x86/tdx: Fix "in-kernel MMIO" check

BugLink: https://bugs.launchpad.net/bugs/2089340

commit d4fc4d01471528da8a9797a065982e05090e1d81 upstream.

TDX only supports kernel-initiated MMIO operations. The handle_mmio()
function checks if the #VE exception occurred in the kernel and rejects
the operation if it did not.

However, userspace can deceive the kernel into performing MMIO on its
behalf. For example, if userspace can point a syscall to an MMIO address,
syscall does get_user() or put_user() on it, triggering MMIO #VE. The
kernel will treat the #VE as in-kernel MMIO.

Ensure that the target MMIO address is within the kernel before decoding
instruction.

Fixes: 31d58c4e55 ("x86/tdx: Handle in-kernel MMIO")
Signed-off-by: Alexey Gladkov (Intel) <legion@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc:stable@vger.kernel.org
Link: https://lore.kernel.org/all/565a804b80387970460a4ebc67c88d1380f61ad1.1726237595.git.legion%40kernel.org
Signed-off-by: Alexey Gladkov (Intel) <legion@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/coco/tdx/tdx.c

@@ -14,6 +14,7 @@
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
 #include <asm/pgtable.h>
+#include <asm/traps.h>
 
 /* MMIO direction */
 #define EPT_READ	0
@@ -429,6 +430,11 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 			return -EINVAL;
 	}
 
+	if (!fault_in_kernel_space(ve->gla)) {
+		WARN_ONCE(1, "Access to userspace address is not supported");
+		return -EINVAL;
+	}
+
 	/*
 	 * Reject EPT violation #VEs that split pages.
 	 *