[PATCHv2 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
The unwanted loads are typically harmless. But, they might be made to
totally unrelated or even unmapped memory. load_unaligned_zeropad()
relies on exception fixup (#PF, #GP and now #VE) to recover from these
unwanted loads.

In TDX guests, the second page can be shared page and VMM may configure
it to trigger #VE.

Kernel assumes that #VE on a shared page is MMIO access and tries to
decode instruction to handle it. In case of load_unaligned_zeropad() it
may result in confusion as it is not MMIO access.

Check fixup table before trying to handle MMIO.

The issue was discovered by analysis. It was not triggered during the
testing.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/coco/tdx/tdx.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 010dc229096a..1a1c8a92cfa5 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -11,6 +11,8 @@
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
 #include <asm/pgtable.h>
+#include <asm/trapnr.h>
+#include <asm/extable.h>
 
 /* TDX module Call Leaf IDs */
 #define TDX_GET_INFO			1
@@ -299,6 +301,24 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	if (WARN_ON_ONCE(user_mode(regs)))
 		return -EFAULT;
 
+	/*
+	 * load_unaligned_zeropad() relies on exception fixups in case of the
+	 * word being a page-crosser and the second page is not accessible.
+	 *
+	 * In TDX guests, the second page can be shared page and VMM may
+	 * configure it to trigger #VE.
+	 *
+	 * Kernel assumes that #VE on a shared page is MMIO access and tries to
+	 * decode instruction to handle it. In case of load_unaligned_zeropad()
+	 * it may result in confusion as it is not MMIO access.
+	 *
+	 * Check fixup table before trying to handle MMIO.
+	 */
+	if (fixup_exception(regs, X86_TRAP_VE, 0, ve->gla)) {
+		/* regs->ip is adjusted by fixup_exception() */
+		return 0;
+	}
+
 	if (copy_from_kernel_nofault(buffer, (void *)regs->ip, MAX_INSN_SIZE))
 		return -EFAULT;
 
-- 
2.35.1