linux-kernel - Re: [PATCH] net: check the return value of the copy_from

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Message-ID: <CANn89iJkfT8=rt23LSp_WkoOibdAKf4pA0uybaWMbb0DJGRY5Q@mail.gmail.com>
Date: Wed, 11 Sep 2024 19:15:27 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: Qianqiang Liu <qianqiang.liu@....com>, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: check the return value of the copy_from_sockptr

On Wed, Sep 11, 2024 at 6:58 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Wed, Sep 11, 2024 at 11:12:24AM +0200, Eric Dumazet wrote:
> > On Wed, Sep 11, 2024 at 10:23 AM Qianqiang Liu <qianqiang.liu@....com> wrote:
> > >
> > > > I do not think it matters, because the copy is performed later, with
> > > > all the needed checks.
> > >
> > > No, there is no checks at all.
> > >
> >
> > Please elaborate ?
> > Why should maintainers have to spend time to provide evidence to
> > support your claims ?
> > Have you thought about the (compat) case ?
> >
> > There are plenty of checks. They were there before Stanislav commit.
> >
> > Each getsockopt() handler must perform the same actions.
>
>
> But in line 2379 we have ops->getsockopt==NULL case:
>
> 2373         if (!compat)
> 2374                 copy_from_sockptr(&max_optlen, optlen, sizeof(int));
> 2375
> 2376         ops = READ_ONCE(sock->ops);
> 2377         if (level == SOL_SOCKET) {
> 2378                 err = sk_getsockopt(sock->sk, level, optname, optval, optlen);
> 2379         } else if (unlikely(!ops->getsockopt)) {
> 2380                 err = -EOPNOTSUPP;         // <--- HERE
> 2381         } else {
> 2382                 if (WARN_ONCE(optval.is_kernel || optlen.is_kernel,
> 2383                               "Invalid argument type"))
> 2384                         return -EOPNOTSUPP;
> 2385
> 2386                 err = ops->getsockopt(sock, level, optname, optval.user,
> 2387                                       optlen.user);
> 2388         }
>
> where we simply continue with calling BPF_CGROUP_RUN_PROG_GETSOCKOPT()
> which actually needs the 'max_optlen' we copied via copy_from_sockptr().
>
> Do I miss anything here?

This is another great reason why we should not change current behavior.

err will be -EOPNOTSUPP, which was the original error code before
Stanislav patch.

Surely the eBPF program will use this value first, and not even look
at max_optlen

Returning -EFAULT might break some user programs, I don't know.

I feel we are making the kernel slower just because we can.