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@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

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index c8d44f4..928dcf7 100644 (file)
--- 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: