On Fri, Jan 19, 2024 at 12:02 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 1/19/24 9:33 AM, Paul Moore wrote: > > Hello all, > > > > I just noticed the recent addition of IORING_OP_FIXED_FD_INSTALL and I > > see that it is currently written to skip the io_uring auditing. > > Assuming I'm understanding the patch correctly, and I'll admit that > > I've only looked at it for a short time today, my gut feeling is that > > we want to audit the FIXED_FD_INSTALL opcode as it could make a > > previously io_uring-only fd generally accessible to userspace. > > We can certainly remove the audit skip, it was mostly done as we're > calling into the security parts anyway later on. But it's not like doing > the extra audit here would cause any concerns on the io_uring front. Great. Do you want to put a patch together for that, or should I? > > I'm also trying to determine how worried we should be about > > io_install_fixed_fd() potentially happening with the current task's > > credentials overridden by the io_uring's personality. Given that this > > io_uring operation inserts a fd into the current process, I believe > > that we should be checking to see if the current task's credentials, > > and not the io_uring's credentials/personality, are allowed to receive > > the fd in receive_fd()/security_file_receive(). I don't see an > > obvious way to filter/block credential overrides on a per-opcode > > basis, but if we don't want to add a mask for io_kiocb::flags in > > io_issue_defs (or something similar), perhaps we can forcibly mask out > > REQ_F_CREDS in io_install_fixed_fd_prep()? I'm very interested to > > hear what others think about this. > > > > Of course if I'm reading the commit or misunderstanding the > > IORING_OP_FIXED_FD_INSTALL operation, corrections are welcome :) > > I think if there are concerns for that, the easiest solution would be to > just fail IORING_OP_FIXED_INSTALL if REQ_F_CREDS is set. I don't really > see a good way to have the security side know about the old creds, as > the task itself is running with the assigned creds. The more I've been thinking about it, yes, I believe there are concerns around FIXED_FD_INSTALL and io_uring personalities for LSMs. Assuming an io_uring with stored credentials for task A, yet accessible via task B, task B could submit an IORING_OP_OPENAT command to open a file using task A's creds and then FIXED_FD_INSTALL that fd into its own (task B's) file descriptor table without a problem as the installer's creds (the io_uring creds, or task A) match the file's creds (also task A since the io_uring opened the file). Following code paths in task B that end up going through security_file_permission() and similar hooks may very well end up catching the mismatch between the file's creds and task B (depending on the LSM), but arguably it is something that should have been caught at receive_fd() time. -- paul-moore.com
