netdev - Re: [PATCH v4 09/11] pidfs, coredump: allow to verify coredump connection

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Message-ID: <20250507.uS1oa0shi0eu@digikod.net>
Date: Wed, 7 May 2025 20:34:13 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Christian Brauner <brauner@...nel.org>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>, linux-fsdevel@...r.kernel.org, 
	Jann Horn <jannh@...gle.com>, Eric Dumazet <edumazet@...gle.com>, 
	Oleg Nesterov <oleg@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Daan De Meyer <daan.j.demeyer@...il.com>, 
	David Rheinsberg <david@...dahead.eu>, Jakub Kicinski <kuba@...nel.org>, Jan Kara <jack@...e.cz>, 
	Lennart Poettering <lennart@...ttering.net>, Luca Boccassi <bluca@...ian.org>, Mike Yuan <me@...dnzj.com>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	Alexander Mikhalitsyn <alexander@...alicyn.com>, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4 09/11] pidfs, coredump: allow to verify coredump
 connection

On Wed, May 07, 2025 at 06:13:42PM +0200, Christian Brauner wrote:
> When a coredump connection is initiated use the socket cookie as the
> coredump cookie and store it in the pidfd. The receiver can now easily
> authenticate that the connection is coming from the kernel.
> 
> Unless the coredump server expects to handle connection from
> non-crashing task it can validate that the connection has been made from
> a crashing task:
> 
>    fd_coredump = accept4(fd_socket, NULL, NULL, SOCK_CLOEXEC);
>    getsockopt(fd_coredump, SOL_SOCKET, SO_PEERPIDFD, &fd_peer_pidfd, &fd_peer_pidfd_len);
> 
>    struct pidfd_info info = {
>            info.mask = PIDFD_INFO_EXIT | PIDFD_INFO_COREDUMP,
>    };
> 
>    ioctl(pidfd, PIDFD_GET_INFO, &info);
>    /* Refuse connections that aren't from a crashing task. */
>    if (!(info.mask & PIDFD_INFO_COREDUMP) || !(info.coredump_mask & PIDFD_COREDUMPED) )
>            close(fd_coredump);
> 
>    /*
>     * Make sure that the coredump cookie matches the connection cookie.
>     * If they don't it's not the coredump connection from the kernel.
>     * We'll get another connection request in a bit.
>     */
>    getsocketop(fd_coredump, SOL_SOCKET, SO_COOKIE, &peer_cookie, &peer_cookie_len);
>    if (!info.coredump_cookie || (info.coredump_cookie != peer_cookie))
>            close(fd_coredump);
> 
> The kernel guarantees that by the time the connection is made the
> coredump info is available.

Nice approach to tie the coredump socket with the coredumped pidfd!
This indeed removes previous race condition.

I guess a socket's cookie is never zero?