PKCS#11 hardware module not work for ssh-add

[hehaoqian]

Prerequisites

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Steps to reproduce

TLDR,

The PKCS#11 of Windows OpenSSH mostly works,
but it lacks ssh-agent -P option,
so I cannot pass PKCS#11 library to ssh-add -s

---

My ssh private key is stored in a YubiKey hardware module,
and I want to use the private key to do ssh login,
and forward the keys to use them in remote machines, using ssh-agent

I can verify that my Yubikey works,
by successfully do the things above on Linux

Step to reproduce on Windows

1. Download yubico-piv-tool from https://developers.yubico.com/yubico-piv-tool/Releases/. Use yubico-piv-tool-2.7.2-win64.msi
2. Install the MSI
3. The installed program is located at C:\Program Files\Yubico\Yubico PIV Tool
4. Copy all DLLs inside C:\Program Files\Yubico\Yubico PIV Tool\bin into C:\Windows\System32\OpenSSH
5. Plug in my USB Yubikey into the computer
6. Use command ssh-keygen -D libykcs11.dll, it works as intended, that it prints out the public key stored in the Yubikey hardware module
7. Use command ssh -I libykcs11.dll username@hostname, it works as intended, that I ssh login without password, using the Yubikey to authenticate
8. ssh-add -s libykcs11.dll or ssh-add -s C:\Windows\System32\OpenSSH\libykcs11.dll, in order to add the hardware module into ssh-agent, DOES NOT WORK. The error message is Could not add card "libykcs11.dll": agent refused operation

From the experience of using ssh agent forwarding on Linux, there is an allowlist for module used by ssh-add -s, where can be specified by ssh-agent -P

The man page of ssh-agent on Linux says:

-P allowed_providers

Specify a pattern-list of acceptable paths for PKCS#11 provider and 
FIDO authenticator middleware shared libraries that may be used with the -S or -s options to ssh-add(1). 
Libraries that do not match the pattern list will be refused. 
See PATTERNS in ssh_config(5) for a description of pattern-list syntax. 
The default list is "/usr/lib/*,/usr/local/lib*"

Pass a -P option to ssh-agent in Linux can be easily done in ~/.bashrc,
but I have no idea how to do this in Windows

The start of ssh-agent on Windows is handled by OpenSSH Authentication Agent Service,
and there is no way to pass argument to it.

Start it manually by ssh-agent.exe -P C:\Windows\System32\OpenSSH\libykcs11.dll does not work either,
that I checked the Command Line by the "Process Explorer" of Sysinternal,
that the Command Line does not include -P

I think PCKS#11 of Windows mostly works,
because ssh-keygen -D and ssh -I works as intended.
Just the ssh-agent -P need some work.

May be change the default allowlist of ssh-agent -P?

Expected behavior

`ssh-add -s libykcs11.dll` succeeds,
and `ssh-add -l` should list the keys stored in Yubikey

Actual behavior

`ssh-add -s libykcs11.dll` errors with
`Could not add card "libykcs11.dll": agent refused operation`

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      5.1.26100.6899
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.26100.6899
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Version

OpenSSH_for_Windows_9.5p2, LibreSSL 3.8.2

Visuals

No response

[hehaoqian]

Well, I checked the release note of [v9.8.0.0p1-Preview](https://github.com/PowerShell/Win32-OpenSSH/releases/tag/v9.8.0.0p1-Preview), it says

[ssh-agent] validate a PKCS11 library path based on allow-list configurable via -P, with default allow-list set to $env:ProgramFiles and $env:ProgramFiles(x86) - https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-38029.

So I put the libykcs11.dll and related dlls inside C:\Program Files, and ssh-add -s works.
I have no idea why Windows choose those paths as the default, that Program Files normally does not contain any dll files

[tgauth]

Pass a -P option to ssh-agent in Linux can be easily done in ~/.bashrc,
but I have no idea how to do this in Windows

To pass the -P option to the ssh-agent service on Windows, you can configure the service startup in the Registry to include it under the HKLM\SYSTEM\CurrentControlSet\Services\ssh-agent key's ImagePath value