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Subject[PATCH] KVM: x86/mmu: Warn on nx_huge_pages for possible problems
Date
Add warning when `nx_huge_pages` is enabled by `auto` for hint that
huge pages may be splited by kernel.

Add warning when CVE-2018-12207 may arise but `nx_huge_pages` is
disabled for hint that malicious guest may cause a CPU lookup.

Signed-off-by: Li Yu <liyu.yukiteru@bytedance.com>
---
arch/x86/kvm/mmu/mmu.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1a64ba5b9437..32026592e566 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -6056,19 +6056,26 @@ static void __set_nx_huge_pages(bool val)
nx_huge_pages = itlb_multihit_kvm_mitigation = val;
}

+#define ITLB_MULTIHIT_NX_ON "iTLB multi-hit CPU bug present and cpu mitigations enabled, huge pages may be splited by kernel for security. See CVE-2018-12207 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/multihit.html for details.\n"
+#define ITLB_MULTIHIT_NX_OFF "iTLB multi-hit CPU bug present and cpu mitigations enabled, malicious guest may cause a CPU lookup. See CVE-2018-12207 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/multihit.html for details.\n"
+
static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
{
bool old_val = nx_huge_pages;
bool new_val;

/* In "auto" mode deploy workaround only if CPU has the bug. */
- if (sysfs_streq(val, "off"))
+ if (sysfs_streq(val, "off")) {
new_val = 0;
- else if (sysfs_streq(val, "force"))
+ if (get_nx_auto_mode() && new_val != old_val)
+ pr_warn(ITLB_MULTIHIT_NX_OFF);
+ } else if (sysfs_streq(val, "force"))
new_val = 1;
- else if (sysfs_streq(val, "auto"))
+ else if (sysfs_streq(val, "auto")) {
new_val = get_nx_auto_mode();
- else if (strtobool(val, &new_val) < 0)
+ if (new_val && new_val != old_val)
+ pr_warn(ITLB_MULTIHIT_NX_ON);
+ } else if (strtobool(val, &new_val) < 0)
return -EINVAL;

__set_nx_huge_pages(new_val);
@@ -6095,8 +6102,11 @@ int kvm_mmu_module_init(void)
{
int ret = -ENOMEM;

- if (nx_huge_pages == -1)
+ if (nx_huge_pages == -1) {
__set_nx_huge_pages(get_nx_auto_mode());
+ if (is_nx_huge_page_enabled())
+ pr_warn_once(ITLB_MULTIHIT_NX_ON);
+ }

/*
* MMU roles use union aliasing which is, generally speaking, an
--
2.11.0
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