lockdep that makes our linux-next CI miserable. Confirmed that this
> patch will fix it.

I appreciate the reports reminding me why I hate touching string handling.

But let us not lose sight of why linux-next exists.  We want to
encourage code to appear there as a sounding board before it goes
mainline, so we can fix things and not pollute mainline git history
with those trivialities.

If you've decided to internalize linux-next as part of your CI, then
great, but do note that does not elevate linux-next to some pristine
status for the world at large.  That only means you have to watch more
closely what is going on.

If you want to declare linux-next unbreakable -- well that would scare
away others to get the multi-arch or multi-config coverage that they may
not be able to do themselves.  We are not going to do that.

I have (hopefully) fixed the "bad commit" in v2 -- as part of the
implicit linux-next rule "you broke it, you better fix it ASAP".

But "bad" and "miserable" can be things that might scare people off of
making use of linux-next for what it is meant to be for.  And I am not
OK with that.

Thanks,
Paul.
--

> 
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>   lib/cpumask.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/cpumask.c b/lib/cpumask.c
>> index 34ecb3005941..cb8a3ef0e73e 100644
>> --- a/lib/cpumask.c
>> +++ b/lib/cpumask.c
>> @@ -185,7 +185,7 @@ int __ref cpulist_parse(const char *buf, struct cpumask
>> *dstp)
>>   {
>>   	int r;
>>   	char *cpulist, last_cpu[5];	/* NR_CPUS <= 9999 */
>> -	size_t len = strlen(buf);
>> +	size_t len = strlen(buf) + 1;
>>   	bool early = !slab_is_available();
>>   
>>   	if (!strcmp(buf, "all")) {
>