Commit 1e776965 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Dave Hansen
Browse files

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: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220614120135.14812-4-kirill.shutemov@linux.intel.com
parent cdd85786

arch/x86/coco/tdx/tdx.c

+14 −1
Original line number Diff line number Diff line
@@ -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: