Re: [RFC PATCH 2/9] audit,io_uring,io-wq: add some basic audit support to io_uring

Am 26.05.21 um 16:38 schrieb Paul Moore: > On Wed, May 26, 2021 at 6:19 AM Pavel Begunkov <asml.silence(a)gmail.com&gt; wrote: >> On 5/26/21 3:04 AM, Paul Moore wrote: >>> On Tue, May 25, 2021 at 9:11 PM Jens Axboe <axboe(a)kernel.dk&gt; wrote: >>>> On 5/24/21 1:59 PM, Paul Moore wrote: >>>>> That said, audit is not for everyone, and we have build time and >>>>> runtime options to help make life easier. Beyond simply disabling >>>>> audit at compile time a number of Linux distributions effectively >>>>> shortcut audit at runtime by adding a "never" rule to the audit >>>>> filter, for example: >>>>> >>>>> % auditctl -a task,never >>>> >>>> As has been brought up, the issue we're facing is that distros have >>>> CONFIG_AUDIT=y and hence the above is the best real world case outside >>>> of people doing custom kernels. My question would then be how much >>>> overhead the above will add, considering it's an entry/exit call per op. >>>> If auditctl is turned off, what is the expectation in turns of overhead? >>> >>> I commented on that case in my last email to Pavel, but I'll try to go >>> over it again in a little more detail. >>> >>> As we discussed earlier in this thread, we can skip the req->opcode >>> check before both the _entry and _exit calls, so we are left with just >>> the bare audit calls in the io_uring code. As the _entry and _exit >>> functions are small, I've copied them and their supporting functions >>> below and I'll try to explain what would happen in CONFIG_AUDIT=y, >>> "task,never" case. >>> >>> + static inline struct audit_context *audit_context(void) >>> + { >>> + return current->audit_context; >>> + } >>> >>> + static inline bool audit_dummy_context(void) >>> + { >>> + void *p = audit_context(); >>> + return !p || *(int *)p; >>> + } >>> >>> + static inline void audit_uring_entry(u8 op) >>> + { >>> + if (unlikely(audit_enabled && audit_context())) >>> + __audit_uring_entry(op); >>> + } >> >> I'd rather agree that it's my cycle-picking. The case I care about >> is CONFIG_AUDIT=y (because everybody enable it), and io_uring >> tracing _not_ enabled at runtime. If enabled let them suffer >> the overhead, it will probably dip down the performance >> >> So, for the case I care about it's two of >> >> if (unlikely(audit_enabled && current->audit_context)) >> >> in the hot path. load-test-jump + current, so it will >> be around 7x2 instructions. We can throw away audit_enabled >> as you say systemd already enables it, that will give >> 4x2 instructions including 2 conditional jumps. > > We've basically got it down to the equivalent of two > "current->audit_context != NULL" checks in the case where audit is > built into the kernel but disabled at runtime, e.g. CONFIG_AUDIT=y and > "task,never". I'm at a loss for how we can lower the overhead any > further, but I'm open to suggestions. > >> That's not great at all. And that's why I brought up >> the question about need of pre and post hooks and whether >> can be combined. Would be just 4 instructions and that is >> ok (ish). > > As discussed previously in this thread that isn't really an option > from an audit perspective. > >>> We would need to check with the current security requirements (there >>> are distro people on the linux-audit list that keep track of that >>> stuff), but looking at the opcodes right now my gut feeling is that >>> most of the opcodes would be considered "security relevant" so >>> selective auditing might not be that useful in practice. It would >>> definitely clutter the code and increase the chances that new opcodes >>> would not be properly audited when they are merged. >> >> I'm curious, why it's enabled by many distros by default? Are there >> use cases they use? > > We've already talked about certain users and environments where audit > is an important requirement, e.g. public sector, health care, > financial institutions, etc.; without audit Linux wouldn't be an > option for these users, at least not without heavy modification, > out-of-tree/ISV patches, etc. I currently don't have any direct ties > to any distros, "Enterprise" or otherwise, but in the past it has been > my experience that distros much prefer to have a single kernel build > to address the needs of all their users. In the few cases I have seen > where a second kernel build is supported it is usually for hardware > enablement. I'm sure there are other cases too, I just haven't seen > them personally; the big distros definitely seem to have a strong > desire to limit the number of supported kernel configs/builds. > >> Tempting to add AUDIT_IOURING=default N, but won't work I guess > > One of the nice things about audit is that it can give you a history > of what a user did on a system, which is very important for a number > of use cases. If we selectively disable audit for certain subsystems > we create a blind spot in the audit log, and in the case of io_uring > this can be a very serious blind spot. I fear that if we can't come > to some agreement here we will need to make io_uring and audit > mutually exclusive at build time which would be awful; forcing many > distros to either make a hard choice or carry out-of-tree patches. I'm wondering why it's not enough to have the native auditing just to happen.

E.g. all (I have checked RECVMSG,SENDMSG,SEND and CONNECT) socket related io_uring opcodes already go via security_socket_{recvmsg,sendmsg,connect}() IORING_OP_OPENAT* goes via do_filp_open() which is in common with the open[at[2]]() syscalls and should also trigger audit_inode() and security_file_open().

So why is there anything special needed for io_uring (now that the native worker threads are used)?

Is there really any io_uring opcode that bypasses the security checks the corresponding native syscall would do? If so, I think that should just be fixed...

Additional LSM based restrictions could be hooked into the io_check_restriction() path and setup at io_uring_setup() or early io_uring_register() time. What do you think? metze