Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

[Alexandre Chartre]

On 5/23/24 14:42, Andrew Cooper wrote:
> On 23/05/2024 1:33 pm, Alexandre Chartre wrote:
> > diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> > index 11c9b8efdc4c..7fa04edc87e9 100644
> > --- a/arch/x86/entry/entry_64_compat.S
> > +++ b/arch/x86/entry/entry_64_compat.S
> > @@ -91,7 +91,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
> >   
> >   	IBRS_ENTER
> >   	UNTRAIN_RET
> > -	CLEAR_BRANCH_HISTORY
> >   
> >   	/*
> >   	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
> > @@ -116,6 +115,12 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
> >   	jnz	.Lsysenter_fix_flags
> >   .Lsysenter_flags_fixed:
> >   
> > +	/*
> > +	 * CLEAR_BRANCH_HISTORY can call other functions. It should be invoked
> > +	 * after making sure TF is cleared because single-step is ignored only
> > +	 * for instructions inside the entry_SYSENTER_compat function.
> > +	 */
> > +	CLEAR_BRANCH_HISTORY
>
> Exactly the same is true of UNTRAIN_RET, although it will only manifest
> in i386 builds running on AMD hardware (SYSENTER is #UD on AMD hardware
> in Long mode.)
>
> #DB is IST so does handle it's own speculation safety.  It should be
> safe to move all the speculation safety logic in the sysenter handler to
> after .Lsysenter_flags_fixed:, I think?

Right, so something like this:

--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -89,10 +89,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 
        cld
 
-       IBRS_ENTER
-       UNTRAIN_RET
-       CLEAR_BRANCH_HISTORY
-
        /*
         * SYSENTER doesn't filter flags, so we need to clear NT and AC
         * ourselves.  To save a few cycles, we can check whether
@@ -116,6 +112,15 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
        jnz     .Lsysenter_fix_flags
 .Lsysenter_flags_fixed:
 
+       /*
+        * CPU bugs mitigations mechanisms can call other functions. They
+        * should be invoked after making sure TF is cleared because
+        * single-step is ignored only for instructions inside the
+        * entry_SYSENTER_compat function.
+        */
+       IBRS_ENTER
+       UNTRAIN_RET
+       CLEAR_BRANCH_HISTORY
        movq    %rsp, %rdi
        call    do_SYSENTER_32
        jmp     sysret32_from_system_call


alex.