sshd fails when using cryptodev-linux to compute hmac

Wed Oct 9 08:36:34 AEDT 2024

I don't know anything about cryptodev-linux, but I assume it's an
openssl engine? If so it's possible sshd's multiprocess model and/or
file descriptor handling is confusing it.

It's not a configuration we test, so you're mostly on your own to debug
it. It's entirely possible there's a bug there; if so, I'd expect it
to be something like a fd being closed while devcrypto is still
depending on it.

I'd suggest turning on LogVerbose=* so you can see which process
(represented by it's PID) is doing what, though that probably won't
be represented in the devcrypto debug messages unless you hack something
similar in.

On Tue, 8 Oct 2024, Peter Rashleigh wrote:

> Hi All,
> 
> I'm having an issue where SSH sessions fail if I enable the cryptodev engine for HMAC. I'd like to confirm if this is a supported configuration and if there are any known bugs.
> 
> HMAC with the cryptodev engine works fine when using the openssl application directly, so I suspect that something in openssh may be the cause of the issue.
> 
> I tried this initially with sshd from openssh 9.6p1, openssl 1.1.1v, and cryptodev-linux 1.12 on a custom buildroot Linux 6.1.53 running on ARMv8.
> My client is PuTTY on Windows using aes256-ctr/hmac-sha2-256 (but I get the same result using openssh on Linux with the same cipher settings).
> 
> When I attempt to connect, I get the following errors from sshd, and the client (PuTTY) reports that the server unexpectedly closed the connection. No login prompt is displayed.
> 
> 1995-11-22T00:18:33.755757+00:00 auth.info sshd[27262]: Corrupted MAC on input. [preauth]
> 1995-11-22T00:18:33.756013+00:00 auth.info sshd[27262]: ssh_dispatch_run_fatal: Connection from 192.168.254.100 port 54167: message authentication code incorrect [preauth]
> 
> Thinking that the issue might be fixed in a newer version, I then tried upgrading to openssh 9.8p1, openssl 3.3.2, and cryptodev-linux 1.13. Now, the session starts and I get a login prompt. After authentication, the MOTD and LastLog are printed, but then the session is terminated. The output of '/usr/sbin/sshd -ddd' is below. Note the "error in devcrypto" message - this happens because of an ioctl call (CIOCCPHASH) for a non-existent session ID (details below).
> 
> ....
> Starting session: shell on pts/0 for engadmin from 192.168.254.45 port 51222 id 0
> debug2: fd 4 setting TCP_NODELAY
> debug3: set_sock_tos: set socket 4 IP_TOS 0x48
> debug2: channel 0: rfd 11 isatty
> debug2: fd 11 setting O_NONBLOCK
> debug3: fd 9 is O_NONBLOCK
> debug1: Setting controlling tty using TIOCSCTTY.
> debug3: send packet: type 99
> channel_output_poll_input_open: channel 0: send data: error in libcrypto
> debug1: do_cleanup
> debug3: PAM: sshpam_thread_cleanup entering
> debug3: mm_request_receive: entering
> debug3: mm_request_receive: monitor fd closed
> debug1: mm_reap: preauth child exited with status 255
> debug1: do_cleanup
> debug1: PAM: cleanup
> debug1: PAM: closing session
> debug1: PAM: deleting credentials
> debug3: PAM: sshpam_thread_cleanup entering
> debug1: session_pty_cleanup2: session 0 release /dev/pts/0
> debug1: audit_event: unhandled event 12
> 
> Looking at the ioctl logging from cryptodev-linux, I can see that a session ID is being closed (CIOCFSESSION) before an attempt is made to copy from it (CIOCCPHASH). As you can see, crypto_destroy_session was called before a crypto_copy_hash_state for the same SID (0x9198779A):
> 
> ....
> 2024-10-08T18:52:23.314088+00:00 debug kernel: [ 1206.105427] cryptodev: sshd-session[2108] (cryptodev_ioctl:960): Got CIOCFSESSION
> 2024-10-08T18:52:23.314104+00:00 debug kernel: [ 1206.105473] cryptodev: sshd-session[2109] (crypto_destroy_session:372): Removed session 0x9198779A
> 2024-10-08T18:52:23.314113+00:00 debug kernel: [ 1206.105483] cryptodev: sshd-session[2109] (crypto_destroy_session:375): freeing space for 32 user pages
> 2024-10-08T18:52:23.314122+00:00 debug kernel: [ 1206.105500] cryptodev: sshd-session[2109] (cryptodev_ioctl:960): Got CIOCFSESSION
> 2024-10-08T18:52:23.314152+00:00 debug kernel: [ 1206.105598] cryptodev: sshd-session[2108] (crypto_destroy_session:372): Removed session 0xDE377830
> 2024-10-08T18:52:23.314229+00:00 debug kernel: [ 1206.105642] cryptodev: sshd-session[2108] (crypto_destroy_session:375): freeing space for 32 user pages
> 2024-10-08T18:52:23.314241+00:00 debug kernel: [ 1206.105662] cryptodev: sshd-session[2108] (cryptodev_ioctl:946): Got CIOCGSESSION
> 2024-10-08T18:52:23.314250+00:00 debug kernel: [ 1206.105680] cryptodev: sshd-session[2108] (crypto_create_session:314): got alignmask 0
> 2024-10-08T18:52:23.314255+00:00 debug kernel: [ 1206.105687] cryptodev: sshd-session[2108] (crypto_create_session:317): preallocating for 32 user pages
> 2024-10-08T18:52:23.314261+00:00 debug kernel: [ 1206.105700] cryptodev: sshd-session[2108] (cryptodev_ioctl:977): Got CIOCCPHASH
> 2024-10-08T18:52:23.314267+00:00 debug kernel: [ 1206.105711] cryptodev: sshd-session[2109] (crypto_destroy_session:372): Removed session 0x62D4D890
> 2024-10-08T18:52:23.314270+00:00 debug kernel: [ 1206.105718] cryptodev: sshd-session[2109] (crypto_destroy_session:375): freeing space for 32 user pages
> 2024-10-08T18:52:23.314273+00:00 debug kernel: [ 1206.105783] cryptodev: sshd-session[2108] (cryptodev_ioctl:960): Got CIOCFSESSION
> 2024-10-08T18:52:23.314315+00:00 debug kernel: [ 1206.105906] cryptodev: sshd-session[2108] (crypto_destroy_session:372): Removed session 0x6FD5C855
> 2024-10-08T18:52:23.314320+00:00 debug kernel: [ 1206.105949] cryptodev: sshd-session[2108] (crypto_destroy_session:375): freeing space for 32 user pages
> 2024-10-08T18:52:23.324801+00:00 debug kernel: [ 1206.115947] cryptodev: sshd-session[2108] (cryptodev_ioctl:946): Got CIOCGSESSION
> 2024-10-08T18:52:23.324821+00:00 debug kernel: [ 1206.115992] cryptodev: sshd-session[2108] (crypto_create_session:314): got alignmask 0
> 2024-10-08T18:52:23.324825+00:00 debug kernel: [ 1206.116000] cryptodev: sshd-session[2108] (crypto_create_session:317): preallocating for 32 user pages
> 2024-10-08T18:52:23.324828+00:00 debug kernel: [ 1206.116013] cryptodev: sshd-session[2108] (cryptodev_ioctl:977): Got CIOCCPHASH
> 2024-10-08T18:52:23.324850+00:00 err kernel: [ 1206.116019] cryptodev: sshd-session[2108] (crypto_copy_hash_state:516): Failed to get sesssions with sid=0x9198779A sid=f7160dd008X!
> ....
> 
> I spent some time tracing through the openssl and openssh code, and I found that If I comment out the call to mac_clear() within kex_free_newkeys(), the issue does not occur. Therefore, I wonder if there is some bug in openssh that causes re-use of a cryptodev session ID, or causes the digest to be cleaned up prematurely. I'd appreciate any feedback or suggestions. I assume that cryptodev should be well-supported and tested, so maybe there's some problem with my configuration. Has anyone got a setup like this working?
> 
> Thanks,
> 
> --
> Peter Rashleigh
> Embedded Developer
> Quester Tangent
> 
> 
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