linux-kernel - Re: [PATCH 3/6] x86/entry_32: Add VERW just before userspace transition

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Message-ID: <20231021012744.3yz7lpo2w6gyvytr@desk>
Date:   Fri, 20 Oct 2023 18:28:11 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        antonio.gomez.iglesias@...ux.intel.com
Subject: Re: [PATCH  3/6] x86/entry_32: Add VERW just before userspace
 transition

On Fri, Oct 20, 2023 at 04:49:34PM -0700, Andi Kleen wrote:
> On Fri, Oct 20, 2023 at 01:45:09PM -0700, Pawan Gupta wrote:
> > As done for entry_64, add support for executing VERW late in exit to
> > user path for 32-bit mode.
> > 
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > ---
> >  arch/x86/entry/entry_32.S | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> > index 6e6af42e044a..bbf77d2aab2e 100644
> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -886,6 +886,9 @@ SYM_FUNC_START(entry_SYSENTER_32)
> >  	popfl
> >  	popl	%eax
> >  
> > +	/* Mitigate CPU data sampling attacks .e.g. MDS */
> > +	USER_CLEAR_CPU_BUFFERS
> > +
> >  	/*
> >  	 * Return back to the vDSO, which will pop ecx and edx.
> >  	 * Don't bother with DS and ES (they already contain __USER_DS).
> 
> Did you forget the INT 0x80 entry point?

I do have VERW in the INT80 path, the diff is showing just the label
restore_all_switch_stack. Below is the sequence:

SYM_FUNC_START(entry_INT80_32)
	ASM_CLAC
	pushl	%eax			/* pt_regs->orig_ax */

	SAVE_ALL pt_regs_ax=$-ENOSYS switch_stacks=1	/* save rest */

	movl	%esp, %eax
	call	do_int80_syscall_32
.Lsyscall_32_done:
	STACKLEAK_ERASE

restore_all_switch_stack:
	SWITCH_TO_ENTRY_STACK
	CHECK_AND_APPLY_ESPFIX

	/* Switch back to user CR3 */
	SWITCH_TO_USER_CR3 scratch_reg=%eax

	BUG_IF_WRONG_CR3

	/* Restore user state */
	RESTORE_REGS pop=4			# skip orig_eax/error_code

	/* Mitigate CPU data sampling attacks .e.g. MDS */
	USER_CLEAR_CPU_BUFFERS
	^^^^^^^^^^^^^^^^^^^^^^