linux-kernel - Re: [PATCH v1 1/2] x86/mm, kexec: Fix memory corruption with SME on successive kexecs

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Message-ID: <20170727071717.aydwpzaji6l2o6xs@gmail.com>
Date:   Thu, 27 Jul 2017 09:17:18 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Young <dyoung@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        kexec@...ts.infradead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v1 1/2] x86/mm, kexec: Fix memory corruption with SME on
 successive kexecs


* Tom Lendacky <thomas.lendacky@....com> wrote:

> After issuing successive kexecs it was found that the SHA hash failed
> verification when booting the kexec'd kernel.  When SME is enabled, the
> change from using pages that were marked encrypted to now being marked as
> not encrypted (through new identify mapped page tables) results in memory
> corruption if there are any cache entries for the previously encrypted
> pages. This is because separate cache entries can exist for the same
> physical location but tagged both with and without the encryption bit.
> 
> To prevent this, issue a wbinvd before copying the pages from the source
> location to the destination location to clear any possible cache entry
> conflicts.
> 
> Cc: <kexec@...ts.infradead.org>
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 98111b3..c11d8bc 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -132,6 +132,13 @@ identity_mapped:
>  	/* Flush the TLB (needed?) */
>  	movq	%r9, %cr3
>  
> +	/*
> +	 * If SME is/was active, there could be old encrypted cache line
> +	 * entries that will conflict with the now unencrypted memory
> +	 * used by kexec. Flush the caches before copying the kernel.
> +	 */
> +	wbinvd

WBINVD is very expensive IIRC - several milliseconds.

So if we change the page table from encrypted to unencrypted we need to do a full 
cache flush sounds pretty broken to me - how can then this be done via an API such 
as mmap() without executing WBINVD?

Thanks,

	Ingo