linux-kernel - Re: [PATCH] net: 9p: fix possible memory leak in p9_check

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Message-ID: <0f6ebc50-06d6-4e3d-b296-1045b0255c8a@gmail.com>
Date:   Fri, 27 Oct 2023 10:41:08 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     asmadeus@...ewreck.org
Cc:     ericvh@...nel.org, lucho@...kov.net, linux_oss@...debyte.com,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, jvrao@...ux.vnet.ibm.com, v9fs@...ts.linux.dev,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: 9p: fix possible memory leak in p9_check_errors()

On 26/10/2023 19:53, asmadeus@...ewreck.org wrote:
> 
> Hangyu Hua wrote on Thu, Oct 26, 2023 at 05:23:51PM +0800:
>> When p9pdu_readf is called with "s?d" attribute, it allocates a pointer
>> that will store a string. But when p9pdu_readf() fails while handling "d"
>> then this pointer will not be freed in p9_check_errors.
> 
> Right, that sounds correct to me.
> 
> Out of curiosity how did you notice this? The leak shouldn't happen with
> any valid server.

I just found that any attributes that require memory allocation are 
prone to errors when mixed with ordinary attributes.

> 
> This cannot break anything so I'll push this to -next tomorrow and
> submit to Linus next week

Agreed.

> 
>> Fixes: ca41bb3e21d7 ("[net/9p] Handle Zero Copy TREAD/RERROR case in !dotl case.")
> 
> This commit moves this code a bit, but the p9pdu_readf call predates
> it -- in this case the Fixes tag is probably not useful; this affects
> all maintained kernels.
> 
>> Signed-off-by: Hangyu Hua <hbh25y@...il.com>
>> ---
>>   net/9p/client.c | 7 +++++--
>>   1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/9p/client.c b/net/9p/client.c
>> index 86bbc7147fc1..6c7cd765b714 100644
>> --- a/net/9p/client.c
>> +++ b/net/9p/client.c
>> @@ -540,12 +540,15 @@ static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
>>   		return 0;
>>   
>>   	if (!p9_is_proto_dotl(c)) {
>> -		char *ename;
>> +		char *ename = NULL;
>>   
>>   		err = p9pdu_readf(&req->rc, c->proto_version, "s?d",
>>   				  &ename, &ecode);
>> -		if (err)
>> +		if (err) {
>> +			if (ename != NULL)
>> +				kfree(ename);
> 
> Don't check for NULL before kfree - kfree does it.
> If that's the only remark you get I can fix it when applying the commit
> on my side.

I get it. I will revise it based on your and Christian's comments and 
send a v2.

Thanks,
Hangyu

> 
> 
>>   			goto out_err;
>> +		}
>>   
>>   		if (p9_is_proto_dotu(c) && ecode < 512)
>>   			err = -ecode;
>