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x86/tdx: Handle load_unaligned_zeropad() page-cross to a shared page

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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@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220614120135.14812-4-kirill.shutemov@linux.intel.com
This commit is contained in:
Kirill A. Shutemov 2022-06-14 15:01:35 +03:00 committed by Dave Hansen
parent cdd85786f4
commit 1e7769653b
1 changed files with 14 additions and 1 deletions
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15
arch/x86/coco/tdx/tdx.c
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@ -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: