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KVM: arm64: Safety check PSTATE when entering guest and handle IL

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This commit adds a paranoid check when entering the guest to make sure
we don't attempt running guest code in an equally or more privilged mode
than the hypervisor.  We also catch other accidental programming of the
SPSR_EL2 which results in an illegal exception return and report this
safely back to the user.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
This commit is contained in:
Christoffer Dall 2018-10-17 20:21:16 +02:00 committed by Marc Zyngier
parent 58bf437ff6
commit e4e11cc0f8
5 changed files with 44 additions and 2 deletions
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1
arch/arm64/include/asm/kvm_asm.h
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@ -30,6 +30,7 @@
#define ARM_EXCEPTION_IRQ 0
#define ARM_EXCEPTION_EL1_SERROR 1
#define ARM_EXCEPTION_TRAP 2
#define ARM_EXCEPTION_IL 3
/* The hyp-stub will return this for any kvm_call_hyp() call */
#define ARM_EXCEPTION_HYP_GONE HVC_STUB_ERR

3
arch/arm64/include/asm/ptrace.h
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@ -25,6 +25,9 @@
#define CurrentEL_EL1 (1 << 2)
#define CurrentEL_EL2 (2 << 2)
/* Additional SPSR bits not exposed in the UABI */
#define PSR_IL_BIT (1 << 20)
/* AArch32-specific ptrace requests */
#define COMPAT_PTRACE_GETREGS 12
#define COMPAT_PTRACE_SETREGS 13

7
arch/arm64/kvm/handle_exit.c
View File

@ -284,6 +284,13 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
*/
run->exit_reason = KVM_EXIT_FAIL_ENTRY;
return 0;
case ARM_EXCEPTION_IL:
/*
* We attempted an illegal exception return. Guest state must
* have been corrupted somehow. Give up.
*/
run->exit_reason = KVM_EXIT_FAIL_ENTRY;
return -EINVAL;
default:
kvm_pr_unimpl("Unsupported exception type: %d",
exception_index);

16
arch/arm64/kvm/hyp/hyp-entry.S
View File

@ -162,6 +162,20 @@ el1_error:
mov x0, #ARM_EXCEPTION_EL1_SERROR
b __guest_exit
el2_sync:
/* Check for illegal exception return, otherwise panic */
mrs x0, spsr_el2
/* if this was something else, then panic! */
tst x0, #PSR_IL_BIT
b.eq __hyp_panic
/* Let's attempt a recovery from the illegal exception return */
get_vcpu_ptr x1, x0
mov x0, #ARM_EXCEPTION_IL
b __guest_exit
el2_error:
ldp x0, x1, [sp], #16
@ -240,7 +254,7 @@ ENTRY(__kvm_hyp_vector)
invalid_vect el2t_fiq_invalid // FIQ EL2t
invalid_vect el2t_error_invalid // Error EL2t
invalid_vect el2h_sync_invalid // Synchronous EL2h
valid_vect el2_sync // Synchronous EL2h
invalid_vect el2h_irq_invalid // IRQ EL2h
invalid_vect el2h_fiq_invalid // FIQ EL2h
valid_vect el2_error // Error EL2h

19
arch/arm64/kvm/hyp/sysreg-sr.c
View File

@ -152,8 +152,25 @@ static void __hyp_text __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
static void __hyp_text
__sysreg_restore_el2_return_state(struct kvm_cpu_context *ctxt)
{
u64 pstate = ctxt->gp_regs.regs.pstate;
u64 mode = pstate & PSR_AA32_MODE_MASK;
/*
* Safety check to ensure we're setting the CPU up to enter the guest
* in a less privileged mode.
*
* If we are attempting a return to EL2 or higher in AArch64 state,
* program SPSR_EL2 with M=EL2h and the IL bit set which ensures that
* we'll take an illegal exception state exception immediately after
* the ERET to the guest. Attempts to return to AArch32 Hyp will
* result in an illegal exception return because EL2's execution state
* is determined by SCR_EL3.RW.
*/
if (!(mode & PSR_MODE32_BIT) && mode >= PSR_MODE_EL2t)
pstate = PSR_MODE_EL2h | PSR_IL_BIT;
write_sysreg_el2(ctxt->gp_regs.regs.pc, elr);
write_sysreg_el2(ctxt->gp_regs.regs.pstate, spsr);
write_sysreg_el2(pstate, spsr);
if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN))
write_sysreg_s(ctxt->sys_regs[DISR_EL1], SYS_VDISR_EL2);