On Mon, Apr 18, 2022 at 3:09 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Fri, 08 Apr 2022 21:03:28 +0100,
> Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> >
> > The hypervisor stacks (for both nVHE Hyp mode and nVHE protected mode)
> > are aligned such that any valid stack address has PAGE_SHIFT bit as 1.
> > This allows us to conveniently check for overflow in the exception entry
> > without corrupting any GPRs. We won't recover from a stack overflow so
> > panic the hypervisor.
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > ---
> >
> > Changes in v7:
> > - Add Fuad's Reviewed-by and Tested-by tags.
> >
> > Changes in v5:
> > - Valid stack addresses now have PAGE_SHIFT bit as 1 instead of 0
> >
> > Changes in v3:
> > - Remove test_sp_overflow macro, per Mark
> > - Add asmlinkage attribute for hyp_panic, hyp_panic_bad_stack, per Ard
> >
> >
> > arch/arm64/kvm/hyp/nvhe/host.S | 24 ++++++++++++++++++++++++
> > arch/arm64/kvm/hyp/nvhe/switch.c | 7 ++++++-
> > 2 files changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
> > index 3d613e721a75..be6d844279b1 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/host.S
> > +++ b/arch/arm64/kvm/hyp/nvhe/host.S
> > @@ -153,6 +153,18 @@ SYM_FUNC_END(__host_hvc)
> >
> > .macro invalid_host_el2_vect
> > .align 7
> > +
> > + /*
> > + * Test whether the SP has overflowed, without corrupting a GPR.
> > + * nVHE hypervisor stacks are aligned so that the PAGE_SHIFT bit
> > + * of SP should always be 1.
> > + */
> > + add sp, sp, x0 // sp' = sp + x0
> > + sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp
> > + tbz x0, #PAGE_SHIFT, .L__hyp_sp_overflow\@
> > + sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0
> > + sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp
> > +
> > /* If a guest is loaded, panic out of it. */
> > stp x0, x1, [sp, #-16]!
> > get_loaded_vcpu x0, x1
> > @@ -165,6 +177,18 @@ SYM_FUNC_END(__host_hvc)
> > * been partially clobbered by __host_enter.
> > */
> > b hyp_panic
> > +
> > +.L__hyp_sp_overflow\@:
> > + /*
> > + * Reset SP to the top of the stack, to allow handling the hyp_panic.
> > + * This corrupts the stack but is ok, since we won't be attempting
> > + * any unwinding here.
> > + */
> > + ldr_this_cpu x0, kvm_init_params + NVHE_INIT_STACK_HYP_VA, x1
> > + mov sp, x0
> > +
> > + bl hyp_panic_bad_stack
>
> Why bl? You clearly don't expect to return here, given that you have
> an ASM_BUG() right below, and that you are calling a __no_return
> function. I think we should be consistent with the rest of the code
> and just do a simple branch.
The idea was to use bl to give the hyp_panic_bad_stack() frame in the
stack trace, which makes it easy to identify overflows. I can add a
comment and drop the redundant ASM_BUG()
Thanks,
Kalesh
>
> It also gives us a chance to preserve an extra register from the
> context.
>
> > + ASM_BUG()
> > .endm
> >
> > .macro invalid_host_el1_vect
> > diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> > index 6410d21d8695..703a5d3f611b 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> > @@ -347,7 +347,7 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> > return exit_code;
> > }
> >
> > -void __noreturn hyp_panic(void)
> > +asmlinkage void __noreturn hyp_panic(void)
> > {
> > u64 spsr = read_sysreg_el2(SYS_SPSR);
> > u64 elr = read_sysreg_el2(SYS_ELR);
> > @@ -369,6 +369,11 @@ void __noreturn hyp_panic(void)
> > unreachable();
> > }
> >
> > +asmlinkage void __noreturn hyp_panic_bad_stack(void)
> > +{
> > + hyp_panic();
> > +}
> > +
> > asmlinkage void kvm_unexpected_el2_exception(void)
> > {
> > return __kvm_unexpected_el2_exception();
> > --
> > 2.35.1.1178.g4f1659d476-goog
> >
> >
>
> Thanks,
>
> M.
>
> --
> Without deviation from the norm, progress is not possible.
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