From: Junio C Hamano <gitster@pobox.com>
To: Fabian Stelzer <fs@gigacodes.de>
Cc: Jeff King <peff@peff.net>,
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, 04 Dec 2021 21:50:38 -0800 [thread overview]
Message-ID: <xmqqk0gjob0x.fsf@gitster.g> (raw)
In-Reply-To: <20211204131149.cvyu7dvf6p66dotq@fs> (Fabian Stelzer's message of "Sat, 4 Dec 2021 14:11:49 +0100")
Fabian Stelzer <fs@gigacodes.de> writes:
> 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 ;).
So instead of the posted patch, we should do something along this
line instead?
trust_size = strcspn(line, "\n"); /* truncate at LF */
if (trust_size && line[trust_size - 1] == '\r')
trust_size--; /* the LF was part of CRLF at the end */
next prev parent reply other threads:[~2021-12-05 5:50 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
2021-12-05 5:50 ` Junio C Hamano [this message]
[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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