On 8 June 2017 at 13:28, Mark Rutland <mark.rutland@xxxxxxx> wrote: > On Thu, Jun 08, 2017 at 01:59:46PM +0100, Mark Rutland wrote: >> On Thu, Jun 08, 2017 at 11:35:48AM +0000, Ard Biesheuvel wrote: >> > @@ -287,10 +288,10 @@ struct page *vmalloc_to_page(const void *vmalloc_addr) >> > if (p4d_none(*p4d)) >> > return NULL; >> > pud = pud_offset(p4d, addr); >> > - if (pud_none(*pud)) >> > + if (pud_none(*pud) || WARN_ON_ONCE(pud_huge(*pud))) >> > return NULL; >> > pmd = pmd_offset(pud, addr); >> > - if (pmd_none(*pmd)) >> > + if (pmd_none(*pmd) || WARN_ON_ONCE(pmd_huge(*pmd))) >> > return NULL; >> >> I think it might be better to use p*d_bad() here, since that doesn't >> depend on CONFIG_HUGETLB_PAGE. >> >> While the cross-arch semantics are a little fuzzy, my understanding is >> those should return true if an entry is not a pointer to a next level of >> table (so pXd_huge(p) implies pXd_bad(p)). > > Ugh; it turns out this isn't universally true. > > I see that at least arch/hexagon's pmd_bad() always returns 0, and they > support CONFIG_HUGETLB_PAGE. > Well, the comment in arch/hexagon/include/asm/pgtable.h suggests otherwise: /* HUGETLB not working currently */ > So I guess there isn't an arch-neutral, always-available way of checking > this. Sorry for having mislead you. > > For arm64, p*d_bad() would still be preferable, so maybe we should check > both? > I am primarily interested in hardening architectures that define CONFIG_HAVE_ARCH_HUGE_VMAP, given that they intentionally create huge mappings in the VMALLOC area which this code may choke on. So whether pmd_bad() always returns 0 on an arch that does not define CONFIG_HAVE_ARCH_HUGE_VMAP does not really matter, because it simply nullifies this change for that particular architecture. So as long as x86 and arm64 [which are the only ones to define CONFIG_HAVE_ARCH_HUGE_VMAP atm] work correctly with pXd_bad(), I think we should use it instead of pXd_huge(), -- Ard. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
