[PATCH v3 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>
Link: https://lore.kernel.org/all/CAKw_Dz96rfSQc6Rn+9QBcUFHhmkK+9zu+P=bxowfZwxrATCBRg@mail.gmail.com/
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: stable@vger.kernel.org
---
 arch/x86/coco/tdx/tdx.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 65119362f9a2..58feca419326 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -703,8 +703,25 @@ 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;
+	/*
+	 * IN writes the result into a sub-register of RAX. Only the
+	 * 32-bit form zero-extends; the smaller forms leave the upper
+	 * bits untouched:
+	 *
+	 *   insn  dest  size  bits written     bits preserved
+	 *   inb   AL    1     RAX[ 7: 0]       RAX[63: 8]
+	 *   inw   AX    2     RAX[15: 0]       RAX[63:16]
+	 *   inl   EAX   4     RAX[63: 0]       (none, zero-extended)
+	 *
+	 * 'mask' only covers the low 'size' bytes, which is exactly the
+	 * range affected for size 1 and 2. For size 4 the write also
+	 * clears RAX[63:32], so widen the clear-mask.
+	 */
+	if (size == 4)
+		regs->ax = 0;
+	else
+		regs->ax &= ~mask;
+
 	if (success)
 		regs->ax |= args.r11 & mask;
 
-- 
2.54.0