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

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Date: Tue, 21 May 2024 10:35:05 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Cc: linux-coco@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	cho@...rosoft.com,
	decui@...rosoft.com,
	John.Starks@...rosoft.com
Subject: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

Currently attempt to do MMIO from userspace in TDX guest leads to
warning about unexpect #VE and SIGSEGV being delivered to the process.

Enlightened userspace might choose to deal with MMIO on their own if
kernel doesn't emulate it.

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 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