[PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

[Kirill A. Shutemov]

Currently, attempting to perform MMIO from userspace in a TDX guest
leads to a warning about an unexpected #VE and SIGSEGV being delivered
to the process.

Enlightened userspace may choose to handle MMIO on their own if the
kernel does not emulate it.

Handle the EPT_VIOLATION exit reason for userspace and deliver SIGBUS
instead of SIGSEGV. SIGBUS is more appropriate for the MMIO situation.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

v2:
  - Rebased;
  - Fix grammar;
---
 arch/x86/coco/tdx/tdx.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index c1cb90369915..d2aa93cebf5a 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -7,6 +7,7 @@
 #include <linux/cpufeature.h>
 #include <linux/export.h>
 #include <linux/io.h>
+#include <linux/sched/signal.h>
 #include <asm/coco.h>
 #include <asm/tdx.h>
 #include <asm/vmx.h>
@@ -630,6 +631,11 @@ void tdx_get_ve_info(struct ve_info *ve)
 	ve->instr_info  = upper_32_bits(args.r10);
 }
 
+static inline bool is_private_gpa(u64 gpa)
+{
+	return gpa == cc_mkenc(gpa);
+}
+
 /*
  * Handle the user initiated #VE.
  *
@@ -641,17 +647,20 @@ static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
 	switch (ve->exit_reason) {
 	case EXIT_REASON_CPUID:
 		return handle_cpuid(regs, ve);
+	case EXIT_REASON_EPT_VIOLATION:
+		if (is_private_gpa(ve->gpa))
+			panic("Unexpected EPT-violation on private memory.");
+
+		force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)ve->gla);
+
+		/* Return 0 to avoid incrementing RIP */
+		return 0;
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		return -EIO;
 	}
 }
 
-static inline bool is_private_gpa(u64 gpa)
-{
-	return gpa == cc_mkenc(gpa);
-}
-
 /*
  * Handle the kernel #VE.
  *
-- 
2.43.0