[PATCH v6 1/6] x86/tdx: Fix "in-kernel MMIO" check

[Alexey Gladkov]

From: "Alexey Gladkov (Intel)" <legion@xxxxxxxxxx>

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.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Alexey Gladkov (Intel) <legion@xxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 078e2bac2553..c90d2fdb5fc4 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -405,6 +405,11 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
 			       EPT_WRITE, addr, val);
 }
 
+static inline bool is_kernel_addr(unsigned long addr)
+{
+	return (long)addr < 0;
+}
+
 static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 {
 	unsigned long *reg, val, vaddr;
@@ -434,6 +439,11 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 			return -EINVAL;
 	}
 
+	if (!user_mode(regs) && !is_kernel_addr(ve->gla)) {
+		WARN_ONCE(1, "Access to userspace address is not supported");
+		return -EINVAL;
+	}
+
 	/*
 	 * Reject EPT violation #VEs that split pages.
 	 *
-- 
2.46.0