linux-kernel - Re: [PATCH v2 0/2] implement OA2_INHERIT

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Date: Wed, 24 Apr 2024 13:57:09 +0300
From: stsp <stsp2@...dex.ru>
To: Andy Lutomirski <luto@...capital.net>
Cc: linux-kernel@...r.kernel.org, Stefan Metzmacher <metze@...ba.org>,
 Eric Biederman <ebiederm@...ssion.com>,
 Alexander Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>,
 Alexander Aring <alex.aring@...il.com>, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
 Christian Göttsche <cgzones@...glemail.com>
Subject: Re: [PATCH v2 0/2] implement OA2_INHERIT_CRED flag for openat2()

23.04.2024 19:44, Andy Lutomirski пишет:
>> On Apr 23, 2024, at 4:02 AM, Stas Sergeev <stsp2@...dex.ru> wrote:
>>
>> This patch-set implements the OA2_INHERIT_CRED flag for openat2() syscall.
>> It is needed to perform an open operation with the creds that were in
>> effect when the dir_fd was opened. This allows the process to pre-open
>> some dirs and switch eUID (and other UIDs/GIDs) to the less-privileged
>> user, while still retaining the possibility to open/create files within
>> the pre-opened directory set.
> I like the concept, as it’s a sort of move toward a capability system. But I think that making a dirfd into this sort of capability would need to be much more explicit. Right now, any program could do this entirely by accident, and applying OA2_INHERIT_CRED to an fd fished out of /proc seems hazardous.

While I still don't quite understand
the threat of /proc symlinks, I posted
v4 which disallows them.

> So perhaps if an open file description for a directory could have something like FMODE_CRED, and if OA2_INHERIT_CRED also blocked .., magic links, symlinks to anywhere above the dirfd (or maybe all symlinks) and absolute path lookups, then this would be okay.

So I think this all is now done.

> Also, there are lots of ways that f_cred could be relevant: fsuid/fsgid, effective capabilities and security labels. And it gets more complex if this ever gets extended to support connecting or sending to a socket or if someone opens a device node.  Does CAP_SYS_ADMIN carry over?
Not any more, nothin is carried but
fsuid, fsgid, groupinfo.

Thank you.