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Fix a race between _pmap_unwire_ptp() and MipsDoTLBMiss().

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MipsDoTLBMiss() will load a segmap entry or pde, check that it isn't
zero, and then chase that pointer to a physical page. If that page has
been freed in the interim, it will read garbage and go on to populate
the TLB with it.

This can happen because pmap_unwire_ptp zeros out the pde and
vm_page_free_zero()s the ptp (or, recursively, zeros out the segmap
entry and vm_page_free_zero()s the pdp) without interlocking against
MipsDoTLBMiss(). The pmap is locked, and pvh_global_lock may or may not
be held, but this is not enough. Solve this issue by inserting TLB
shootdowns within _pmap_unwire_ptp(); as MipsDoTLBMiss() runs with IRQs
deferred, the IPIs involved in TLB shootdown are sufficient to ensure
that MipsDoTLBMiss() sees either a zero segmap entry / pde or a non-zero
entry and the pointed-to page still not freed.

Submitted by:	Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D24491
This commit is contained in:
Mark Johnston 2020-04-24 21:21:23 +00:00
parent b6fd3860b5
commit 117d10f3c1
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=360280
1 changed files with 97 additions and 21 deletions
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118
sys/mips/mips/pmap.c
View File

@ -1004,18 +1004,26 @@ static void
_pmap_unwire_ptp(pmap_t pmap, vm_offset_t va, vm_page_t m)
{
pd_entry_t *pde;
vm_offset_t sva, eva;
PMAP_LOCK_ASSERT(pmap, MA_OWNED);
/*
* unmap the page table page
*/
#ifdef __mips_n64
if (m->pindex < NUPDE)
if (m->pindex < NUPDE) {
pde = pmap_pde(pmap, va);
else
sva = va & ~PDRMASK;
eva = sva + NBPDR;
} else {
pde = pmap_segmap(pmap, va);
sva = va & ~SEGMASK;
eva = sva + NBSEG;
}
#else
pde = pmap_pde(pmap, va);
sva = va & ~SEGMASK;
eva = sva + NBSEG;
#endif
*pde = 0;
pmap->pm_stats.resident_count--;
@ -1026,12 +1034,22 @@ _pmap_unwire_ptp(pmap_t pmap, vm_offset_t va, vm_page_t m)
vm_page_t pdpg;
/*
* Recursively decrement next level pagetable refcount
* Recursively decrement next level pagetable refcount.
* Either that shoots down a larger range from TLBs (below)
* or we're to shoot down just the page in question.
*/
pdp = (pd_entry_t *)*pmap_segmap(pmap, va);
pdpg = PHYS_TO_VM_PAGE(MIPS_DIRECT_TO_PHYS(pdp));
pmap_unwire_ptp(pmap, va, pdpg);
if (!pmap_unwire_ptp(pmap, va, pdpg)) {
pmap_invalidate_range(pmap, sva, eva);
}
} else {
/* Segmap entry shootdown */
pmap_invalidate_range(pmap, sva, eva);
}
#else
/* Segmap entry shootdown */
pmap_invalidate_range(pmap, sva, eva);
#endif
/*
@ -1485,7 +1503,15 @@ pmap_pv_reclaim(pmap_t locked_pmap)
if (TAILQ_EMPTY(&m->md.pv_list))
vm_page_aflag_clear(m, PGA_WRITEABLE);
pc->pc_map[field] |= 1UL << bit;
pmap_unuse_pt(pmap, va, *pde);
/*
* For simplicity, we will unconditionally shoot
* down TLBs either at the end of this function
* or at the top of the loop above if we switch
* to a different pmap.
*/
(void)pmap_unuse_pt(pmap, va, *pde);
freed++;
}
}
@ -1714,6 +1740,23 @@ pmap_try_insert_pv_entry(pmap_t pmap, vm_page_t mpte, vm_offset_t va,
/*
* pmap_remove_pte: do the things to unmap a page in a process
*
* Returns true if this was the last PTE in the PT (and possibly the last PT in
* the PD, and possibly the last PD in the segmap), in which case...
*
* 1) the TLB has been invalidated for the whole PT's span (at least),
* already, to ensure that MipsDoTLBMiss does not attempt to follow a
* dangling pointer into a freed page. No additional TLB shootdown is
* required.
*
* 2) if this removal was part of a sweep to remove PTEs, it is safe to jump
* to the PT span boundary and continue.
*
* 3) The given pde may now point onto a freed page and must not be
* dereferenced
*
* If the return value is false, the TLB has not been shot down (and the segmap
* entry, PD, and PT all remain in place).
*/
static int
pmap_remove_pte(struct pmap *pmap, pt_entry_t *ptq, vm_offset_t va,
@ -1782,8 +1825,12 @@ pmap_remove_page(struct pmap *pmap, vm_offset_t va)
if (!pte_test(ptq, PTE_V))
return;
(void)pmap_remove_pte(pmap, ptq, va, *pde);
pmap_invalidate_page(pmap, va);
/*
* Remove this PTE from the PT. If this is the last one, then
* the TLB has already been shot down, so don't bother again
*/
if (!pmap_remove_pte(pmap, ptq, va, *pde))
pmap_invalidate_page(pmap, va);
}
/*
@ -1797,7 +1844,9 @@ pmap_remove(pmap_t pmap, vm_offset_t sva, vm_offset_t eva)
{
pd_entry_t *pde, *pdpe;
pt_entry_t *pte;
vm_offset_t va, va_next;
vm_offset_t va_next;
vm_offset_t va_init, va_fini;
bool need_tlb_shootdown;
/*
* Perform an unsynchronized read. This is, however, safe.
@ -1826,6 +1875,8 @@ pmap_remove(pmap_t pmap, vm_offset_t sva, vm_offset_t eva)
continue;
}
#endif
/* Scan up to the end of the page table pointed to by pde */
va_next = (sva + NBPDR) & ~PDRMASK;
if (va_next < sva)
va_next = eva;
@ -1842,25 +1893,44 @@ pmap_remove(pmap_t pmap, vm_offset_t sva, vm_offset_t eva)
if (va_next > eva)
va_next = eva;
va = va_next;
need_tlb_shootdown = false;
va_init = sva;
va_fini = va_next;
for (pte = pmap_pde_to_pte(pde, sva); sva != va_next; pte++,
sva += PAGE_SIZE) {
/* Skip over invalid entries; no need to shootdown */
if (!pte_test(pte, PTE_V)) {
if (va != va_next) {
pmap_invalidate_range(pmap, va, sva);
va = va_next;
}
/*
* If we have not yet found a valid entry, then
* we can move the lower edge of the region to
* invalidate to the next PTE.
*/
if (!need_tlb_shootdown)
va_init = sva + PAGE_SIZE;
continue;
}
if (va == va_next)
va = sva;
/*
* A valid entry; the range we are shooting down must
* include this page. va_fini is used instead of sva
* so that if the range ends with a run of !PTE_V PTEs,
* but doesn't clear out so much that pmap_remove_pte
* removes the entire PT, we won't include these !PTE_V
* entries in the region to be shot down.
*/
va_fini = sva + PAGE_SIZE;
if (pmap_remove_pte(pmap, pte, sva, *pde)) {
sva += PAGE_SIZE;
/* Entire PT removed and TLBs shot down. */
need_tlb_shootdown = false;
break;
} else {
need_tlb_shootdown = true;
}
}
if (va != va_next)
pmap_invalidate_range(pmap, va, sva);
if (need_tlb_shootdown)
pmap_invalidate_range(pmap, va_init, va_fini);
}
out:
rw_wunlock(&pvh_global_lock);
@ -1930,10 +2000,11 @@ pmap_remove_all(vm_page_t m)
__func__, (void *)pv->pv_va, (uintmax_t)tpte));
vm_page_dirty(m);
}
pmap_invalidate_page(pmap, pv->pv_va);
if (!pmap_unuse_pt(pmap, pv->pv_va, *pde))
pmap_invalidate_page(pmap, pv->pv_va);
TAILQ_REMOVE(&m->md.pv_list, pv, pv_list);
pmap_unuse_pt(pmap, pv->pv_va, *pde);
free_pv_entry(pmap, pv);
PMAP_UNLOCK(pmap);
}
@ -2812,7 +2883,12 @@ pmap_remove_pages(pmap_t pmap)
TAILQ_REMOVE(&m->md.pv_list, pv, pv_list);
if (TAILQ_EMPTY(&m->md.pv_list))
vm_page_aflag_clear(m, PGA_WRITEABLE);
pmap_unuse_pt(pmap, pv->pv_va, *pde);
/*
* For simplicity, unconditionally call
* pmap_invalidate_all(), below.
*/
(void)pmap_unuse_pt(pmap, pv->pv_va, *pde);
}
}
if (allfree) {