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

Jim Mattson
2017-02-17T17:49:42Z

Can we use the read-only GDT here? When expanding the virtual address
for 64-bit system descriptors, isn't it sufficient to check (d->s == 0
&& d->type != 0)?

On Tue, Feb 14, 2017 at 11:42 AM, Thomas Garnier <thgarnie@google.com> wrote:
> 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
>