linux-kernel - Re: [PATCH v3] KVM: arm64: Use BTI for nvhe

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Date:   Tue, 4 Jul 2023 14:41:36 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Mostafa Saleh <smostafa@...gle.com>
Cc:     maz@...nel.org, oliver.upton@...ux.dev,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, tabba@...gle.com, qperret@...gle.com,
        will@...nel.org, catalin.marinas@....com, yuzenghui@...wei.com,
        suzuki.poulose@....com, james.morse@....com, bgardon@...gle.com,
        gshan@...hat.com
Subject: Re: [PATCH v3] KVM: arm64: Use BTI for nvhe

On Tue, May 30, 2023 at 03:08:45PM +0000, Mostafa Saleh wrote:
> CONFIG_ARM64_BTI_KERNEL compiles the kernel to support ARMv8.5-BTI.
> However, the nvhe code doesn't make use of it as it doesn't map any
> pages with Guarded Page(GP) bit.
> 
> kvm pgtable code is modified to map executable pages with GP bit
> if BTI is enabled for the kernel.
> 
> At hyp init, SCTLR_EL2.BT is set to 1 to match EL1 configuration
> (SCTLR_EL1.BT1) set in bti_enable().
> 
> One difference between kernel and nvhe code, is that the kernel maps
> .text with GP while nvhe maps all the executable pages, this makes
> nvhe code need to deal with special initialization code coming from
> other executable sections (.idmap.text).
> For this we need to add bti instruction at the beginning of
> __kvm_handle_stub_hvc as it can be called by  __host_hvc through
> branch instruction(br) and unlike SYM_FUNC_START, SYM_CODE_START
> doesn’t add bti instruction at the beginning, and it can’t be modified
> to add it as it is used with vector tables.
> Another solution which is more intrusive is to convert
> __kvm_handle_stub_hvc to a function and inject “bti jc” instead of
> “bti c” in SYM_FUNC_START
>

I was chasing a bug in linux-next yesterday with protected nVHE(pKVM) and
cpuidle enabled. The system fails to boot. I just bisected the issue to this
patch and also saw this patch landed in the linus tree yesterday/today.
Not sure if this is something to do with the fact that pKVM skips to
__kvm_handle_stub_hvc in __host_hvc.

Let me know if you want be to try something.

-- 
Regards,
Sudeep