[PATCH 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

According to x86 architecture rules, 32-bit operations zero-extend the
result to 64 bits. The current implementation of handle_in() only masks
the lower 32 bits, which preserves the upper 32 bits of RAX when a
32-bit port IN instruction is emulated.

Update handle_in() to zero out the entire RAX register when the I/O size
is 4 bytes to ensure correct zero-extension. For smaller sizes (1 or 2
bytes), continue to preserve the unaffected upper bits.

Fixes: 03149948832a ("x86/tdx: Port I/O: Add runtime hypercalls")
Reported-by: Borys Tsyrulnikov <tsyrulnikov.borys@gmail.com>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/x86/coco/tdx/tdx.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 4d7f71d50122..b9b9a2d75119 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -703,8 +703,17 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
 	 */
 	success = !__tdx_hypercall(&args);
 
-	/* Update part of the register affected by the emulated instruction */
-	regs->ax &= ~mask;
+	/*
+	 * Update part of the register affected by the emulated instruction.
+	 *
+	 * 32-bit operands generate a 32-bit result, zero-extended to a 64-bit
+	 * result.
+	 */
+	if (size < 4)
+		regs->ax &= ~mask;
+	else
+		regs->ax = 0;
+
 	if (success)
 		regs->ax |= args.r11 & mask;
 
-- 
2.51.2