On Mon, Mar 11, 2024 at 2:27 PM James Houghton jthoughton@google.com wrote:
On Mon, Mar 11, 2024 at 12:28 PM Peter Xu peterx@redhat.com wrote:
On Mon, Mar 11, 2024 at 11:59:59AM -0700, Axel Rasmussen wrote:
I'd prefer not to require root or CAP_SYS_ADMIN or similar for UFFDIO_POISON, because those control access to lots more things besides, which we don't necessarily want the process using UFFD to be able to do. :/
I agree; UFFDIO_POISON should not require CAP_SYS_ADMIN.
+1.
Ratelimiting seems fairly reasonable to me. I do see the concern about dropping some addresses though.
Do you know how much could an admin rely on such addresses? How frequent would MCE generate normally in a sane system?
I'm not sure about how much admins rely on the address themselves. +cc Jiaqi Yan
I think admins mostly care about MCEs from **real** hardware. For example they may choose to perform some maintenance if the number of hardware DIMM errors, keyed by PFN, exceeds some threshold. And I think mcelog or /sys/devices/system/node/node${X}/memory_failure are better tools than dmesg. In the case all memory errors are emulated by hypervisor after a live migration, these dmesgs may confuse admins to think there is dimm error on host but actually it is not the case. In this sense, silencing these emulated by UFFDIO_POISON makes sense (if not too complicated to do).
SIGBUS (and logged "MCE: Killing %s:%d due to hardware memory corruption fault at %lx\n") emit by fault handler due to UFFDIO_POISON are less useful to admins AFAIK. They are for sure crucial to userspace / vmm / hypervisor, but the SIGBUS sent already contains the poisoned address (in si_addr from force_sig_mceerr).
It's possible for a sane hypervisor dealing with a buggy guest / guest userspace to trigger lots of these pr_errs. Consider the case where a guest userspace uses HugeTLB-1G, finds poison (which HugeTLB used to ignore), and then ignores SIGBUS. It will keep getting MCEs / SIGBUSes.
The sane hypervisor will use UFFDIO_POISON to prevent the guest from re-accessing *real* poison, but we will still get the pr_err, and we still keep injecting MCEs into the guest. We have observed scenarios like this before.
Perhaps we can mitigate that concern by defining our own ratelimit interval/burst configuration?
Any details?
Another idea would be to only ratelimit it if !CONFIG_DEBUG_VM or similar. Not sure if that's considered valid or not. :)
This, OTOH, sounds like an overkill..
I just checked again on the detail of ratelimit code, where we by default it has:
#define DEFAULT_RATELIMIT_INTERVAL (5 * HZ) #define DEFAULT_RATELIMIT_BURST 10
So it allows a 10 times burst rather than 2.. IIUC it means even if there're continous 10 MCEs it won't get suppressed, until the 11th came, in 5 seconds interval. I think it means it's possibly even less of a concern to directly use pr_err_ratelimited().
I'm okay with any rate limiting everyone agrees on. IMO, silencing these pr_errs if they came from UFFDIO_POISON (or, perhaps, if they did not come from real hardware MCE events) sounds like the most correct thing to do, but I don't mind. Just don't make UFFDIO_POISON require CAP_SYS_ADMIN. :)
Thanks.