Messages in this thread Patch in this message | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/speculation: Allow overriding seccomp speculation disable | Date | Sat, 21 Mar 2020 15:46:29 +0100 |
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Andi Kleen <andi@firstfloor.org> writes:
Cc+: Seccomp maintainers ....
> From: Andi Kleen <ak@linux.intel.com> > > seccomp currently force enables the SSBD and IB mitigations, > which disable certain features in the CPU to avoid speculation > attacks at a performance penalty. > > This is a heuristic to detect applications that may run untrusted code > (such as web browsers) and provide mitigation for them. > > At least for SSBD the mitigation is really only for side channel > leaks inside processes. > > There are two cases when the heuristic has problems: > > - The seccomp user has a superior mitigation and doesn't need the > CPU level disables. For example for a Web Browser this is using > site isolation, which separates different sites in different > processes, so side channel leaks inside a process are not > of a concern. > > - Another case are seccomp users who don't run untrusted code, > such as sshd, and don't really benefit from SSBD > > As currently implemented seccomp force enables the mitigation > so it's not possible for processes to opt-in that they don't > need mitigations (such as when they already use site isolation). > > In some cases we're seeing significant performance penalties > of enabling the SSBD mitigation on web workloads. > > This patch changes the seccomp code to not force enable,
I'm sure I asked you to do
git grep "This patch" Documentation/process/
before.
> but merely enable, the SSBD and IB mitigations. > > This allows processes to use the PR_SET_SPECULATION prctl > after running seccomp and reenable SSBD and/or IB > if they don't need any extra mitigation. > > The effective default has not changed, it just allows > processes to opt-out of the default. > > It's not clear to me what the use case for the force > disable is anyways. Certainly if someone controls the process, > and can run prctl(), they can leak data in all kinds of > ways anyways, or just read the whole memory map. > > Longer term we probably need to discuss if the seccomp heuristic > is still warranted and should be perhaps changed. It seemed > like a good idea when these vulnerabilities were new, and > no web browsers supported site isolation. But with site isolation > widely deployed -- Chrome has it on by default, and as I understand > it, Firefox is going to enable it by default soon. And other seccomp > users (like sshd or systemd) probably don't really need it. > Given that it's not clear the default heuristic is still a good > idea. > > But anyways this patch doesn't change any defaults, just > let's applications override it.
It changes the enforcement and I really want the seccomp people to have a say here.
Thanks,
tglx
> Signed-off-by: Andi Kleen <ak@linux.intel.com> > --- > arch/x86/kernel/cpu/bugs.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c > index ed54b3b21c39..f15ae9bfd7ad 100644 > --- a/arch/x86/kernel/cpu/bugs.c > +++ b/arch/x86/kernel/cpu/bugs.c > @@ -1215,9 +1215,9 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which, > void arch_seccomp_spec_mitigate(struct task_struct *task) > { > if (ssb_mode == SPEC_STORE_BYPASS_SECCOMP) > - ssb_prctl_set(task, PR_SPEC_FORCE_DISABLE); > + ssb_prctl_set(task, PR_SPEC_DISABLE); > if (spectre_v2_user == SPECTRE_V2_USER_SECCOMP) > - ib_prctl_set(task, PR_SPEC_FORCE_DISABLE); > + ib_prctl_set(task, PR_SPEC_DISABLE); > } > #endif > > -- > 2.24.1
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