netdev - Re: [net-next v2] tcp: Explicitly mark reserved field in tcp_zerocopy

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Date:   Tue, 9 Feb 2021 08:59:38 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Arjun Roy <arjunroy.kdev@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org, arjunroy@...gle.com, edumazet@...gle.com,
        soheil@...gle.com, David Ahern <dsahern@...il.com>
Subject: Re: [net-next v2] tcp: Explicitly mark reserved field in
 tcp_zerocopy_receive args.

On Tue, 9 Feb 2021 08:15:11 +0200 Leon Romanovsky wrote:
> At least in my tree, we have the length check:
>   4155                 if (len > sizeof(zc)) {
>   4156                         len = sizeof(zc);
>   4157                         if (put_user(len, optlen))
>   4158                                 return -EFAULT;
>   4159                 }
> 
> 
> Ad David wrote below, the "if (zc.reserved)" is enough.
> 
> We have following options:
> 1. Old kernel that have sizeof(sz) upto .reserved and old userspace
> -> len <= sizeof(sz) - works correctly.  
> 2. Old kernel that have sizeof(sz) upto .reserved and new userspace that
> sends larger struct -> "f (len > sizeof(zc))" will return -EFAULT

Based on the code you quoted? I don't see how. Maybe I need a vacation.
put_user() just copies len back to user space after truncation.

> 3. New kernel that have sizeof(sz) beyond reserved and old userspace
> -> any new added field to struct sz should be checked and anyway it is the same as item 1.  
> 4. New kernel and new userspace
> -> standard flow.