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Peter Anvin" , Rick Edgecombe , Kuppuswamy Sathyanarayanan , Kai Huang , Borys Tsyrulnikov , linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, stable@vger.kernel.org, "Kiryl Shutsemau (Meta)" Subject: [PATCH v2 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O Date: Tue, 28 Apr 2026 13:56:32 +0100 Message-ID: <20260428125632.129770-3-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20260428125632.129770-1-kas@kernel.org> References: <20260428125632.129770-1-kas@kernel.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Link: https://lore.kernel.org/all/CAKw_Dz96rfSQc6Rn+9QBcUFHhmkK+9zu+P=bxowfZwxrATCBRg@mail.gmail.com/ Signed-off-by: Kiryl Shutsemau (Meta) Reviewed-by: Kai Huang Reviewed-by: Kuppuswamy Sathyanarayanan 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 65119362f9a2..e09636564237 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 = 0; + else + regs->ax &= ~mask; + if (success) regs->ax |= args.r11 & mask; -- 2.51.2