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Message-Id: <20220614120135.14812-4-kirill.shutemov@linux.intel.com>
Date: Tue, 14 Jun 2022 15:01:35 +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: [PATCHv4 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 split page MMIO accesses 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 | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 7d6d484a6d28..3bcaf2170ede 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -333,8 +333,8 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
{
+ unsigned long *reg, val, vaddr;
char buffer[MAX_INSN_SIZE];
- unsigned long *reg, val;
struct insn insn = {};
enum mmio_type mmio;
int size, extend_size;
@@ -360,6 +360,19 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
return -EINVAL;
}
+ /*
+ * Reject EPT violation #VEs that split pages.
+ *
+ * MMIO accesses suppose to be naturally aligned and therefore never
+ * cross a page boundary. Seeing split page accesses indicates a bug
+ * or load_unaligned_zeropad() that steps into unmapped shared page.
+ *
+ * load_unaligned_zeropad() will recover using exception fixups.
+ */
+ vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
+ if (vaddr / PAGE_SIZE != (vaddr + size) / PAGE_SIZE)
+ return -EFAULT;
+
/* Handle writes first */
switch (mmio) {
case MMIO_WRITE:
--
2.35.1
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