From ed57cac83e05f2e93567e4b5c57ee58a1bf8a582 Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Sun, 3 Dec 2017 18:22:49 +0000 Subject: [PATCH] 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 Reviewed-by: James Morse Signed-off-by: Marc Zyngier --- arch/arm64/include/asm/kvm_mmu.h | 12 ++++- arch/arm64/kvm/va_layout.c | 76 +++++++++++++++++++++++++++++--- virt/kvm/arm/mmu.c | 2 +- 3 files changed, 82 insertions(+), 8 deletions(-) diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h index 3836641dd6f0..a84e2dd14ebe 100644 --- 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 7998d1a60916..3d41a480b6a5 100644 --- a/arch/arm64/kvm/va_layout.c +++ b/arch/arm64/kvm/va_layout.c @@ -16,24 +16,60 @@ */ #include +#include +#include #include #include #include #include +/* + * 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 d8ea68b78e9c..394f41a65732 100644 --- 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, -- GitLab