From 3f2e4f5dd83478b41ee273a500477cbf20266d23 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 17 Aug 2018 10:27:36 -0700 Subject: [PATCH] x86/speculation/l1tf: Exempt zeroed PTEs from inversion commit f19f5c49bbc3ffcc9126cc245fc1b24cc29f4a37 upstream. It turns out that we should *not* invert all not-present mappings, because the all zeroes case is obviously special. 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. [ This is a very modified version of Sean's original patch, but all credit goes to Sean for doing this and also pointing out that sometimes the __pte_needs_invert() function only gets the protection bits, not the full eventual pte. But zero remains special even in just protection bits, so that's ok. - Linus ] Fixes: f22cc87f6c1f ("x86/speculation/l1tf: Invert all not present mappings") Signed-off-by: Sean Christopherson Acked-by: Andi Kleen Cc: Thomas Gleixner Cc: Josh Poimboeuf Cc: Michal Hocko Cc: Vlastimil Babka Cc: Dave Hansen Cc: Greg Kroah-Hartman Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- arch/x86/include/asm/pgtable-invert.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/pgtable-invert.h b/arch/x86/include/asm/pgtable-invert.h index 44b1203ece12..a0c1525f1b6f 100644 --- a/arch/x86/include/asm/pgtable-invert.h +++ b/arch/x86/include/asm/pgtable-invert.h @@ -4,9 +4,18 @@ #ifndef __ASSEMBLY__ +/* + * A clear pte value is special, and doesn't get inverted. + * + * Note that even users that only pass a pgprot_t (rather + * than a full pte) won't trigger the special zero case, + * because even PAGE_NONE has _PAGE_PROTNONE | _PAGE_ACCESSED + * set. So the all zero case really is limited to just the + * cleared page table entry case. + */ static inline bool __pte_needs_invert(u64 val) { - return !(val & _PAGE_PRESENT); + return val && !(val & _PAGE_PRESENT); } /* Get a mask to xor with the page table entry to get the correct pfn. */ -- 2.20.1