Messages in this thread | | | Date | Thu, 4 Jan 2018 11:27:04 +0100 | From | Willy Tarreau <> | Subject | Re: Linux 4.15-rc6 |
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On Wed, Jan 03, 2018 at 01:20:00PM -0800, Andres Freund wrote: > On 2018-01-03 13:57:25 +0100, Willy Tarreau wrote: > > I think we should start to think about an option to > > disable this per process. We could imagine for example a prctl() > > requiring CAP_SYS_ADMIN to disable it. This would at least allow > > processes started as root to disable it when they consider themselves > > irrelevant to this kind of protection (mostly I/O intensive or network > > intensive applications). > > That might not be a bad idea. If so, it'd be a good idea to keep it > separate from CAP_SYS_ADMIN. E.g. postgres refuses to run as root,
There's a difference between "running as" and "starting as" (eg in haproxy we encourage to *start as root* but not to *run as root*, this allows the process to chroot to /var/empty and drop all privileges). But I get your point, it's important to adapt to what various programs will require.
> but > setcap'ing to allow CAP_SYS_LIVE_AND_LET_LIVE_SYSCALL or such would > work.
yes probably.
> But I suspect this isn't something easily done on a capability/prctl > level? Seems not uncomplicated to change this after a process has > already been created - so maybe it'd be easier to force this via > personality()?
I don't know. One solution when you perform changes that effect the running process' VMA is to re-exec itself after the change :
if (pti_protection_enabled && prctl(PR_SET_PTI, PR_PTI_DISABLE) == 0) exit(execve(argv[0], argv, envp));
> > > > This isn't a complaint, I just thought it might be useful > > > > information. If it helps for anything/anybody, I'm happy to run > > > > additional benchmarks / provide additional information. > > > > > > Note that it will depend heavily on the hardware too. Older CPU's > > > without PCID will be impacted more by the isolation. > > > > Interesting. This CPU has PCID, so it's possible that older hardware > > may indeed be hit a bit more. > > The post linked above has numbers with nopcid disabling pcid use, and > indeed, the difference is quite measurable.
I'm going to re-run the tests on an Atom C2518 now, which doesn't have pcid, I don't even know if it's affected by the issue.
Cheers, Willy
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