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

* Tom Lendacky <thomas.lendacky@amd.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@lists.infradead.org>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.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

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