Re: [PATCH] ssh signing: better error message when key not in agent

From: Fabian Stelzer <fs@gigacodes.de>
Date: Jan 23, 2023, 10:02 UTC
Message-ID: <20230123100245.3qbscxkgvbnh7ilt@fs>
In-Reply-To: <6e57bef8-7387-3341-5ed5-4bcfa7ded7a0@dunelm.org.uk>

On 23.01.2023 09:33, Phillip Wood wrote:

Show 43 quoted lines

>On 20/01/2023 09:03, Fabian Stelzer wrote:
>>On 18.01.2023 16:29, Phillip Wood wrote:
>>>Hi Adam
>>>
>>>I've cc'd Fabian who knows more about the ssh signing code that I do.
>>>
>>>On 18/01/2023 15:28, Adam Szkoda wrote:
>>>>Hi Phillip,
>>>>
>>>>Good point!  My first thought is to try doing a stat() syscall on the
>>>>path from 'user.signingKey' to see if it exists and if not, treat it
>>>>as a public key (and pass the -U option).  If that sounds reasonable,
>>>>I can update the patch.
>>>
>>>My reading of the documentation is that user.signingKey may point 
>>>to a public or private key so I'm not sure how stat()ing would 
>>>help. Looking at the code in sign_buffer_ssh() we have a function 
>>>is_literal_ssh_key() that checks if the config value is a public 
>>>key. When the user passes the path to a key we could read the file 
>>>check use is_literal_ssh_key() to check if it is a public key (or 
>>>possibly just check if the file begins with "ssh-"). Fabian - does 
>>>that sound reasonable?
>>
>>Hi,
>>I have encountered the mentioned problem before as well and tried to 
>>fix it but did not find a good / reasonable way to do so. Git just 
>>passes the user.signingKey to ssh-keygen which states:
>>`The key used for signing is specified using the -f option and may 
>>refer to either a private key, or a public key with the private half 
>>available via ssh-agent(1)`
>>
>>I don't think it's a good idea for git to parse the key and try to 
>>determine if it's public or private. The fix should probably be in 
>>openssh (different error message) but when looking into it last time 
>>i remember that the logic for using the key is quite deeply embedded 
>>into the ssh code and not easily adjusted for the signing use case. 
>>At the moment I don't have the time to look into it but the openssh 
>>code for signing is quite readable so feel free to give it a try. 
>>Maybe you find a good way.
>
>Thanks Fabian, perhaps the easiest way forward is for us to only pass 
>"-U" when we have a literal key in user.signingKey as we know it must 
>a be public key in that case.

Yes, i think that's a good idea as long as the `-U` flag is ignored in older ssh versions and shouldn't be too hard to implement. And it should work just as well when using `defaultKeyCommand`.

Best, Fabian

Show 44 quoted lines

>
>Best Wishes
>
>Phillip
>
>>Best regards,
>>Fabian
>>
>>>
>>>Best Wishes
>>>
>>>Phillip
>>>
>>>>Best
>>>>— Adam
>>>>
>>>>
>>>>On Wed, Jan 18, 2023 at 3:34 PM Phillip Wood 
>>>><phillip.wood123@gmail.com> wrote:
>>>>>
>>>>>On 18/01/2023 11:10, Phillip Wood wrote:
>>>>>>>the agent [1].  A fix is scheduled to be released in OpenSSH 9.1. All
>>>>>>>that
>>>>>>>needs to be done is to pass an additional backward-compatible option
>>>>>>>-U to
>>>>>>>'ssh-keygen -Y sign' call.  With '-U', ssh-keygen always interprets
>>>>>>>the file
>>>>>>>as public key and expects to find the private key in the agent.
>>>>>>
>>>>>>The documentation for user.signingKey says
>>>>>>
>>>>>>  If gpg.format is set to ssh this can contain the path to either your
>>>>>>private ssh key or the public key when ssh-agent is used.
>>>>>>
>>>>>>If I've understood correctly passing -U will prevent users 
>>>>>>from setting
>>>>>>this to a private key.
>>>>>
>>>>>If there is an easy way to tell if the user has given us a public key
>>>>>then we could pass "-U" in that case.
>>>>>
>>>>>Best Wishes
>>>>>
>>>>>Phillip