[PATCH v3 6/7] x86/tdx: Add a restriction on access to MMIO address

[Alexey Gladkov (Intel)]

In the case of userspace MMIO, if the user instruction + MAX_INSN_SIZE
straddles page, then the "fetch" in the kernel could trigger a #VE. In
this case the kernel would handle this second #VE as a !user_mode() MMIO.
That way, additional address verifications can be avoided.

The scenario of accessing userspace MMIO addresses from kernelspace does
not seem appropriate under normal circumstances. Until there is a
specific usecase for such a scenario it can be disabled.

Signed-off-by: Alexey Gladkov (Intel) <legion@xxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index dfadb085d2d3..5b3421a89998 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -411,6 +411,11 @@ static inline bool is_private_gpa(u64 gpa)
 	return gpa == cc_mkenc(gpa);
 }
 
+static inline bool is_kernel_addr(unsigned long addr)
+{
+	return (long)addr < 0;
+}
+
 static int get_phys_addr(unsigned long addr, phys_addr_t *phys_addr, bool *writable)
 {
 	unsigned int level;
@@ -641,6 +646,7 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	unsigned long vaddr;
 	int size, ret;
 
+
 	ret = decode_insn_struct(&insn, regs);
 	if (ret)
 		return ret;
@@ -661,6 +667,9 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 		ret = valid_vaddr(ve, mmio, size, vaddr);
 		if (ret)
 			goto unlock;
+	} else if (!is_kernel_addr(ve->gla)) {
+		WARN_ONCE(1, "Access to userspace address is not supported");
+		return -EINVAL;
 	}
 
 	/*
-- 
2.45.2