Commit f3176ec9 authored by Andrey Ryabinin's avatar Andrey Ryabinin Committed by Thomas Gleixner

x86/kasan: Fix boot with 5-level paging and KASAN

Since commit d52888aa ("x86/mm: Move LDT remap out of KASLR region on
5-level paging") kernel doesn't boot with KASAN on 5-level paging machines.
The bug is actually in early_p4d_offset() and introduced by commit
12a8cc7f ("x86/kasan: Use the same shadow offset for 4- and 5-level paging")

early_p4d_offset() tries to convert pgd_val(*pgd) value to a physical
address. This doesn't make sense because pgd_val() already contains the
physical address.

It did work prior to commit d52888aa because the result of
"__pa_nodebug(pgd_val(*pgd)) & PTE_PFN_MASK" was the same as "pgd_val(*pgd)
& PTE_PFN_MASK". __pa_nodebug() just set some high bits which were masked
out by applying PTE_PFN_MASK.

After the change of the PAGE_OFFSET offset in commit d52888aa
__pa_nodebug(pgd_val(*pgd)) started to return a value with more high bits
set and PTE_PFN_MASK wasn't enough to mask out all of them. So it returns a
wrong not even canonical address and crashes on the attempt to dereference

Switch back to pgd_val() & PTE_PFN_MASK to cure the issue.

Fixes: 12a8cc7f ("x86/kasan: Use the same shadow offset for 4- and 5-level paging")
Reported-by: default avatarKirill A. Shutemov <>
Signed-off-by: default avatarAndrey Ryabinin <>
Signed-off-by: default avatarThomas Gleixner <>
Cc: Borislav Petkov <>
Cc: "H. Peter Anvin" <>
Cc: Alexander Potapenko <>
Cc: Dmitry Vyukov <>
Cc: <>
parent 8d3289f2
......@@ -199,7 +199,7 @@ static inline p4d_t *early_p4d_offset(pgd_t *pgd, unsigned long addr)
if (!pgtable_l5_enabled())
return (p4d_t *)pgd;
p4d = __pa_nodebug(pgd_val(*pgd)) & PTE_PFN_MASK;
p4d = pgd_val(*pgd) & PTE_PFN_MASK;
p4d += __START_KERNEL_map - phys_base;
return (p4d_t *)p4d + p4d_index(addr);
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