[Patch v3 05/14] x86/mm: Handle decryption/re-encryption of bss_decrypted consistently

Michael Kelley
2022-11-16T18:44:02Z

Current code in sme_postprocess_startup() decrypts the bss_decrypted
section when sme_me_mask is non-zero. But code in
mem_encrypt_free_decrytped_mem() re-encrypts the unused portion based
on CC_ATTR_MEM_ENCRYPT. In a Hyper-V guest VM using vTOM, these
conditions are not equivalent as sme_me_mask is always zero when
using vTOM. Consequently, mem_encrypt_free_decrypted_mem() attempts
to re-encrypt memory that was never decrypted.

Fix this in mem_encrypt_free_decrypted_mem() by conditioning the
re-encryption on the same test for non-zero sme_me_mask. Hyper-V
guests using vTOM don't need the bss_decrypted section to be
decrypted, so skipping the decryption/re-encryption doesn't cause
a problem.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
---
arch/x86/mm/mem_encrypt_amd.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index 9c4d8db..5a51343 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -513,10 +513,14 @@ void __init mem_encrypt_free_decrypted_mem(void)
npages = (vaddr_end - vaddr) >> PAGE_SHIFT;

/*
- * The unused memory range was mapped decrypted, change the encryption
- * attribute from decrypted to encrypted before freeing it.
+ * If the unused memory range was mapped decrypted, change the encryption
+ * attribute from decrypted to encrypted before freeing it. Base the
+ * re-encryption on the same condition used for the decryption in
+ * sme_postprocess_startup(). Higher level abstractions, such as
+ * CC_ATTR_MEM_ENCRYPT, aren't necessarily equivalent in a Hyper-V VM
+ * using vTOM, where sme_me_mask is always zero.
*/
- if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) {
+ if (sme_get_me_mask()) {
r = set_memory_encrypted(vaddr, npages);
if (r) {
pr_warn("failed to free unused decrypted pages\n");
--
1.8.3.1