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

[Kirill A. Shutemov]

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 guest 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.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 03deb4d6920d..5fbdda2f2b86 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
@@ -296,6 +298,26 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	if (WARN_ON_ONCE(user_mode(regs)))
 		return false;
 
+	/*
+	 * 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 guest 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() */
+		ve->instr_len = 0;
+
+		return true;
+	}
+
 	if (copy_from_kernel_nofault(buffer, (void *)regs->ip, MAX_INSN_SIZE))
 		return false;
 
-- 
2.35.1