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bus1 / dbus-broker

Linux D-Bus Message Broker
https://github.com/bus1/dbus-broker/wiki
Apache License 2.0
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Client terminated while trying to send an L2CAP socket #364

Closed ford-prefect closed 1 month ago

ford-prefect commented 2 months ago

This may or may not be an actual bug in dbus-broker, but any pointers to help debug this would be most appreciated.

I have DBus method call that returns a file descriptor -- a socket corresponding to L2CAP LE connection-oriented channel (created with socket(PF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_L2CAP);).

If I try to put this fd in a message reply, dbus-broker silently terminates the connection (nothing is logged). If I replace this fd with something like open("/dev/null", O_RDONLY);, the reply works correctly.

dvdhrm commented 2 months ago

Hi! Thanks for the report. Do you have SELinux enabled? Does it work with it disabled?

Vudentz commented 2 months ago

@dvdhrm yeah, I suspect it needs some policy to let the fd be passed, or have SELinux disabled. @ford-prefect try with: sudo setenforce 0

ford-prefect commented 2 months ago

Oh, indeed, that was it! Thank you both. Happy to dig further to see what we'd need to allow.

dvdhrm commented 1 month ago

To clarify: The linux kernel usually reports SELinux denials via audit-messages. Furthermore, it returns EPERM so user-space can deal with it. Unfortunately, this is NOT the case if SELinux denies transmission of a file-descriptor. Instead, it will simply drop the respective file-descriptor from the message with NO indication of doing so.

dbus-broker sees the message and notices a mismatch between the intended number of file-descriptors, and the actual number of file-descriptors, and it has no way to tell whether this is a fault of the client, or an LSM denial.

At some point the kernel should really fix this, but I am unaware of a backwards-compatible way to solve this. It will likely require some additional annotation somewhere in the ancillary data.

I will close this bug now. Feel free to re-open if you feel like there is more need for discussion!

ford-prefect commented 1 month ago

This makes sense, thanks @dvdhrm!