[PATCHv4 3/3] 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 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.

Fix it by detecting split page MMIO accesses and fail them.
load_unaligned_zeropad() will recover using exception fixups.

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 7d6d484a6d28..3bcaf2170ede 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -333,8 +333,8 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
 
 static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 {
+	unsigned long *reg, val, vaddr;
 	char buffer[MAX_INSN_SIZE];
-	unsigned long *reg, val;
 	struct insn insn = {};
 	enum mmio_type mmio;
 	int size, extend_size;
@@ -360,6 +360,19 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 			return -EINVAL;
 	}
 
+	/*
+	 * Reject EPT violation #VEs that split pages.
+	 *
+	 * MMIO accesses suppose to be naturally aligned and therefore never
+	 * cross a page boundary. Seeing split page accesses indicates a bug
+	 * or load_unaligned_zeropad() that steps into unmapped shared page.
+	 *
+	 * load_unaligned_zeropad() will recover using exception fixups.
+	 */
+	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
+	if (vaddr / PAGE_SIZE != (vaddr + size) / PAGE_SIZE)
+		return -EFAULT;
+
 	/* Handle writes first */
 	switch (mmio) {
 	case MMIO_WRITE:
-- 
2.35.1