[PATCH 6.6.y] x86/tdx: Fix zero-extension for 32-bit port I/O

[Kiryl Shutsemau <kas@kernel.org>]

From: "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.

Use insn_assign_reg() to write the result back into RAX with proper
partial-register-write semantics: 1- and 2-byte forms leave the upper
bits untouched, the 4-byte form zero-extends to the full register.

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>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/all/CAKw_Dz96rfSQc6Rn+9QBcUFHhmkK+9zu+P=bxowfZwxrATCBRg@mail.gmail.com/
Cc:stable@vger.kernel.org
Link: https://patch.msgid.link/20260713133753.223947-4-kirill@shutemov.name
(cherry picked from commit 941370fc93cc3474e26811f4d3b062903eefe2cf)
[ kas: adjust for the __tdx_hypercall_ret() spelling of the
  output-returning hypercall in this tree; no other change ]
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 arch/x86/coco/tdx/tdx.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index de56b69a0c27..504923e2f012 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -580,8 +580,8 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
 		.r13 = PORT_READ,
 		.r14 = port,
 	};
-	u64 mask = GENMASK(BITS_PER_BYTE * size - 1, 0);
 	bool success;
+	u64 val;
 
 	/*
 	 * Emulate the I/O read via hypercall. More info about ABI can be found
@@ -589,11 +589,9 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
 	 * "TDG.VP.VMCALL<Instruction.IO>".
 	 */
 	success = !__tdx_hypercall_ret(&args);
+	val = success ? args.r11 : 0;
 
-	/* Update part of the register affected by the emulated instruction */
-	regs->ax &= ~mask;
-	if (success)
-		regs->ax |= args.r11 & mask;
+	insn_assign_reg(&regs->ax, val, size);
 
 	return success;
 }
-- 
2.55.0