[tip: x86/urgent] x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

[tip-bot2 for Kirill A. Shutemov]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     1e7769653b06b56b7ea7d56911d2d5b2957750cd
Gitweb:        https://git.kernel.org/tip/1e7769653b06b56b7ea7d56911d2d5b2957750cd
Author:        Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
AuthorDate:    Tue, 14 Jun 2022 15:01:35 +03:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Fri, 17 Jun 2022 15:37:33 -07:00

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 a VMM may configure
it to trigger #VE.

The kernel assumes that #VE on a shared page is an 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 failing them.
load_unaligned_zeropad() will recover using exception fixups.

The issue was discovered by analysis and reproduced artificially. It was
not triggered during testing.

[ dhansen: fix up changelogs and comments for grammar and clarity,
	   plus incorporate Kirill's off-by-one fix]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20220614120135.14812-4-kirill.shutemov@xxxxxxxxxxxxxxx
---
 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 c8d44f4..928dcf7 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 are supposed to be naturally aligned and therefore
+	 * never cross page boundaries. Seeing split page accesses indicates
+	 * a bug or a load_unaligned_zeropad() that stepped into an MMIO page.
+	 *
+	 * load_unaligned_zeropad() will recover using exception fixups.
+	 */
+	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
+	if (vaddr / PAGE_SIZE != (vaddr + size - 1) / PAGE_SIZE)
+		return -EFAULT;
+
 	/* Handle writes first */
 	switch (mmio) {
 	case MMIO_WRITE: