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x86/pti: Make unpoison of pgd for trusted boot work for real
commit445b69e3b7
upstream. The inital fix for trusted boot and PTI potentially misses the pgd clearing if pud_alloc() sets a PGD. It probably works in *practice* because for two adjacent calls to map_tboot_page() that share a PGD entry, the first will clear NX, *then* allocate and set the PGD (without NX clear). The second call will *not* allocate but will clear the NX bit. Defer the NX clearing to a point after it is known that all top-level allocations have occurred. Add a comment to clarify why. [ tglx: Massaged changelog ] Fixes:262b6b3008
("x86/tboot: Unbreak tboot with PTI enabled") Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Cc: Jon Masters <jcm@redhat.com> Cc: "Tim Chen" <tim.c.chen@linux.intel.com> Cc: gnomes@lxorguk.ukuu.org.uk Cc: peterz@infradead.org Cc: ning.sun@intel.com Cc: tboot-devel@lists.sourceforge.net Cc: andi@firstfloor.org Cc: luto@kernel.org Cc: law@redhat.com Cc: pbonzini@redhat.com Cc: torvalds@linux-foundation.org Cc: gregkh@linux-foundation.org Cc: dwmw@amazon.co.uk Cc: nickc@redhat.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 changed files with 11 additions and 1 deletions
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@ -127,7 +127,6 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
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p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
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if (!p4d)
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return -1;
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pgd->pgd &= ~_PAGE_NX;
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pud = pud_alloc(&tboot_mm, p4d, vaddr);
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if (!pud)
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return -1;
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@ -139,6 +138,17 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
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return -1;
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set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot));
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pte_unmap(pte);
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/*
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* PTI poisons low addresses in the kernel page tables in the
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* name of making them unusable for userspace. To execute
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* code at such a low address, the poison must be cleared.
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*
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* Note: 'pgd' actually gets set in p4d_alloc() _or_
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* pud_alloc() depending on 4/5-level paging.
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*/
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pgd->pgd &= ~_PAGE_NX;
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return 0;
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}
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