arm64: KVM: Introduce EL2 VA randomisation

The main idea behind randomising the EL2 VA is that we usually have
a few spare bits between the most significant bit of the VA mask
and the most significant bit of the linear mapping.

Those bits could be a bunch of zeroes, and could be useful
to move things around a bit. Of course, the more memory you have,
the less randomisation you get...

Alternatively, these bits could be the result of KASLR, in which
case they are already random. But it would be nice to have a
*different* randomization, just to make the job of a potential
attacker a bit more difficult.

Inserting these random bits is a bit involved. We don't have a spare
register (short of rewriting all the kern_hyp_va call sites), and
the immediate we want to insert is too random to be used with the
ORR instruction. The best option I could come up with is the following
sequence:

	and x0, x0, #va_mask
	ror x0, x0, #first_random_bit
	add x0, x0, #(random & 0xfff)
	add x0, x0, #(random >> 12), lsl #12
	ror x0, x0, #(63 - first_random_bit)

making it a fairly long sequence, but one that a decent CPU should
be able to execute without breaking a sweat. It is of course NOPed
out on VHE. The last 4 instructions can also be turned into NOPs
if it appears that there is no free bits to use.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 3836641dd6f065ffb9921e5fba1e67fc386370e4..a84e2dd14ebe82dfd1e94ec30257889d3c3e0636 100644 (file)
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -85,7 +85,11 @@
  */
 .macro kern_hyp_va     reg
 alternative_cb kvm_update_va_mask
-       and     \reg, \reg, #1
+       and     \reg, \reg, #1          /* mask with va_mask */
+       ror     \reg, \reg, #1          /* rotate to the first tag bit */
+       add     \reg, \reg, #0          /* insert the low 12 bits of the tag */
+       add     \reg, \reg, #0, lsl 12  /* insert the top 12 bits of the tag */
+       ror     \reg, \reg, #63         /* rotate back */
 alternative_cb_end
 .endm
 
@@ -102,7 +106,11 @@ void kvm_update_va_mask(struct alt_instr *alt,
 
 static inline unsigned long __kern_hyp_va(unsigned long v)
 {
-       asm volatile(ALTERNATIVE_CB("and %0, %0, #1\n",
+       asm volatile(ALTERNATIVE_CB("and %0, %0, #1\n"
+                                   "ror %0, %0, #1\n"
+                                   "add %0, %0, #0\n"
+                                   "add %0, %0, #0, lsl 12\n"
+                                   "ror %0, %0, #63\n",
                                    kvm_update_va_mask)
                     : "+r" (v));
        return v;

diff --git a/arch/arm64/kvm/va_layout.c b/arch/arm64/kvm/va_layout.c
index 7998d1a60916241c4ad9f793b64b7d4ab1f9622a..3d41a480b6a52865cd256c0cadb6d913e37ad287 100644 (file)
--- a/arch/arm64/kvm/va_layout.c
+++ b/arch/arm64/kvm/va_layout.c
@@ -16,24 +16,60 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/random.h>
+#include <linux/memblock.h>
 #include <asm/alternative.h>
 #include <asm/debug-monitors.h>
 #include <asm/insn.h>
 #include <asm/kvm_mmu.h>
 
+/*
+ * The LSB of the random hyp VA tag or 0 if no randomization is used.
+ */
+static u8 tag_lsb;
+/*
+ * The random hyp VA tag value with the region bit if hyp randomization is used
+ */
+static u64 tag_val;
 static u64 va_mask;
 
 static void compute_layout(void)
 {
        phys_addr_t idmap_addr = __pa_symbol(__hyp_idmap_text_start);
        u64 hyp_va_msb;
+       int kva_msb;
 
        /* Where is my RAM region? */
        hyp_va_msb  = idmap_addr & BIT(VA_BITS - 1);
        hyp_va_msb ^= BIT(VA_BITS - 1);
 
-       va_mask  = GENMASK_ULL(VA_BITS - 2, 0);
-       va_mask |= hyp_va_msb;
+       kva_msb = fls64((u64)phys_to_virt(memblock_start_of_DRAM()) ^
+                       (u64)(high_memory - 1));
+
+       if (kva_msb == (VA_BITS - 1)) {
+               /*
+                * No space in the address, let's compute the mask so
+                * that it covers (VA_BITS - 1) bits, and the region
+                * bit. The tag stays set to zero.
+                */
+               va_mask  = BIT(VA_BITS - 1) - 1;
+               va_mask |= hyp_va_msb;
+       } else {
+               /*
+                * We do have some free bits to insert a random tag.
+                * Hyp VAs are now created from kernel linear map VAs
+                * using the following formula (with V == VA_BITS):
+                *
+                *  63 ... V |     V-1    | V-2 .. tag_lsb | tag_lsb - 1 .. 0
+                *  ---------------------------------------------------------
+                * | 0000000 | hyp_va_msb |    random tag  |  kern linear VA |
+                */
+               tag_lsb = kva_msb;
+               va_mask = GENMASK_ULL(tag_lsb - 1, 0);
+               tag_val = get_random_long() & GENMASK_ULL(VA_BITS - 2, tag_lsb);
+               tag_val |= hyp_va_msb;
+               tag_val >>= tag_lsb;
+       }
 }
 
 static u32 compute_instruction(int n, u32 rd, u32 rn)
@@ -46,6 +82,33 @@ static u32 compute_instruction(int n, u32 rd, u32 rn)
                                                          AARCH64_INSN_VARIANT_64BIT,
                                                          rn, rd, va_mask);
                break;
+
+       case 1:
+               /* ROR is a variant of EXTR with Rm = Rn */
+               insn = aarch64_insn_gen_extr(AARCH64_INSN_VARIANT_64BIT,
+                                            rn, rn, rd,
+                                            tag_lsb);
+               break;
+
+       case 2:
+               insn = aarch64_insn_gen_add_sub_imm(rd, rn,
+                                                   tag_val & GENMASK(11, 0),
+                                                   AARCH64_INSN_VARIANT_64BIT,
+                                                   AARCH64_INSN_ADSB_ADD);
+               break;
+
+       case 3:
+               insn = aarch64_insn_gen_add_sub_imm(rd, rn,
+                                                   tag_val & GENMASK(23, 12),
+                                                   AARCH64_INSN_VARIANT_64BIT,
+                                                   AARCH64_INSN_ADSB_ADD);
+               break;
+
+       case 4:
+               /* ROR is a variant of EXTR with Rm = Rn */
+               insn = aarch64_insn_gen_extr(AARCH64_INSN_VARIANT_64BIT,
+                                            rn, rn, rd, 64 - tag_lsb);
+               break;
        }
 
        return insn;
@@ -56,8 +119,7 @@ void __init kvm_update_va_mask(struct alt_instr *alt,
 {
        int i;
 
-       /* We only expect a single instruction in the alternative sequence */
-       BUG_ON(nr_inst != 1);
+       BUG_ON(nr_inst != 5);
 
        if (!has_vhe() && !va_mask)
                compute_layout();
@@ -68,8 +130,12 @@ void __init kvm_update_va_mask(struct alt_instr *alt,
                /*
                 * VHE doesn't need any address translation, let's NOP
                 * everything.
+                *
+                * Alternatively, if we don't have any spare bits in
+                * the address, NOP everything after masking that
+                * kernel VA.
                 */
-               if (has_vhe()) {
+               if (has_vhe() || (!tag_lsb && i > 0)) {
                        updptr[i] = cpu_to_le32(aarch64_insn_gen_nop());
                        continue;
                }

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index d8ea68b78e9cb59b6a8b31cdfcf6aa397deedcc7..394f41a657329f16eecec2d0f6b424a719c1e83c 100644 (file)
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1892,7 +1892,7 @@ int kvm_mmu_init(void)
                  kern_hyp_va((unsigned long)high_memory - 1));
 
        if (hyp_idmap_start >= kern_hyp_va(PAGE_OFFSET) &&
-           hyp_idmap_start <  kern_hyp_va(~0UL) &&
+           hyp_idmap_start <  kern_hyp_va((unsigned long)high_memory - 1) &&
            hyp_idmap_start != (unsigned long)__hyp_idmap_text_start) {
                /*
                 * The idmap page is intersecting with the VA space,