KVM: arm64: Survive synchronous exceptions caused by AT instructions

commit 88a84ccccb3966bcc3f309cdb76092a9892c0260 upstream.

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>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index d929b4c4df1fb17823409e6dd01b49f86b0c327a..64d79b2884344059fe73246f7379dde58a1b6fd5 100644 (file)
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -88,6 +88,34 @@ extern u32 __kvm_get_mdcr_el2(void);
                *__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 c8cdadb3871983104ef1df6500a3b3d9e0d1866e..f36aad0f207bb582206e89cbe75c914df8c80032 100644 (file)
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -166,13 +166,19 @@ 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/switch.c b/arch/arm64/kvm/hyp/switch.c
index 5bb4e13d3c226cb94ee3333b35e4610f2ca8f833..65660b6144740f664adbf3768c1f095553d7d509 100644 (file)
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -261,10 +261,10 @@ static bool __hyp_text __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))