KVM: arm64: Survive synchronous exceptions caused by AT instructions

KVM doesn't expect any synchronous exceptions when executing, any such
exception leads to a panic(). AT instructions access the guest page
tables, and can cause a synchronous external abort to be taken.

The arm-arm is unclear on what should happen if the guest has configured
the hardware update of the access-flag, and a memory type in TCR_EL1 that
does not support atomic operations. B2.2.6 "Possible implementation
restrictions on using atomic instructions" from DDI0487F.a lists
synchronous external abort as a possible behaviour of atomic instructions
that target memory that isn't writeback cacheable, but the page table
walker may behave differently.

Make KVM robust to synchronous exceptions caused by AT instructions.
Add a get_user() style helper for AT instructions that returns -EFAULT
if an exception was generated.

While KVM's version of the exception table mixes synchronous and
asynchronous exceptions, only one of these can occur at each location.

Re-enter the guest when the AT instructions take an exception on the
assumption the guest will take the same exception. This isn't guaranteed
to make forward progress, as the AT instructions may always walk the page
tables, but guest execution may use the translation cached in the TLB.

This isn't a problem, as since commit 5dcd0fdbb492 ("KVM: arm64: Defer guest
entry when an asynchronous exception is pending"), KVM will return to the
host to process IRQs allowing the rest of the system to keep running.

Cc: stable@vger.kernel.org # <v5.3: 5dcd0fdbb492 ("KVM: arm64: Defer guest entry when an asynchronous exception is pending")
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 63350f0..6f98fbd 100644 (file)
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -169,6 +169,34 @@ extern char __smccc_workaround_1_smc[__SMCCC_WORKAROUND_1_SMC_SZ];
                *__hyp_this_cpu_ptr(sym);                               \
         })
 
+#define __KVM_EXTABLE(from, to)                                                \
+       "       .pushsection    __kvm_ex_table, \"a\"\n"                \
+       "       .align          3\n"                                    \
+       "       .long           (" #from " - .), (" #to " - .)\n"       \
+       "       .popsection\n"
+
+
+#define __kvm_at(at_op, addr)                                          \
+( {                                                                    \
+       int __kvm_at_err = 0;                                           \
+       u64 spsr, elr;                                                  \
+       asm volatile(                                                   \
+       "       mrs     %1, spsr_el2\n"                                 \
+       "       mrs     %2, elr_el2\n"                                  \
+       "1:     at      "at_op", %3\n"                                  \
+       "       isb\n"                                                  \
+       "       b       9f\n"                                           \
+       "2:     msr     spsr_el2, %1\n"                                 \
+       "       msr     elr_el2, %2\n"                                  \
+       "       mov     %w0, %4\n"                                      \
+       "9:\n"                                                          \
+       __KVM_EXTABLE(1b, 2b)                                           \
+       : "+r" (__kvm_at_err), "=&r" (spsr), "=&r" (elr)                \
+       : "r" (addr), "i" (-EFAULT));                                   \
+       __kvm_at_err;                                                   \
+} )
+
+
 #else /* __ASSEMBLY__ */
 
 .macro hyp_adr_this_cpu reg, sym, tmp

diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index e47547e..46b4dab 100644 (file)
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -167,13 +167,19 @@ el1_error:
        b       __guest_exit
 
 el2_sync:
-       /* Check for illegal exception return, otherwise panic */
+       /* Check for illegal exception return */
        mrs     x0, spsr_el2
+       tbnz    x0, #20, 1f
 
-       /* if this was something else, then panic! */
-       tst     x0, #PSR_IL_BIT
-       b.eq    __hyp_panic
+       save_caller_saved_regs_vect
+       stp     x29, x30, [sp, #-16]!
+       bl      kvm_unexpected_el2_exception
+       ldp     x29, x30, [sp], #16
+       restore_caller_saved_regs_vect
+
+       eret
 
+1:
        /* Let's attempt a recovery from the illegal exception return */
        get_vcpu_ptr    x1, x0
        mov     x0, #ARM_EXCEPTION_IL

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 6d8d4ed..5b6b8fa 100644 (file)
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -146,10 +146,10 @@ static inline bool __translate_far_to_hpfar(u64 far, u64 *hpfar)
         * saved the guest context yet, and we may return early...
         */
        par = read_sysreg(par_el1);
-       asm volatile("at s1e1r, %0" : : "r" (far));
-       isb();
-
-       tmp = read_sysreg(par_el1);
+       if (!__kvm_at("s1e1r", far))
+               tmp = read_sysreg(par_el1);
+       else
+               tmp = SYS_PAR_EL1_F; /* back to the guest */
        write_sysreg(par, par_el1);
 
        if (unlikely(tmp & SYS_PAR_EL1_F))