linux-kernel - Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions

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Date:   Fri, 12 Jul 2019 07:36:18 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions

On Fri, Jul 12, 2019 at 05:14:54AM +0100, Al Viro wrote:

> That's not quite guaranteed (it is possible to bind a symlink on top
> of a regular file, and you will get LOOKUP_JUMPED on the entry into
> trailing_symlink() when looking the result up).  Moreover, why bother
> with LOOKUP_JUMPED here?  See that
> 	nd->last_type = LAST_BIND;
> several lines prior?  That's precisely to be able to recognize those
> suckers.

... except that this won't work these days (once upon a time it used
to, but that had been a long time ago).  However, that doesn't change
the fact that the test is really wrong.  So let's do it right:

* set a new flag along with LOOKUP_JUMPED in nd_jump_link()
* clear it in get_link() right before
	res = READ_ONCE(inode->i_link);
* check it in trailing_symlink() (or callers thereof)

The rest of comments stand, AFAICS.