netdev - Re: [PATCH v5 4/7] pidfd: Replace open-coded partial fd_install

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Message-ID: <20200706130713.n6r3vhn4hn2lodex@wittgenstein>
Date:   Mon, 6 Jul 2020 15:07:13 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian@...uner.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Laight <David.Laight@...LAB.COM>,
        Christoph Hellwig <hch@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        Matt Denton <mpdenton@...gle.com>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Robert Sesek <rsesek@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
        netdev@...r.kernel.org, containers@...ts.linux-foundation.org,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 4/7] pidfd: Replace open-coded partial
 fd_install_received()

On Wed, Jun 17, 2020 at 03:03:24PM -0700, Kees Cook wrote:
> The sock counting (sock_update_netprioidx() and sock_update_classid()) was
> missing from pidfd's implementation of received fd installation. Replace
> the open-coded version with a call to the new fd_install_received()
> helper.
> 
> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  kernel/pid.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index f1496b757162..24924ec5df0e 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -635,18 +635,9 @@ static int pidfd_getfd(struct pid *pid, int fd)
>  	if (IS_ERR(file))
>  		return PTR_ERR(file);
>  
> -	ret = security_file_receive(file);
> -	if (ret) {
> -		fput(file);
> -		return ret;
> -	}
> -
> -	ret = get_unused_fd_flags(O_CLOEXEC);
> +	ret = fd_install_received(file, O_CLOEXEC);
>  	if (ret < 0)
>  		fput(file);
> -	else
> -		fd_install(ret, file);

So someone just sent a fix for pidfd_getfd() that was based on the
changes done here.

I've been on vacation so didn't have a change to review this series and
I see it's already in linux-next. This introduces a memory leak and
actually proves a point I tried to stress when adding this helper:
fd_install_received() in contrast to fd_install() does _not_ consume a
reference because it takes one before it calls into fd_install(). That
means, you need an unconditional fput() here both in the failure and
error path.
I strongly suggest though that we simply align the behavior between
fd_install() and fd_install_received() and have the latter simply
consume a reference when it succeeds! Imho, this bug proves that I was
right to insist on this before. ;)

Thanks!
Christian