Disallow preemptive creation of wired superpage mappings.

There are some unusual cases where a process may cause an mlock()ed
range of memory to be unmapped.  If the application subsequently
faults on that region, the handler may attempt to create a superpage
mapping backed by the resident, wired pages.  However, the pmap code
responsible for creating such a mapping (pmap_enter_pde() on i386
and amd64) does not ensure that a leaf page table page is available
if the superpage is later demoted; the demotion operation must therefore
perform a non-blocking page allocation and must unmap the entire
superpage if the allocation fails.  The pmap layer ensures that this
can never happen for wired mappings, and so the case described above
breaks that invariant.

For now, simply ensure that the MI fault handler never attempts to
create a wired superpage except via promotion.

Reviewed by:	kib
Reported by:	syzbot+292d3b0416c27c131505@syzkaller.appspotmail.com
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19670

sys/amd64/amd64/pmap.c

@@ -5308,6 +5308,8 @@ pmap_enter_pde(pmap_t pmap, vm_offset_t va, pd_entry_t newpde, u_int flags,
 	pt_entry_t PG_G, PG_RW, PG_V;
 	vm_page_t mt, pdpg;
 
+	KASSERT(pmap == kernel_pmap || (newpde & PG_W) == 0,
+	    ("pmap_enter_pde: cannot create wired user mapping"));
 	PG_G = pmap_global_bit(pmap);
 	PG_RW = pmap_rw_bit(pmap);
 	KASSERT((newpde & (pmap_modified_bit(pmap) | PG_RW)) != PG_RW,

sys/i386/i386/pmap.c

@@ -3882,6 +3882,8 @@ pmap_enter_pde(pmap_t pmap, vm_offset_t va, pd_entry_t newpde, u_int flags,
 	rw_assert(&pvh_global_lock, RA_WLOCKED);
 	KASSERT((newpde & (PG_M | PG_RW)) != PG_RW,
 	    ("pmap_enter_pde: newpde is missing PG_M"));
+	KASSERT(pmap == kernel_pmap || (newpde & PG_W) == 0,
+	    ("pmap_enter_pde: cannot create wired user mapping"));
 	PMAP_LOCK_ASSERT(pmap, MA_OWNED);
 	pde = pmap_pde(pmap, va);
 	oldpde = *pde;

sys/vm/vm_fault.c

@@ -294,7 +294,7 @@ vm_fault_soft_fast(struct faultstate *fs, vm_offset_t vaddr, vm_prot_t prot,
 	    rounddown2(vaddr, pagesizes[m_super->psind]) >= fs->entry->start &&
 	    roundup2(vaddr + 1, pagesizes[m_super->psind]) <= fs->entry->end &&
 	    (vaddr & (pagesizes[m_super->psind] - 1)) == (VM_PAGE_TO_PHYS(m) &
-	    (pagesizes[m_super->psind] - 1)) &&
+	    (pagesizes[m_super->psind] - 1)) && !wired &&
 	    pmap_ps_enabled(fs->map->pmap)) {
 		flags = PS_ALL_VALID;
 		if ((prot & VM_PROT_WRITE) != 0) {
@@ -469,7 +469,7 @@ vm_fault_populate(struct faultstate *fs, vm_prot_t prot, int fault_type,
 		psind = m->psind;
 		if (psind > 0 && ((vaddr & (pagesizes[psind] - 1)) != 0 ||
 		    pidx + OFF_TO_IDX(pagesizes[psind]) - 1 > pager_last ||
-		    !pmap_ps_enabled(fs->map->pmap)))
+		    !pmap_ps_enabled(fs->map->pmap) || wired))
 			psind = 0;
 #else
 		psind = 0;