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| author |   Kirill A. Shutemov <kirill.shutemov@linux.intel.com> | 2015-11-06 16:29:54 -0800 |
|---|---|---|
| committer |   Linus Torvalds <torvalds@linux-foundation.org> | 2015-11-06 17:50:42 -0800 |
| commit | 1d798ca3f16437c71ff63e36597ff07f9c12e4d6 (patch) | |
| tree | 4b70d32439fb18ef699175413e4b82c4af206f81 /mm/page_alloc.c | |
| parent | mm: pack compound_dtor and compound_order into one word in struct page (diff) | |
| download | wireguard-linux-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.tar.xz wireguard-linux-1d798ca3f16437c71ff63e36597ff07f9c12e4d6.zip | |
mm: make compound_head() robust
Hugh has pointed that compound_head() call can be unsafe in some
context. There's one example:
CPU0 CPU1
isolate_migratepages_block()
page_count()
compound_head()
!!PageTail() == true
put_page()
tail->first_page = NULL
head = tail->first_page
alloc_pages(__GFP_COMP)
prep_compound_page()
tail->first_page = head
__SetPageTail(p);
!!PageTail() == true
<head == NULL dereferencing>
The race is pure theoretical. I don't it's possible to trigger it in
practice. But who knows.
We can fix the race by changing how encode PageTail() and compound_head()
within struct page to be able to update them in one shot.
The patch introduces page->compound_head into third double word block in
front of compound_dtor and compound_order. Bit 0 encodes PageTail() and
the rest bits are pointer to head page if bit zero is set.
The patch moves page->pmd_huge_pte out of word, just in case if an
architecture defines pgtable_t into something what can have the bit 0
set.
hugetlb_cgroup uses page->lru.next in the second tail page to store
pointer struct hugetlb_cgroup. The patch switch it to use page->private
in the second tail page instead. The space is free since ->first_page is
removed from the union.
The patch also opens possibility to remove HUGETLB_CGROUP_MIN_ORDER
limitation, since there's now space in first tail page to store struct
hugetlb_cgroup pointer. But that's out of scope of the patch.
That means page->compound_head shares storage space with:
- page->lru.next;
- page->next;
- page->rcu_head.next;
That's too long list to be absolutely sure, but looks like nobody uses
bit 0 of the word.
page->rcu_head.next guaranteed[1] to have bit 0 clean as long as we use
call_rcu(), call_rcu_bh(), call_rcu_sched(), or call_srcu(). But future
call_rcu_lazy() is not allowed as it makes use of the bit and we can
get false positive PageTail().
[1] http://lkml.kernel.org/g/20150827163634.GD4029@linux.vnet.ibm.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/page_alloc.c')
| -rw-r--r-- | mm/page_alloc.c | 48 |
1 files changed, 31 insertions, 17 deletions
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index fae1bd6f9f37..e361001519d3 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -445,15 +445,15 @@ out: /* * Higher-order pages are called "compound pages". They are structured thusly: * - * The first PAGE_SIZE page is called the "head page". + * The first PAGE_SIZE page is called the "head page" and have PG_head set. * - * The remaining PAGE_SIZE pages are called "tail pages". + * The remaining PAGE_SIZE pages are called "tail pages". PageTail() is encoded + * in bit 0 of page->compound_head. The rest of bits is pointer to head page. * - * All pages have PG_compound set. All tail pages have their ->first_page - * pointing at the head page. + * The first tail page's ->compound_dtor holds the offset in array of compound + * page destructors. See compound_page_dtors. * - * The first tail page's ->lru.next holds the address of the compound page's - * put_page() function. Its ->lru.prev holds the order of allocation. + * The first tail page's ->compound_order holds the order of allocation. * This usage means that zero-order pages may not be compound. */ @@ -473,10 +473,7 @@ void prep_compound_page(struct page *page, unsigned long order) for (i = 1; i < nr_pages; i++) { struct page *p = page + i; set_page_count(p, 0); - p->first_page = page; - /* Make sure p->first_page is always valid for PageTail() */ - smp_wmb(); - __SetPageTail(p); + set_compound_head(p, page); } } @@ -854,17 +851,30 @@ static void free_one_page(struct zone *zone, static int free_tail_pages_check(struct page *head_page, struct page *page) { - if (!IS_ENABLED(CONFIG_DEBUG_VM)) - return 0; + int ret = 1; + + /* + * We rely page->lru.next never has bit 0 set, unless the page + * is PageTail(). Let's make sure that's true even for poisoned ->lru. + */ + BUILD_BUG_ON((unsigned long)LIST_POISON1 & 1); + + if (!IS_ENABLED(CONFIG_DEBUG_VM)) { + ret = 0; + goto out; + } if (unlikely(!PageTail(page))) { bad_page(page, "PageTail not set", 0); - return 1; + goto out; } - if (unlikely(page->first_page != head_page)) { - bad_page(page, "first_page not consistent", 0); - return 1; + if (unlikely(compound_head(page) != head_page)) { + bad_page(page, "compound_head not consistent", 0); + goto out; } - return 0; + ret = 0; +out: + clear_compound_head(page); + return ret; } static void __meminit __init_single_page(struct page *page, unsigned long pfn, @@ -931,6 +941,10 @@ void __meminit reserve_bootmem_region(unsigned long start, unsigned long end) struct page *page = pfn_to_page(start_pfn); init_reserved_page(start_pfn); + + /* Avoid false-positive PageTail() */ + INIT_LIST_HEAD(&page->lru); + SetPageReserved(page); } } |