From b08600e2d6b58eca29d72f6cb6672a5cba5baed4 Mon Sep 17 00:00:00 2001 From: Jan Beulich Date: Mon, 12 Dec 2022 12:27:30 +0100 Subject: [PATCH] x86/tboot: actually wipe contexts Especially with our use of __builtin_memset() to implement memset() the compiler is free to eliminate instances when it can prove that the affected object is dead. Introduce a small helper function accompanying the memset() with a construct forcing the compiler to retain the clearing of (stack) memory. Fixes: c021c95498d9 ("x86: Replace our own specialised versions of memset and memcpy with") Signed-off-by: Jan Beulich Reviewed-by: Jason Andryuk Acked-by: Andrew Cooper --- xen/arch/x86/tboot.c | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) diff --git a/xen/arch/x86/tboot.c b/xen/arch/x86/tboot.c index 68bee222dd..90f6e805a9 100644 --- a/xen/arch/x86/tboot.c +++ b/xen/arch/x86/tboot.c @@ -179,6 +179,17 @@ static void update_iommu_mac(vmac_ctx_t *ctx, uint64_t pt_maddr, int level) #define is_page_in_use(page) \ (page_state_is(page, inuse) || page_state_is(page, offlining)) +/* Wipe ctx to ensure key is not left in memory. */ +static void wipe_ctx(vmac_ctx_t *ctx) +{ + memset(ctx, 0, sizeof(*ctx)); + /* + * Make sure the compiler won't optimize out the memset(), for the local + * variable (at the call sites) going out of scope right afterwards. + */ + asm volatile ( "" :: "m" (*ctx) ); +} + static void tboot_gen_domain_integrity(const uint8_t key[TB_KEY_SIZE], vmac_t *mac) { @@ -216,8 +227,7 @@ static void tboot_gen_domain_integrity(const uint8_t key[TB_KEY_SIZE], *mac = vmac(NULL, 0, nonce, NULL, &ctx); - /* wipe ctx to ensure key is not left in memory */ - memset(&ctx, 0, sizeof(ctx)); + wipe_ctx(&ctx); } static void tboot_gen_xenheap_integrity(const uint8_t key[TB_KEY_SIZE], @@ -247,8 +257,7 @@ static void tboot_gen_xenheap_integrity(const uint8_t key[TB_KEY_SIZE], } *mac = vmac(NULL, 0, nonce, NULL, &ctx); - /* wipe ctx to ensure key is not left in memory */ - memset(&ctx, 0, sizeof(ctx)); + wipe_ctx(&ctx); } static void tboot_gen_frametable_integrity(const uint8_t key[TB_KEY_SIZE], @@ -276,8 +285,7 @@ static void tboot_gen_frametable_integrity(const uint8_t key[TB_KEY_SIZE], *mac = vmac(NULL, 0, nonce, NULL, &ctx); - /* wipe ctx to ensure key is not left in memory */ - memset(&ctx, 0, sizeof(ctx)); + wipe_ctx(&ctx); } void tboot_shutdown(uint32_t shutdown_type)