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

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.