netdev - RE: [PATCH net-next 5/6] igb: Do not free q_vector unless new one was allocated

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Message-ID: <DM5PR11MB1324FDF4D4399A6A99727B5EC13C9@DM5PR11MB1324.namprd11.prod.outlook.com>
Date:   Mon, 7 Nov 2022 13:55:58 +0000
From:   "Ruhl, Michael J" <michael.j.ruhl@...el.com>
To:     Leon Romanovsky <leon@...nel.org>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        "G, GurucharanX" <gurucharanx.g@...el.com>
Subject: RE: [PATCH net-next 5/6] igb: Do not free q_vector unless new one was
 allocated

>-----Original Message-----
>From: Leon Romanovsky <leon@...nel.org>
>Sent: Monday, November 7, 2022 2:03 AM
>To: Nguyen, Anthony L <anthony.l.nguyen@...el.com>
>Cc: davem@...emloft.net; kuba@...nel.org; pabeni@...hat.com;
>edumazet@...gle.com; Kees Cook <keescook@...omium.org>;
>netdev@...r.kernel.org; Brandeburg, Jesse <jesse.brandeburg@...el.com>;
>intel-wired-lan@...ts.osuosl.org; Ruhl, Michael J <michael.j.ruhl@...el.com>;
>Keller, Jacob E <jacob.e.keller@...el.com>; G, GurucharanX
><gurucharanx.g@...el.com>
>Subject: Re: [PATCH net-next 5/6] igb: Do not free q_vector unless new one
>was allocated
>
>On Fri, Nov 04, 2022 at 01:54:13PM -0700, Tony Nguyen wrote:
>> From: Kees Cook <keescook@...omium.org>
>>
>> Avoid potential use-after-free condition under memory pressure. If the
>> kzalloc() fails, q_vector will be freed but left in the original
>> adapter->q_vector[v_idx] array position.
>>
>> Cc: Jesse Brandeburg <jesse.brandeburg@...el.com>
>> Cc: Tony Nguyen <anthony.l.nguyen@...el.com>
>> Cc: "David S. Miller" <davem@...emloft.net>
>> Cc: Eric Dumazet <edumazet@...gle.com>
>> Cc: Jakub Kicinski <kuba@...nel.org>
>> Cc: Paolo Abeni <pabeni@...hat.com>
>> Cc: intel-wired-lan@...ts.osuosl.org
>> Cc: netdev@...r.kernel.org
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@...el.com>
>> Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>
>> Tested-by: Gurucharan <gurucharanx.g@...el.com> (A Contingent worker
>at Intel)
>
>You should use first and last names here.
>
>> Signed-off-by: Tony Nguyen <anthony.l.nguyen@...el.com>
>> ---
>>  drivers/net/ethernet/intel/igb/igb_main.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c
>b/drivers/net/ethernet/intel/igb/igb_main.c
>> index d6c1c2e66f26..c2bb658198bf 100644
>> --- a/drivers/net/ethernet/intel/igb/igb_main.c
>> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
>> @@ -1202,8 +1202,12 @@ static int igb_alloc_q_vector(struct igb_adapter
>*adapter,
>>  	if (!q_vector) {
>>  		q_vector = kzalloc(size, GFP_KERNEL);
>>  	} else if (size > ksize(q_vector)) {
>> -		kfree_rcu(q_vector, rcu);
>> -		q_vector = kzalloc(size, GFP_KERNEL);
>> +		struct igb_q_vector *new_q_vector;
>> +
>> +		new_q_vector = kzalloc(size, GFP_KERNEL);
>> +		if (new_q_vector)
>> +			kfree_rcu(q_vector, rcu);
>> +		q_vector = new_q_vector;
>
>I wonder if this is correct.
>1. if new_q_vector is NULL, you will overwrite q_vector without releasing it.
>2. kfree_rcu() doesn't immediately release memory, but after grace
>period, but here you are overwriting the pointer which is not release
>yet.

The actual pointer is: adapter->q_vector[v_idx]

q_vector is just a convenience pointer.

If the allocation succeeds, the q_vector[v_idx] will be replaced (later in the code).

If the allocation fails, this is not being freed.  The original code freed the adapter
pointer but didn't not remove the pointer.

If q_vector is NULL,  (i.e. the allocation failed), the function exits, but the original
pointer is left in place.

I think this logic is correct.

The error path leaves the original allocation in place.  If this is incorrect behavior,
a different change would be:

	q_vector = adapter->q_vector[v_idx];
	adapter->q_vector[v_idx] = NULL;
	... the original code...

But I am not sure if that is what is desired?

Mike

>
>>  	} else {
>>  		memset(q_vector, 0, size);
>>  	}
>> --
>> 2.35.1
>>