From: Fabian Stelzer <fs@gigacodes.de>
To: Jeff King <peff@peff.net>
Cc: Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
git@vger.kernel.org,
Johannes Schindelin <johannes.schindelin@gmx.de>,
pedro martelletto <pedro@yubico.com>,
Damien Miller <djm@mindrot.org>
Subject: Re: [PATCH] gpg-interface: trim CR from ssh-keygen -Y find-principals
Date: Sat, 4 Dec 2021 14:11:49 +0100 [thread overview]
Message-ID: <20211204131149.cvyu7dvf6p66dotq@fs> (raw)
In-Reply-To: <Yao+l0ckDWZNf4AE@coredump.intra.peff.net>
On 03.12.2021 10:58, Jeff King wrote:
>On Fri, Dec 03, 2021 at 01:31:16PM +0000, Johannes Schindelin via GitGitGadget wrote:
>
>> We need to trim \r from the output of 'ssh-keygen -Y find-principals' on
>> Windows, or we end up calling 'ssh-keygen -Y verify' with a bogus signer
>> identity. ssh-keygen.c:2841 contains a call to puts(3), which confirms this
>> hypothesis. Signature verification passes with the fix.
>> [...]
>> @@ -497,7 +497,7 @@ static int verify_ssh_signed_buffer(struct signature_check *sigc,
>> if (!*line)
>> break;
>>
>> - trust_size = strcspn(line, "\n");
>> + trust_size = strcspn(line, "\r\n");
>> principal = xmemdupz(line, trust_size);
>
>Just playing devil's advocate for a moment: this parsing is kind of
>loose. Is there any chance that I could smuggle a CR into my principal
>name, and make "a principal\rthat is fake" now get parsed as "a
>principal"? Our strcspn() here would cut off at the first CR.
>
>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.
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?
From a security perspective I don't think this is problem. The principal
does not come from any user input but is actually looked up in the allowed
signers file using the signatures public key (with ssh-keygen -Y
find-principals). If I could manipulate this file I could change the key as
well.
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 ;).
Thanks
next prev parent reply other threads:[~2021-12-04 13:11 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-03 13:31 [PATCH] gpg-interface: trim CR from ssh-keygen -Y find-principals Johannes Schindelin via GitGitGadget
2021-12-03 14:18 ` Fabian Stelzer
2021-12-03 15:58 ` Jeff King
2021-12-04 13:11 ` Fabian Stelzer [this message]
2021-12-05 5:50 ` Junio C Hamano
[not found] ` <CABPYr=y+sDDko9zPxQTOM6Tz4E7CafH7hJc6oB1zv7XYA9KH1A@mail.gmail.com>
2021-12-09 16:33 ` Fabian Stelzer
[not found] ` <CABPYr=xfotWvTQK9k1eKHa0kP4SsB=TKKuM0d8cpMb5BtuUZLA@mail.gmail.com>
2021-12-09 17:20 ` Fabian Stelzer
2021-12-30 10:25 ` Fabian Stelzer
2021-12-05 23:06 ` Damien Miller
2021-12-06 8:39 ` Fabian Stelzer
2022-01-03 9:53 ` [PATCH v2] gpg-interface: trim CR from ssh-keygen Fabian Stelzer
2022-01-03 17:17 ` Eric Sunshine
2022-01-03 23:34 ` Junio C Hamano
2022-01-04 0:41 ` Eric Sunshine
2022-01-04 1:19 ` Junio C Hamano
2022-01-04 3:06 ` Eric Sunshine
2022-01-04 12:55 ` Fabian Stelzer
2022-01-04 19:33 ` Junio C Hamano
2022-01-05 7:09 ` Eric Sunshine
2022-01-05 10:36 ` Fabian Stelzer
2022-01-05 20:40 ` Junio C Hamano
2022-01-06 10:26 ` Fabian Stelzer
2022-01-06 17:50 ` Junio C Hamano
2022-01-09 20:49 ` Eric Sunshine
2022-01-10 12:28 ` Fabian Stelzer
2022-01-07 9:07 ` [PATCH v3] " Fabian Stelzer
2022-01-09 21:37 ` Eric Sunshine
2022-01-10 12:59 ` Fabian Stelzer
2022-01-10 17:51 ` Junio C Hamano
2022-01-10 17:03 ` Junio C Hamano
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