Re: [PATCH] gpg-interface: trim CR from ssh-keygen -Y find-principals

From: Fabian Stelzer <fs@gigacodes.de>
Date: Dec 6, 2021, 08:39 UTC
Message-ID: <20211206083925.tuy2w3wzlgpc36bj@fs>
In-Reply-To: <70cee773-9547-e3cf-9327-ac0213d327e@mindrot.org>

On 06.12.2021 10:06, Damien Miller wrote:

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>On Sat, 4 Dec 2021, Fabian Stelzer wrote:
>
>> > I'm guessing probably not, but when it comes to something with security
>> > implications like this, it pays to be extra careful. I'm hoping somebody
>> > familiar with the ssh-keygen side and how the rest of the parsing works
>> > (like Fabian) can verify that this is OK.
>> >
>>
>> A good point. I just tested this and CR is a valid character to use in a
>> principal name in the allowed signers file and as of now the principal will be
>> passed to the verify call `as is` and everything works just fine. When we
>> introduce the patch above a principal with a CR in it will fail to verify.
>
>Are you sure? I thought that we split principals in allowed_signers on
>most whitespace, including \r. Follow:
>
>https://github.com/openssh/openssh-portable/blob/e9c7149/sshsig.c#L742
>https://github.com/openssh/openssh-portable/blob/e9c7149/misc.c#L452
>https://github.com/openssh/openssh-portable/blob/e9c7149/misc.c#L408
>

Sorry, I should have mentioned that I quoted the principal. Within the quotes whitespace (and \r) works. Since find-principals then returns one principal per line the line ending issue can come up.

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>> I've added Damien Miller to this thread. He knows more about what the expected
>> behaviour for the principal would/should be. I think at the moment almost
>> everything except \n or \0 goes. Maybe restricting \r as well would make life
>> easier for other uses too?
>
>IMO sensible content for the principals section would be printable, non-
>whitespace characters, excluding wildcards ('*', '?'). ssh-keygen mostly
>assumes that the file is in good order, but maybe it could be stricter.
>

Ok, I think we can make sure of that when adding principals and use Junios suggested patch for trimming the \r at the line ending.

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>> If we add `trust on first use` in a future series I would assume we use the
>> email address from the commit/tag author ident when adding a new principal to
>> the file. Can the ident contain a CR?
>> Even if it did, I would only allow a list of allowed alphanumeric chars to be
>> added anyway since a principal can contain wildcards which we obviously don't
>> want to trust on first use ;).
>
>Yeah. my mental model for the allowed_signers file is that it's similar
>to ~/.ssh/authorized_keys in that it directly controls authn/authz
>decisions, and if you put bad stuff in there then you're going to have
>a bad day...
>

Thanks for your input.