[kernel-hardening] [PATCH v3 4/4] KVM: VMX: Simplify segment_base

Thomas Garnier
2017-02-14T19:42:59Z

The KVM segment_base function is confusing. This patch replaces integers
with appropriate flags, simplify constructs and add comments.

Signed-off-by: Thomas Garnier <thgarnie@google.com>
---
Based on next-20170213
---
arch/x86/kvm/vmx.c | 26 ++++++++++++++++++--------
1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 99167f20bc34..edb8326108dd 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2062,25 +2062,35 @@ static unsigned long segment_base(u16 selector)
struct desc_struct *d;
unsigned long table_base;
unsigned long v;
+ u32 high32;

- if (!(selector & ~3))
+ if (!(selector & ~SEGMENT_RPL_MASK))
return 0;

- table_base = get_current_gdt_rw_vaddr();
-
- if (selector & 4) { /* from ldt */
+ /* LDT selector */
+ if ((selector & SEGMENT_TI_MASK) == SEGMENT_LDT) {
u16 ldt_selector = kvm_read_ldt();

- if (!(ldt_selector & ~3))
+ if (!(ldt_selector & ~SEGMENT_RPL_MASK))
return 0;

table_base = segment_base(ldt_selector);
+ } else {
+ table_base = get_current_gdt_rw_vaddr();
}
- d = (struct desc_struct *)(table_base + (selector & ~7));
+
+ d = (struct desc_struct *)table_base + (selector >> 3);
v = get_desc_base(d);
#ifdef CONFIG_X86_64
- if (d->s == 0 && (d->type == 2 || d->type == 9 || d->type == 11))
- v |= ((unsigned long)((struct ldttss_desc64 *)d)->base3) << 32;
+ /*
+ * Extend the virtual address if we have a system descriptor entry for
+ * LDT or TSS (available or busy).
+ */
+ if (d->s == 0 && (d->type == DESC_LDT || d->type == DESC_TSS ||
+ d->type == 11/*Busy TSS */)) {
+ high32 = ((struct ldttss_desc64 *)d)->base3;
+ v |= (u64)high32 << 32;
+ }
#endif
return v;
}
--
2.11.0.483.g087da7b7c-goog