linux-kernel - Re: [PATCH 3/5] x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path

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Date:   Mon, 19 Oct 2020 17:22:47 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>, x86@...nel.org,
        Joerg Roedel <jroedel@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Martin Radev <martin.b.radev@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] x86/boot/compressed/64: Check SEV encryption in
 64-bit boot-path

On Mon, Oct 19, 2020 at 10:33:45PM +0200, Joerg Roedel wrote:
> Hi Arvind,
> 
> On Mon, Oct 19, 2020 at 01:00:08PM -0400, Arvind Sankar wrote:
> > On Mon, Oct 19, 2020 at 05:11:19PM +0200, Joerg Roedel wrote:
> > > +
> > > +	/* Store value to memory and keep it in %r10 */
> > > +	movq	%r10, sev_check_data(%rip)
> > > +
> > 
> > Does there need to be a cache flush/invalidation between this and the
> > read below to avoid just reading back from cache, or will the hardware
> > take care of that?
> 
> No, a cache flush is not needed. When the C bit position is correct,
> then the data will be mapped encrypted with the old and the new
> page-table. If the C bit position is wrong, the access goes to a
> different physical address.

Ok.

> 
> > > +	/* Backup current %cr3 value to restore it later */
> > > +	movq	%cr3, %r11
> > > +
> > > +	/* Switch to new %cr3 - This might unmap the stack */
> > > +	movq	%rdi, %cr3
> > 
> > Does there need to be a TLB flush after this? When executed from the
> > main kernel's head code, CR4.PGE is enabled, and if the original page
> > mapping had the global bit set (the decompressor stub sets that in the
> > identity mapping), won't the read below still use the original encrypted
> > mapping if we don't explicitly flush it?
> 
> The check only really matters for the boot CPU, not for the secondary
> CPUs. IIRC at this point in boot CR4.PGE is still off.
> 
> Regards,
> 
> 	Joerg
> 

The boot cpu also enables CR4.PGE -- that code is shared between boot
and secondary cpus. The boot cpu jumps to the first "1" label below,
just before the call to sev_verify_cbit you're adding.

	/* Form the CR3 value being sure to include the CR3 modifier */
	addq	$(init_top_pgt - __START_KERNEL_map), %rax
1:

	/* Enable PAE mode, PGE and LA57 */
	movl	$(X86_CR4_PAE | X86_CR4_PGE), %ecx
#ifdef CONFIG_X86_5LEVEL
	testl	$1, __pgtable_l5_enabled(%rip)
	jz	1f
	orl	$X86_CR4_LA57, %ecx
1:
#endif
	movq	%rcx, %cr4

	/* Setup early boot stage 4-/5-level pagetables. */
	addq	phys_base(%rip), %rax
	movq	%rax, %cr3