Re: [PATCHv2 2/3] x86/tdx: Fix race between set_memory_encrypted() and load_unaligned_zeropad()
From: Tom Lendacky
Date: Tue May 30 2023 - 08:58:16 EST
On 5/29/23 19:57, Kirill A. Shutemov wrote:
On Fri, May 26, 2023 at 03:10:56PM -0700, Sathyanarayanan Kuppuswamy wrote:
On 5/26/23 5:02 AM, Kirill A. Shutemov wrote:
Touching privately mapped GPA that is not properly converted to private
with MapGPA and accepted leads to unrecoverable exit to VMM.
load_unaligned_zeropad() can touch memory that is not owned by the
caller, but just happened to next after the owned memory.
/s/to/to be ?
Yep, my bad.
This load_unaligned_zeropad() behaviour makes it important when kernel
asks VMM to convert a GPA from shared to private or back. Kernel must
never have a page mapped into direct mapping (and aliases) as private
when the GPA is already converted to shared or when GPA is not yet
converted to private.
I am wondering whether this issue exist in the AMD code?
IMO, you can add some info on the window in set_memory_encrypted()
where this race exists.
I don't think AMD affected by load_unaligned_zeropad() the same way as
Intel does. But I'm not sure.
Tom, do you have any comments?
Right, shouldn't be an issue for SNP.
Thanks,
Tom
guest.enc_status_change_prepare() called before adjusting direct mapping
and therefore it is responsible for converting the memory to private.
guest.enc_status_change_finish() called after adjusting direct mapping
and it converts the memory to shared.
It is okay to have a shared mapping of memory that is not converted
properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
stepping on it.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Fixes: 7dbde7631629 ("x86/mm/cpa: Add support for TDX shared memory")
Cc: stable@xxxxxxxxxxxxxxx
---
arch/x86/coco/tdx/tdx.c | 56 ++++++++++++++++++++++++++++++++++++++---
1 file changed, 53 insertions(+), 3 deletions(-)
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index e146b599260f..59cc13e41aa6 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -840,6 +840,30 @@ static bool tdx_enc_status_changed(unsigned long vaddr, int numpages, bool enc)
return true;
}
+static bool tdx_enc_status_change_prepare(unsigned long vaddr, int numpages,
+ bool enc)
+{
+ /*
+ * Only handle shared->private conversion here.
+ * See the comment in tdx_early_init().
+ */
+ if (enc)
+ return tdx_enc_status_changed(vaddr, numpages, enc);
+ return true;
+}
+
+static bool tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
+ bool enc)
+{
+ /*
+ * Only handle private->shared conversion here.
+ * See the comment in tdx_early_init().
+ */
+ if (!enc)
+ return tdx_enc_status_changed(vaddr, numpages, enc);
+ return true;
+}
+
void __init tdx_early_init(void)
{
u64 cc_mask;
@@ -867,9 +891,35 @@ void __init tdx_early_init(void)
*/
physical_mask &= cc_mask - 1;
- x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
- x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
I think you don't need to change the order here.
I wanted to emphasise that the comment is for _prepare/_finish callbacks
and I hoped re-order would help with this.
- x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
+ /*
+ * Touching privately mapped GPA that is not properly converted to
+ * private with MapGPA and accepted leads to unrecoverable exit
+ * to VMM.
+ *
+ * load_unaligned_zeropad() can touch memory that is not owned by
+ * the caller, but just happened to next after the owned memory.
+ * This load_unaligned_zeropad() behaviour makes it important when
+ * kernel asks VMM to convert a GPA from shared to private or back.
+ * Kernel must never have a page mapped into direct mapping (and
+ * aliases) as private when the GPA is already converted to shared or
+ * when GPA is not yet converted to private.
+ *
+ * guest.enc_status_change_prepare() called before adjusting direct
+ * mapping and therefore it is responsible for converting the memory
+ * to private.
+ *
+ * guest.enc_status_change_finish() called after adjusting direct
+ * mapping and it converts the memory to shared.
+ *
+ * It is okay to have a shared mapping of memory that is not converted
+ * properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
+ * stepping on it.
+ */
+ x86_platform.guest.enc_status_change_prepare = tdx_enc_status_change_prepare;
+ x86_platform.guest.enc_status_change_finish = tdx_enc_status_change_finish;
+
+ x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
+ x86_platform.guest.enc_tlb_flush_required = tdx_tlb_flush_required;
pr_info("Guest detected\n");
}
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer