Messages in this thread | | | From | Lecopzer Chen <> | Subject | Re: [PATCH] ARM: mm: harden branch predictor before opening interrupts during fault | Date | Wed, 27 Jan 2021 00:03:03 +0800 |
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> On Tue, Jan 26, 2021 at 11:01:50PM +0800, Lecopzer Chen wrote: > > > On 2021-01-26 10:59:32 [+0000], Russell King - ARM Linux admin wrote: > > > > On Tue, Jan 26, 2021 at 05:17:08PM +0800, Lecopzer Chen wrote: > > > > > Hi all, > > > > > > > > > > I don't see any fix for this issue now(maybe I missed it..?), > > > > > could we fix this if there is better solution? > > > > > This issue exists almost two years. > > > > > > > > I don't think anyone provided an acceptable patch. > > > > > > > > The first patch moved the hardening out of the translation/section > > > > fault handling. Since the kernel is mapped with sections, these > > > > are above TASK_SIZE, and the whole point of the branch prediction > > > > hardening is to prevent the prediction in the kernel being exploited, > > > > missing the hardening effectively makes the mitigation useless. > > > > > > > > The discussion in February 2019 never concluded from what I can see. > > > > > > My memory is that I never got a reply which I understood. > > > Let me try again this week with the information above. > > > > > > NOTE: > > Before sending this mail, I had searched the relative threads and > > there are two solutions in general: > > 1. Add get_pcpu()/put_cpu() https://lkml.org/lkml/2019/6/3/426 > > Reject by Marc: > > > The right fix would be to move the call to a point where we haven't > > > enabled preemption yet. > > > > 2. Move out like the patch from Sebastian: > > This seems follow the concept of 1. > > (move the call to a point where we haven't enabled preemption yet). > > But I can't find any reply in the thread. > > > > Now the CONFIG_HARDEN_BRANCH_PREDICTOR has already backported to LTS, > > and after upgrading ARM CONFIG_CPU_V7 products to latest LTS, the > > CONFIG_HARDEN_BRANCH_PREDICTOR will be default y and this issue makes > > our devices panic and we have to either disable HARDEN_BRANCH_PREDICTOR > > or hack in-house to avoid the kernel panic. > > It does _not_ cause the kernel to panic, ever. A kernel panic takes > out the system. This is not the case here. > > It merely causes a noisy message to be emitted in the kernel log, and > the system survives. That is way more preferable than breaking the > effect of branch predictor hardening. > > If it is taking out your kernel with a real panic, then there is > something wrong elsewhere - and this is _not_ something that should > be happening during normal system operation.
Oh, yes, you're right;
After reread the panic log, our panic happened because -> invalid userspace memory access -> debug_preempt log -> the program seg fault -> main service need the program but it crash -> panic
Sorry for wrong information and thanks a lot for the correctness. I think I have to see why the in-house hacking is working...
Thanks!!
BRs, Lecopzer
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