clear_page() does not undergo the XOR logic to invert the address bits, i.e. PTE, PMD and PUD entries that have not been individually written will have val=0 and so will trigger __pte_needs_invert(). As a result, {pte,pmd,pud}_pfn() will return the wrong PFN value, i.e. all ones (adjusted by the max PFN mask) instead of zero. A zeroed entry is ok because the page at physical address 0 is reserved early in boot specifically to mitigate L1TF, so explicitly exempt them from the inversion when reading the PFN.
Manifested as an unexpected mprotect(..., PROT_NONE) failure when called on a VMA that has VM_PFNMAP and was mmap'd to as something other than PROT_NONE but never used. mprotect() sends the PROT_NONE request down prot_none_walk(), which walks the PTEs to check the PFNs. prot_none_pte_entry() gets the bogus PFN from pte_pfn() and returns -EACCES because it thinks mprotect() is trying to adjust a high MMIO address. Fixes: 6b28baca9b1f ("x86/speculation/l1tf: Protect PROT_NONE PTEs against speculation") Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com> Cc: Andi Kleen <a...@linux.intel.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Josh Poimboeuf <jpoim...@redhat.com> Cc: Michal Hocko <mho...@suse.com> Cc: Vlastimil Babka <vba...@suse.cz> Cc: Dave Hansen <dave.han...@intel.com> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- arch/x86/include/asm/pgtable.h | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index e4ffa565a69f..f21a1df4ca89 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -195,21 +195,24 @@ static inline u64 protnone_mask(u64 val); static inline unsigned long pte_pfn(pte_t pte) { phys_addr_t pfn = pte_val(pte); - pfn ^= protnone_mask(pfn); + if (pfn) + pfn ^= protnone_mask(pfn); return (pfn & PTE_PFN_MASK) >> PAGE_SHIFT; } static inline unsigned long pmd_pfn(pmd_t pmd) { phys_addr_t pfn = pmd_val(pmd); - pfn ^= protnone_mask(pfn); + if (pfn) + pfn ^= protnone_mask(pfn); return (pfn & pmd_pfn_mask(pmd)) >> PAGE_SHIFT; } static inline unsigned long pud_pfn(pud_t pud) { phys_addr_t pfn = pud_val(pud); - pfn ^= protnone_mask(pfn); + if (pfn) + pfn ^= protnone_mask(pfn); return (pfn & pud_pfn_mask(pud)) >> PAGE_SHIFT; } -- 2.18.0