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

[Ingo Molnar]

* Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  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