On 1/19/24 10:20 AM, Paul Moore wrote: > 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? Either way - I'd say if you have time to do it, please do! Probably just include the REQ_F_CREDS change too. FWIW, I'd add that in io_uring/openclose.c:io_install_fixed_fd_prep() - just check for REQ_F_CREDS in there and return -EPERM (I think that would be appropriate?) and that should disallow any IORING_OP_FIXED_FD_INSTALL if creds have been reassigned. >>> 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. If there are any concerns, then I say let's just explicitly disable it rather than rely on maybe something in the security checking catching it. Especially because I don't think there's a valid use case for doing this, other than perhaps trying to bypass checks you'd normally hit. Better to err on the side of caution then. See above for a HOWTO, if in doubt. Thanks for looking into this! -- Jens Axboe
