[PATCH v15 04/23] x86/cpu: Detect TDX partial write machine check erratum

Kai Huang
2023-11-09T11:56:41Z

TDX memory has integrity and confidentiality protections. Violations of
this integrity protection are supposed to only affect TDX operations and
are never supposed to affect the host kernel itself. In other words,
the host kernel should never, itself, see machine checks induced by the
TDX integrity hardware.

Alas, the first few generations of TDX hardware have an erratum. A
partial write to a TDX private memory cacheline will silently "poison"
the line. Subsequent reads will consume the poison and generate a
machine check. According to the TDX hardware spec, neither of these
things should have happened.

Virtually all kernel memory accesses operations happen in full
cachelines. In practice, writing a "byte" of memory usually reads a 64
byte cacheline of memory, modifies it, then writes the whole line back.
Those operations do not trigger this problem.

This problem is triggered by "partial" writes where a write transaction
of less than cacheline lands at the memory controller. The CPU does
these via non-temporal write instructions (like MOVNTI), or through
UC/WC memory mappings. The issue can also be triggered away from the
CPU by devices doing partial writes via DMA.

With this erratum, there are additional things need to be done. To
prepare for those changes, add a CPU bug bit to indicate this erratum.
Note this bug reflects the hardware thus it is detected regardless of
whether the kernel is built with TDX support or not.

Signed-off-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/intel.c | 17 +++++++++++++++++
2 files changed, 18 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 4af140cf5719..d097e558e079 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -495,6 +495,7 @@
#define X86_BUG_EIBRS_PBRSB X86_BUG(28) /* EIBRS is vulnerable to Post Barrier RSB Predictions */
#define X86_BUG_SMT_RSB X86_BUG(29) /* CPU is vulnerable to Cross-Thread Return Address Predictions */
#define X86_BUG_GDS X86_BUG(30) /* CPU is affected by Gather Data Sampling */
+#define X86_BUG_TDX_PW_MCE X86_BUG(31) /* CPU may incur #MC if non-TD software does partial write to TDX private memory */

/* BUG word 2 */
#define X86_BUG_SRSO X86_BUG(1*32 + 0) /* AMD SRSO bug */
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index a927a8fc9624..1304d29c0660 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -184,6 +184,21 @@ static bool bad_spectre_microcode(struct cpuinfo_x86 *c)
return false;
}

+static void check_tdx_erratum(struct cpuinfo_x86 *c)
+{
+ /*
+ * These CPUs have an erratum. A partial write from non-TD
+ * software (e.g. via MOVNTI variants or UC/WC mapping) to TDX
+ * private memory poisons that memory, and a subsequent read of
+ * that memory triggers #MC.
+ */
+ switch (c->x86_model) {
+ case INTEL_FAM6_SAPPHIRERAPIDS_X:
+ case INTEL_FAM6_EMERALDRAPIDS_X:
+ setup_force_cpu_bug(X86_BUG_TDX_PW_MCE);
+ }
+}
+
static void early_init_intel(struct cpuinfo_x86 *c)
{
u64 misc_enable;
@@ -322,6 +337,8 @@ static void early_init_intel(struct cpuinfo_x86 *c)
*/
if (detect_extended_topology_early(c) < 0)
detect_ht_early(c);
+
+ check_tdx_erratum(c);
}

static void bsp_init_intel(struct cpuinfo_x86 *c)
--
2.41.0