Re: [PATCH v3 49/75] x86/sev-es: Handle instruction fetches from user-space

Borislav Petkov
2020-05-23T09:24:01Z

On Tue, Apr 28, 2020 at 05:16:59PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> When a #VC exception is triggered by user-space the instruction decoder
> needs to read the instruction bytes from user addresses. Enhance
> vc_decode_insn() to safely fetch kernel and user instructions.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/kernel/sev-es.c | 31 ++++++++++++++++++++++---------
> 1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index 85027fb4177e..c2223c2a28c2 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -165,17 +165,30 @@ static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
> enum es_result ret;
> int res;
>
> - res = vc_fetch_insn_kernel(ctxt, buffer);
> - if (unlikely(res == -EFAULT)) {

Let's also test for 0 in case the probe_read* guts get changed and start
returning so other errval besides -EFAULT.

> - ctxt->fi.vector = X86_TRAP_PF;
> - ctxt->fi.error_code = 0;
> - ctxt->fi.cr2 = ctxt->regs->ip;
> - return ES_EXCEPTION;
> + if (!user_mode(ctxt->regs)) {

Flip that check so that it reads more naturally:

if (user_mode(..)
insn_fetch_from_user()

...
} else {
vc_fetch_insn_kernel()

}

> + res = vc_fetch_insn_kernel(ctxt, buffer);

> + if (unlikely(res == -EFAULT)) {
> + ctxt->fi.vector = X86_TRAP_PF;
> + ctxt->fi.error_code = 0;
> + ctxt->fi.cr2 = ctxt->regs->ip;
> + return ES_EXCEPTION;
> + }
> +
> + insn_init(&ctxt->insn, buffer, MAX_INSN_SIZE - res, 1);
> + insn_get_length(&ctxt->insn);
> + } else {
> + res = insn_fetch_from_user(ctxt->regs, buffer);
> + if (res == 0) {

if (!res)

--
Regards/Gruss,
Boris.