Booting x86_64 with CONFIG_EFI_PGT_DUMP=y shows messages of the form "mm/pgtable-generic.c:53: bad pmd (____ptrval____)(8000000100077061)". EFI_PGT_DUMP dumps all of efi_mm, including the espfix area, which is set up with pmd entries which fit the pmd_bad() check: so 0d940a9b270b warns and clears those entries, which would ruin running Win16 binaries. The failing pte_offset_map() stopped such a kernel from even booting, until a few commits later be872f83bf57 changed the pagewalk to tolerate that: but it needs to be even more careful, to not spoil those entries. I might have preferred to change init_espfix_ap() not to use "bad" pmd entries; or to leave them out of the efi_mm dump. But there is great value in staying away from there, and a pagewalk check of address against TASK_SIZE may protect from other such aberrations too. Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx> Closes: https://lore.kernel.org/linux-mm/CABXGCsN3JqXckWO=V7p=FhPU1tK03RE1w9UE6xL5Y86SMk209w@xxxxxxxxxxxxxx/ Fixes: 0d940a9b270b ("mm/pgtable: allow pte_offset_map[_lock]() to fail") Fixes: be872f83bf57 ("mm/pagewalk: walk_pte_range() allow for pte_offset_map()") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- mm/pagewalk.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/mm/pagewalk.c b/mm/pagewalk.c index 64437105fe0d..2022333805d3 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c @@ -48,8 +48,11 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end, if (walk->no_vma) { /* * pte_offset_map() might apply user-specific validation. + * Indeed, on x86_64 the pmd entries set up by init_espfix_ap() + * fit its pmd_bad() check (_PAGE_NX set and _PAGE_RW clear), + * and CONFIG_EFI_PGT_DUMP efi_mm goes so far as to walk them. */ - if (walk->mm == &init_mm) + if (walk->mm == &init_mm || addr >= TASK_SIZE) pte = pte_offset_kernel(pmd, addr); else pte = pte_offset_map(pmd, addr); -- 2.35.3
