2014-10-10 05:07+0300, Nadav Amit:
> When read access is performed using a readable code segment, the "conforming"
> and "non-conforming" checks should not be done. As a result, read using
> non-conforming readable code segment fails.
>
> This is according to Intel SDM 5.6.1 ("Accessing Data in Code Segments").
>
> One exception is the case of conforming code segment. The SDM says: "Use a
> code-segment override prefix (CS) to read a readable... [it is] valid because
> the DPL of the code segment selected by the CS register is the same as the
> CPL." This is misleading since CS.DPL may be lower (numerically) than CPL, and
> CS would still be accessible. The emulator should avoid privilage level checks
> for data reads using CS.
Ah, after stripping faulty presumptions, I'm not sure this change is
enough ... shouldn't we also skip the check on conforming code segments?
Method 2 is always valid because the privilege level of a conforming
code segment is effectively the same as the CPL, regardless of its DPL.
And we couldn't read it from less privileged modes.
> The fix is not to perform the "non-conforming" checks if the access is not a
> fetch, and never to perform the checks for CS.
>
> ---
> v1->v2: Privilage level checks are always skipped for CS
>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> ---
> arch/x86/kvm/emulate.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index a46207a..0fee0a0 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -661,9 +661,9 @@ static int __linearize(struct x86_emulate_ctxt *ctxt,
> goto bad;
> }
> cpl = ctxt->ops->cpl(ctxt);
> - if (!(desc.type & 8)) {
> - /* data segment */
> - if (cpl > desc.dpl)
> + if (!fetch) {
> + /* data segment or readable code segment */
> + if (cpl > desc.dpl && addr.seg != VCPU_SREG_CS)
> goto bad;
> } else if ((desc.type & 8) && !(desc.type & 4)) {
> /* nonconforming code segment */
> --
> 1.9.1
>
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