From: Dave Hansen dave.hansen@linux.intel.com

commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a 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: 262b6b30087 ("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 Cc: Jiri Kosina jkosina@suse.cz Signed-off-by: Hugh Dickins hughd@google.com

hughd notes: I have not tested tboot, but this looks to me as necessary and as safe in old-Kaiser backports as it is upstream; I'm not submitting the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e, and makes conflict trouble because of 5-level's p4d versus 4-level's pgd. --- arch/x86/kernel/tboot.c | 10 ++++++++++ 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c index 91a4496db434..c77ab1f51fbe 100644 --- a/arch/x86/kernel/tboot.c +++ b/arch/x86/kernel/tboot.c @@ -140,6 +140,16 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn, return -1; set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot)); pte_unmap(pte); + + /* + * PTI poisons low addresses in the kernel page tables in the + * name of making them unusable for userspace. To execute + * code at such a low address, the poison must be cleared. + * + * Note: 'pgd' actually gets set in pud_alloc(). + */ + pgd->pgd &= ~_PAGE_NX; + return 0; }