[PATCH 3/4] x86/tdx: Relax SEPT_VE_DISABLE check for debug TD

Kirill A. Shutemov
2022-12-09T13:25:45Z

SEPT_VE_DISABLE check is required to keep the TD protected from VMM
attacks, but it makes harder to debug guest kernel bugs. If guest
touches unaccepted memory the TD will get terminated without any
traces on what has happened.

Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
the #VE handler on EPT-violation on private memory. It will produce
useful backtrace.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 8ad04d101270..0e47846ff8ff 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -38,6 +38,7 @@
#define VE_GET_PORT_NUM(e) ((e) >> 16)
#define VE_IS_IO_STRING(e) ((e) & BIT(4))

+#define ATTR_DEBUG BIT(0)
#define ATTR_SEPT_VE_DISABLE BIT(28)

/* TDX Module call error codes */
@@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
* TD-private memory. Only VMM-shared memory (MMIO) will #VE.
*/
td_attr = out.rdx;
- if (!(td_attr & ATTR_SEPT_VE_DISABLE))
- tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
+ if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
+ const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
+
+ /* Relax SEPT_VE_DISABLE check for debug TD. */
+ if (td_attr & ATTR_DEBUG)
+ pr_warn("%s\n", msg);
+ else
+ tdx_panic(msg);
+ }
}

/*
@@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
case EXIT_REASON_CPUID:
return handle_cpuid(regs, ve);
case EXIT_REASON_EPT_VIOLATION:
+ if (ve->gpa != cc_mkdec(ve->gpa))
+ panic("Unexpected EPT-violation on private memory.");
return handle_mmio(regs, ve);
case EXIT_REASON_IO_INSTRUCTION:
return handle_io(regs, ve);
--
2.38.0