Commit message: KVM: MTRR: remove MSR 0x2f8

MSR 0x2f8 accessed the 124th Variable Range MTRR ever since MTRR support
was introduced by 9ba075a664df ("KVM: MTRR support").

0x2f8 became harmful when 910a6aae4e2e ("KVM: MTRR: exactly define the
size of variable MTRRs") shrinked the array of VR MTRRs from 256 to 8,
which made access to index 124 out of bounds.  The surrounding code only
WARNs in this situation, thus the guest gained a limited read/write
access to struct kvm_arch_vcpu.

0x2f8 is not a valid VR MTRR MSR, because KVM has/advertises only 16 VR
MTRR MSRs, 0x200-0x20f.  Every VR MTRR is set up using two MSRs, 0x2f8
was treated as a PHYSBASE and 0x2f9 would be its PHYSMASK, but 0x2f9 was
not implemented in KVM, therefore 0x2f8 could never do anything useful
and getting rid of it is safe.

This fixes CVE-2016-3713.

Fixes: 910a6aae4e2e ("KVM: MTRR: exactly define the size of variable MTRRs")
Cc: stable@vger.kernel.org
Reported-by: David Matlack <dmatlack@google.com>
Signed-off-by: Andy Honig <ahonig@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
CVE: CVE-2016-3713
CWE: CWE-284
Vulnerability description: The msr_mtrr_valid function in arch/x86/kvm/mtrr.c in the Linux kernel before 4.6.1 supports MSR 0x2f8, which allows guest OS users to read or write to the kvm_arch_vcpu data structure, and consequently obtain sensitive information or cause a denial of service (system crash), via a crafted ioctl call.

Project: linux
File: mtrr.c

static void __mtrr_lookup_var_next(struct mtrr_iter *iter)
{
	struct kvm_mtrr *mtrr_state = iter->mtrr_state;

	list_for_each_entry_continue(iter->range, &mtrr_state->head, node)
		if (match_var_range(iter, iter->range))
			return;

	iter->range = NULL;
	iter->partial_map |= iter->start_max < iter->end;
}

Project: linux
File: mtrr.c

static int fixed_msr_to_range_index(u32 msr)
{
	int seg, unit;

	if (!fixed_msr_to_seg_unit(msr, &seg, &unit))
		return -1;

	return fixed_mtrr_seg_unit_range_index(seg, unit);
}

Project: linux
File: mtrr.c

static int fixed_mtrr_addr_seg_to_range_index(u64 addr, int seg)
{
	struct fixed_mtrr_segment *mtrr_seg;
	int index;

	mtrr_seg = &fixed_seg_table[seg];
	index = mtrr_seg->range_start;
	index += (addr - mtrr_seg->start) >> mtrr_seg->range_shift;
	return index;
}

Project: linux
File: mtrr.c

static int fixed_mtrr_addr_to_seg(u64 addr)
{
	struct fixed_mtrr_segment *mtrr_seg;
	int seg, seg_num = ARRAY_SIZE(fixed_seg_table);

	for (seg = 0; seg < seg_num; seg++) {
		mtrr_seg = &fixed_seg_table[seg];
		if (mtrr_seg->start <= addr && addr < mtrr_seg->end)
			return seg;
	}

	return -1;
}

Project: linux
File: mtrr.c

static u64 fixed_mtrr_range_end_addr(int seg, int index)
{
	struct fixed_mtrr_segment *mtrr_seg = &fixed_seg_table[seg];
	int pos = index - mtrr_seg->range_start;

	return mtrr_seg->start + ((pos + 1) << mtrr_seg->range_shift);
}

Project: linux
File: mtrr.c

static int fixed_mtrr_seg_end_range_index(int seg)
{
	struct fixed_mtrr_segment *mtrr_seg = &fixed_seg_table[seg];
	int n;

	n = (mtrr_seg->end - mtrr_seg->start) >> mtrr_seg->range_shift;
	return mtrr_seg->range_start + n - 1;
}

Project: linux
File: mtrr.c

static int fixed_mtrr_seg_unit_range_index(int seg, int unit)
{
	struct fixed_mtrr_segment *mtrr_seg = &fixed_seg_table[seg];

	WARN_ON(mtrr_seg->start + unit * fixed_mtrr_seg_unit_size(seg)
		> mtrr_seg->end);

	/* each unit has 8 ranges. */
	return mtrr_seg->range_start + 8 * unit;
}

Project: linux
File: mtrr.c

static u64 fixed_mtrr_seg_unit_size(int seg)
{
	return 8 << fixed_seg_table[seg].range_shift;
}

Project: linux
File: mtrr.c

bool kvm_mtrr_check_gfn_range_consistency(struct kvm_vcpu *vcpu, gfn_t gfn,
					  int page_num)
{
	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
	struct mtrr_iter iter;
	u64 start, end;
	int type = -1;

	start = gfn_to_gpa(gfn);
	end = gfn_to_gpa(gfn + page_num);
	mtrr_for_each_mem_type(&iter, mtrr_state, start, end) {
		if (type == -1) {
			type = iter.mem_type;
			continue;
		}

		if (type != iter.mem_type)
			return false;
	}

	if (iter.mtrr_disabled)
		return true;

	if (!iter.partial_map)
		return true;

	if (type == -1)
		return true;

	return type == mtrr_default_type(mtrr_state);
}

Project: linux
File: mtrr.c

u8 kvm_mtrr_get_guest_memory_type(struct kvm_vcpu *vcpu, gfn_t gfn)
{
	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
	struct mtrr_iter iter;
	u64 start, end;
	int type = -1;
	const int wt_wb_mask = (1 << MTRR_TYPE_WRBACK)
			       | (1 << MTRR_TYPE_WRTHROUGH);

	start = gfn_to_gpa(gfn);
	end = start + PAGE_SIZE;

	mtrr_for_each_mem_type(&iter, mtrr_state, start, end) {
		int curr_type = iter.mem_type;

		/*
		 * Please refer to Intel SDM Volume 3: 11.11.4.1 MTRR
		 * Precedences.
		 */

		if (type == -1) {
			type = curr_type;
			continue;
		}

		/*
		 * If two or more variable memory ranges match and the
		 * memory types are identical, then that memory type is
		 * used.
		 */
		if (type == curr_type)
			continue;

		/*
		 * If two or more variable memory ranges match and one of
		 * the memory types is UC, the UC memory type used.
		 */
		if (curr_type == MTRR_TYPE_UNCACHABLE)
			return MTRR_TYPE_UNCACHABLE;

		/*
		 * If two or more variable memory ranges match and the
		 * memory types are WT and WB, the WT memory type is used.
		 */
		if (((1 << type) & wt_wb_mask) &&
		      ((1 << curr_type) & wt_wb_mask)) {
			type = MTRR_TYPE_WRTHROUGH;
			continue;
		}

		/*
		 * For overlaps not defined by the above rules, processor
		 * behavior is undefined.
		 */

		/* We use WB for this undefined behavior. :( */
		return MTRR_TYPE_WRBACK;
	}

	if (iter.mtrr_disabled)
		return mtrr_disabled_type(vcpu);

	/* not contained in any MTRRs. */
	if (type == -1)
		return mtrr_default_type(mtrr_state);

	/*
	 * We just check one page, partially covered by MTRRs is
	 * impossible.
	 */
	WARN_ON(iter.partial_map);

	return type;
}

Project: linux
File: mtrr.c

int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
{
	int index;

	/* MSR_MTRRcap is a readonly MSR. */
	if (msr == MSR_MTRRcap) {
		/*
		 * SMRR = 0
		 * WC = 1
		 * FIX = 1
		 * VCNT = KVM_NR_VAR_MTRR
		 */
		*pdata = 0x500 | KVM_NR_VAR_MTRR;
		return 0;
	}

	if (!msr_mtrr_valid(msr))
		return 1;

	index = fixed_msr_to_range_index(msr);
	if (index >= 0)
		*pdata = *(u64 *)&vcpu->arch.mtrr_state.fixed_ranges[index];
	else if (msr == MSR_MTRRdefType)
		*pdata = vcpu->arch.mtrr_state.deftype;
	else if (msr == MSR_IA32_CR_PAT)
		*pdata = vcpu->arch.pat;
	else {	/* Variable MTRRs */
		int is_mtrr_mask;

		index = (msr - 0x200) / 2;
		is_mtrr_mask = msr - 0x200 - 2 * index;
		if (!is_mtrr_mask)
			*pdata = vcpu->arch.mtrr_state.var_ranges[index].base;
		else
			*pdata = vcpu->arch.mtrr_state.var_ranges[index].mask;

		*pdata &= (1ULL << cpuid_maxphyaddr(vcpu)) - 1;
	}

	return 0;
}

Project: linux
File: mtrr.c

int kvm_mtrr_set_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data)
{
	int index;

	if (!kvm_mtrr_valid(vcpu, msr, data))
		return 1;

	index = fixed_msr_to_range_index(msr);
	if (index >= 0)
		*(u64 *)&vcpu->arch.mtrr_state.fixed_ranges[index] = data;
	else if (msr == MSR_MTRRdefType)
		vcpu->arch.mtrr_state.deftype = data;
	else if (msr == MSR_IA32_CR_PAT)
		vcpu->arch.pat = data;
	else
		set_var_mtrr_msr(vcpu, msr, data);

	update_mtrr(vcpu, msr);
	return 0;
}

Project: linux
File: mtrr.c

void kvm_vcpu_mtrr_init(struct kvm_vcpu *vcpu)
{
	INIT_LIST_HEAD(&vcpu->arch.mtrr_state.head);
}

Project: linux
File: mtrr.c

static bool match_var_range(struct mtrr_iter *iter,
			    struct kvm_mtrr_range *range)
{
	u64 start, end;

	var_mtrr_range(range, &start, &end);
	if (!(start >= iter->end || end <= iter->start)) {
		iter->range = range;

		/*
		 * the function is called when we do kvm_mtrr.head walking.
		 * Range has the minimum base address which interleaves
		 * [looker->start_max, looker->end).
		 */
		iter->partial_map |= iter->start_max < start;

		/* update the max address has been covered. */
		iter->start_max = max(iter->start_max, end);
		return true;
	}

	return false;
}

Project: linux
File: mtrr.c

static u8 mtrr_default_type(struct kvm_mtrr *mtrr_state)
{
	return mtrr_state->deftype & IA32_MTRR_DEF_TYPE_TYPE_MASK;
}

Project: linux
File: mtrr.c

static u8 mtrr_disabled_type(struct kvm_vcpu *vcpu)
{
	/*
	 * Intel SDM 11.11.2.2: all MTRRs are disabled when
	 * IA32_MTRR_DEF_TYPE.E bit is cleared, and the UC
	 * memory type is applied to all of physical memory.
	 *
	 * However, virtual machines can be run with CPUID such that
	 * there are no MTRRs.  In that case, the firmware will never
	 * enable MTRRs and it is obviously undesirable to run the
	 * guest entirely with UC memory and we use WB.
	 */
	if (guest_cpuid_has_mtrr(vcpu))
		return MTRR_TYPE_UNCACHABLE;
	else
		return MTRR_TYPE_WRBACK;
}

Project: linux
File: mtrr.c

static bool mtrr_is_enabled(struct kvm_mtrr *mtrr_state)
{
	return !!(mtrr_state->deftype & IA32_MTRR_DEF_TYPE_E);
}

Project: linux
File: mtrr.c

static void mtrr_lookup_fixed_next(struct mtrr_iter *iter)
{
	/* terminate the lookup. */
	if (fixed_mtrr_range_end_addr(iter->seg, iter->index) >= iter->end) {
		iter->fixed = false;
		iter->range = NULL;
		return;
	}

	iter->index++;

	/* have looked up for all fixed MTRRs. */
	if (iter->index >= ARRAY_SIZE(iter->mtrr_state->fixed_ranges))
		return mtrr_lookup_var_start(iter);

	/* switch to next segment. */
	if (iter->index > fixed_mtrr_seg_end_range_index(iter->seg))
		iter->seg++;
}

Project: linux
File: mtrr.c

static bool mtrr_lookup_fixed_start(struct mtrr_iter *iter)
{
	int seg, index;

	if (!fixed_mtrr_is_enabled(iter->mtrr_state))
		return false;

	seg = fixed_mtrr_addr_to_seg(iter->start);
	if (seg < 0)
		return false;

	iter->fixed = true;
	index = fixed_mtrr_addr_seg_to_range_index(iter->start, seg);
	iter->index = index;
	iter->seg = seg;
	return true;
}

Project: linux
File: mtrr.c

static void mtrr_lookup_init(struct mtrr_iter *iter,
			     struct kvm_mtrr *mtrr_state, u64 start, u64 end)
{
	iter->mtrr_state = mtrr_state;
	iter->start = start;
	iter->end = end;
	iter->mtrr_disabled = false;
	iter->partial_map = false;
	iter->fixed = false;
	iter->range = NULL;

	mtrr_lookup_start(iter);
}

Project: linux
File: mtrr.c

static void mtrr_lookup_next(struct mtrr_iter *iter)
{
	if (iter->fixed)
		mtrr_lookup_fixed_next(iter);
	else
		mtrr_lookup_var_next(iter);
}

Project: linux
File: mtrr.c

static bool mtrr_lookup_okay(struct mtrr_iter *iter)
{
	if (iter->fixed) {
		iter->mem_type = iter->mtrr_state->fixed_ranges[iter->index];
		return true;
	}

	if (iter->range) {
		iter->mem_type = iter->range->base & 0xff;
		return true;
	}

	return false;
}

Project: linux
File: mtrr.c

static void mtrr_lookup_start(struct mtrr_iter *iter)
{
	if (!mtrr_is_enabled(iter->mtrr_state)) {
		iter->mtrr_disabled = true;
		return;
	}

	if (!mtrr_lookup_fixed_start(iter))
		mtrr_lookup_var_start(iter);
}

Project: linux
File: mtrr.c

static void mtrr_lookup_var_next(struct mtrr_iter *iter)
{
	__mtrr_lookup_var_next(iter);
}

Project: linux
File: mtrr.c

static void mtrr_lookup_var_start(struct mtrr_iter *iter)
{
	struct kvm_mtrr *mtrr_state = iter->mtrr_state;

	iter->fixed = false;
	iter->start_max = iter->start;
	iter->range = list_prepare_entry(iter->range, &mtrr_state->head, node);

	__mtrr_lookup_var_next(iter);
}

Project: linux
File: mtrr.c

static void set_var_mtrr_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data)
{
	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
	struct kvm_mtrr_range *tmp, *cur;
	int index, is_mtrr_mask;

	index = (msr - 0x200) / 2;
	is_mtrr_mask = msr - 0x200 - 2 * index;
	cur = &mtrr_state->var_ranges[index];

	/* remove the entry if it's in the list. */
	if (var_mtrr_range_is_valid(cur))
		list_del(&mtrr_state->var_ranges[index].node);

	/* Extend the mask with all 1 bits to the left, since those
	 * bits must implicitly be 0.  The bits are then cleared
	 * when reading them.
	 */
	if (!is_mtrr_mask)
		cur->base = data;
	else
		cur->mask = data | (-1LL << cpuid_maxphyaddr(vcpu));

	/* add it to the list if it's enabled. */
	if (var_mtrr_range_is_valid(cur)) {
		list_for_each_entry(tmp, &mtrr_state->head, node)
			if (cur->base >= tmp->base)
				break;
		list_add_tail(&cur->node, &tmp->node);
	}
}

Project: linux
File: mtrr.c

static void update_mtrr(struct kvm_vcpu *vcpu, u32 msr)
{
	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
	gfn_t start, end;
	int index;

	if (msr == MSR_IA32_CR_PAT || !tdp_enabled ||
	      !kvm_arch_has_noncoherent_dma(vcpu->kvm))
		return;

	if (!mtrr_is_enabled(mtrr_state) && msr != MSR_MTRRdefType)
		return;

	/* fixed MTRRs. */
	if (fixed_msr_to_range(msr, &start, &end)) {
		if (!fixed_mtrr_is_enabled(mtrr_state))
			return;
	} else if (msr == MSR_MTRRdefType) {
		start = 0x0;
		end = ~0ULL;
	} else {
		/* variable range MTRRs. */
		index = (msr - 0x200) / 2;
		var_mtrr_range(&mtrr_state->var_ranges[index], &start, &end);
	}

	kvm_zap_gfn_range(vcpu->kvm, gpa_to_gfn(start), gpa_to_gfn(end));
}

Project: linux
File: mtrr.c

static void var_mtrr_range(struct kvm_mtrr_range *range, u64 *start, u64 *end)
{
	u64 mask;

	*start = range->base & PAGE_MASK;

	mask = range->mask & PAGE_MASK;

	/* This cannot overflow because writing to the reserved bits of
	 * variable MTRRs causes a #GP.
	 */
	*end = (*start | ~mask) + 1;
}

Project: linux
File: mtrr.c

static bool var_mtrr_range_is_valid(struct kvm_mtrr_range *range)
{
	return (range->mask & (1 << 11)) != 0;
}