4.9-stable review patch. If anyone has any objections, please let me know.
------------------ From: Dave Hansen <dave.han...@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 ] [ 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.] Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled") Signed-off-by: Dave Hansen <dave.han...@linux.intel.com> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Reviewed-by: Andrea Arcangeli <aarca...@redhat.com> Cc: Jon Masters <j...@redhat.com> Cc: Tim Chen <tim.c.c...@linux.intel.com> Cc: gno...@lxorguk.ukuu.org.uk Cc: pet...@infradead.org Cc: ning....@intel.com Cc: tboot-de...@lists.sourceforge.net Cc: a...@firstfloor.org Cc: l...@kernel.org Cc: l...@redhat.com Cc: pbonz...@redhat.com Cc: torva...@linux-foundation.org Cc: gre...@linux-foundation.org Cc: d...@amazon.co.uk Cc: ni...@redhat.com Link: https://lkml.kernel.org/r/20180110224939.2695c...@viggo.jf.intel.com Cc: Jiri Kosina <jkos...@suse.cz> Signed-off-by: Hugh Dickins <hu...@google.com> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- arch/x86/kernel/tboot.c | 10 ++++++++++ 1 file changed, 10 insertions(+) --- a/arch/x86/kernel/tboot.c +++ b/arch/x86/kernel/tboot.c @@ -134,6 +134,16 @@ static int map_tboot_page(unsigned long 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; }