On Wed, Feb 17, 2021 at 01:01:42PM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
>
> The code in the NMI handler to adjust the #VC handler IST stack is
> needed in case an NMI hits when the #VC handler is still using its IST
> stack.
> But the check for this condition also needs to look if the regs->sp
> value is trusted, meaning it was not set by user-space. Extend the
> check to not use regs->sp when the NMI interrupted user-space code or
> the SYSCALL gap.
>
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
> Cc: stable@xxxxxxxxxxxxxxx # 5.10+
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
> arch/x86/kernel/sev-es.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index 84c1821819af..0df38b185d53 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -144,7 +144,9 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
> old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
>
> /* Make room on the IST stack */
> - if (on_vc_stack(regs->sp))
> + if (on_vc_stack(regs->sp) &&
> + !user_mode(regs) &&
> + !from_syscall_gap(regs))
Why not add those checks to on_vc_stack() directly? Because in it, you
can say:
on_vc_stack():
/* user mode rSP is not trusted */
if (user_mode())
return false;
/* ditto */
if (ip_within_syscall_gap())
return false;
...
?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
