linux-kernel - [PATCHv3 3/3] x86/tdx: Handle load_unaligned

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Date:   Wed, 25 May 2022 01:10:12 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...el.com, luto@...nel.org, peterz@...radead.org
Cc:     ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        thomas.lendacky@....com, x86@...nel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv3 3/3] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

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 unaligned MMIO accesses (it includes page-crossings)
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@...ux.intel.com>
---
 arch/x86/coco/tdx/tdx.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 94e447e7f103..4e566ed67db8 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -331,6 +331,17 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 			return -EINVAL;
 	}

+	/*
+	 * MMIO accesses suppose to be naturally aligned and therefore never
+	 * cross a page boundary. Seeing unaligned accesses indicates a bug or
+	 * load_unaligned_zeropad() that steps into unmapped shared page.
+	 *
+	 * In both cases fail the #VE handling. load_unaligned_zeropad() will
+	 * recover using exception fixups.
+	 */
+	if ((unsigned long)insn_get_addr_ref(&insn, regs) % size)
+		return -EFAULT;
+
 	/* Handle writes first */
 	switch (mmio) {
 	case MMIO_WRITE:
-- 
2.35.1