linux-kernel - Re: [PATCH net] net: 9p: initialize sun_server.sun_path to have addr's value only when addr is valid

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Date:   Mon, 12 Oct 2020 14:47:50 +0530
From:   Anant Thazhemadam <anant.thazhemadam@...il.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     ericvh@...il.com, lucho@...kov.net, davem@...emloft.net,
        kuba@...nel.org, v9fs-developer@...ts.sourceforge.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
Subject: Re: [PATCH net] net: 9p: initialize sun_server.sun_path to have
 addr's value only when addr is valid


On 12-10-2020 13:29, Dominique Martinet wrote:
> Anant Thazhemadam wrote on Mon, Oct 12, 2020:
>> In p9_fd_create_unix, checking is performed to see if the addr (passed
>> as an argument) is NULL or not.
>> However, no check is performed to see if addr is a valid address, i.e.,
>> it doesn't entirely consist of only 0's.
>> The initialization of sun_server.sun_path to be equal to this faulty
>> addr value leads to an uninitialized variable, as detected by KMSAN.
>> Checking for this (faulty addr) and returning a negative error number
>> appropriately, resolves this issue.
> I'm not sure I agree a fully zeroed address is faulty but I agree we can
> probably refuse it given userspace can't pass useful abstract addresses
> here.


Understood. It's  probably a better that I modify the commit message a little and
send a v2 so it becomes more accurate.

> Just one nitpick but this is otherwise fine - good catch!

Thank you!

>
>> Reported-by: syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
>> Tested-by: syzbot+75d51fe5bf4ebe988518@...kaller.appspotmail.com
>> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@...il.com>
>> ---
>>  net/9p/trans_fd.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
>> index c0762a302162..8f528e783a6c 100644
>> --- a/net/9p/trans_fd.c
>> +++ b/net/9p/trans_fd.c
>> @@ -1023,7 +1023,7 @@ p9_fd_create_unix(struct p9_client *client, const char *addr, char *args)
>>  
>>  	csocket = NULL;
>>  
>> -	if (addr == NULL)
>> +	if (!addr || !strlen(addr))
> Since we don't care about the actual length here, how about checking for
> addr[0] directly?
> That'll spare a strlen() call in the valid case.
>
You mentioned how a fully zeroed address isn't exactly faulty. By extension, wouldn't that
mean that an address that simply begins with a 0 isn't faulty as well?
This is an interesting point, because if the condition is modified to checking for addr[0] directly,
addresses that simply begin with 0 (but have more non-zero content following) wouldn't be
copied over either, right?
In the end, it comes down to what you define as a "valid" value that sun_path can have.
We've already agreed that a fully zeroed address wouldn't qualify as a valid value for sun_path.
Are addresses that aren't fully zeroed, but only begin with a 0 also to be considered as an
unacceptable value for sun_path?

Thanks,
Anant