sshd fails when using cryptodev-linux to compute hmac

[Peter Rashleigh]

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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