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Date: Wed, 10 Aug 2022 20:01:58 +0530
From: Charan Teja Kalla <quic_charante@...cinc.com>
To: Vlastimil Babka <vbabka@...e.cz>, <akpm@...ux-foundation.org>,
<mhocko@...e.com>, <david@...hat.com>, <pasha.tatashin@...een.com>,
<sieberf@...zon.com>, <shakeelb@...gle.com>, <sjpark@...zon.de>,
<dhowells@...hat.com>, <willy@...radead.org>,
<quic_pkondeti@...cinc.com>
CC: <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH V3] mm: fix use-after free of page_ext after race with
memory-offline
Thanks Vlastimil for the inputs!!
On 8/10/2022 5:10 PM, Vlastimil Babka wrote:
>> --- a/mm/page_owner.c
>> +++ b/mm/page_owner.c
>> @@ -141,7 +141,7 @@ void __reset_page_owner(struct page *page,
>> unsigned short order)
>> struct page_owner *page_owner;
>> u64 free_ts_nsec = local_clock();
>> - page_ext = lookup_page_ext(page);
>> + page_ext = page_ext_get(page);
>> if (unlikely(!page_ext))
>> return;
>> @@ -153,6 +153,7 @@ void __reset_page_owner(struct page *page,
>> unsigned short order)
>> page_owner->free_ts_nsec = free_ts_nsec;
>> page_ext = page_ext_next(page_ext);
>> }
>> + page_ext_put();
>> }
>> static inline void __set_page_owner_handle(struct page_ext *page_ext,
>> @@ -183,19 +184,26 @@ static inline void
>> __set_page_owner_handle(struct page_ext *page_ext,
>> noinline void __set_page_owner(struct page *page, unsigned short order,
>> gfp_t gfp_mask)
>> {
>> - struct page_ext *page_ext = lookup_page_ext(page);
>> + struct page_ext *page_ext = page_ext_get(page);
>> depot_stack_handle_t handle;
>> if (unlikely(!page_ext))
>> return;
>> + page_ext_put();
>> handle = save_stack(gfp_mask);
>> +
>> + /* Ensure page_ext is valid after page_ext_put() above */
>> + page_ext = page_ext_get(page);
>
> Why not simply do the save_stack() first and then page_ext_get() just
> once? It should be really rare that it's NULL, so I don't think we save
> much by avoiding an unnecessary save_stack(), while the overhead of
> doing two get/put instead of one will affect every call.
>
I am under the assumption that save_stack can take time when it goes for
GFP_KERNEL allocations over page_ext which is mere rcu_lock + few
arithmetic operations. Am I wrong here?
But yes it is rare that page_ext can be NULL here, so I am fine to
follow your suggestion, which atleast improve the code readability, IMO.
>> + if (unlikely(!page_ext))
>> + return;
>> __set_page_owner_handle(page_ext, handle, order, gfp_mask);
>> + page_ext_put();
>> }
>> void __set_page_owner_migrate_reason(struct page *page, int reason)
>> {
>> - struct page_ext *page_ext = lookup_page_ext(page);
>> + struct page_ext *page_ext = page_ext_get(page);
>> struct page_owner *page_owner;
>> if (unlikely(!page_ext))
>> @@ -203,12 +211,13 @@ void __set_page_owner_migrate_reason(struct page
>> *page, int reason)
>> page_owner = get_page_owner(page_ext);
>> page_owner->last_migrate_reason = reason;
>> + page_ext_put();
>> }
>> void __split_page_owner(struct page *page, unsigned int nr)
>> {
>> int i;
>> - struct page_ext *page_ext = lookup_page_ext(page);
>> + struct page_ext *page_ext = page_ext_get(page);
>> struct page_owner *page_owner;
>> if (unlikely(!page_ext))
>> @@ -219,16 +228,24 @@ void __split_page_owner(struct page *page,
>> unsigned int nr)
>> page_owner->order = 0;
>> page_ext = page_ext_next(page_ext);
>> }
>> + page_ext_put();
>> }
>> void __folio_copy_owner(struct folio *newfolio, struct folio *old)
>> {
>> - struct page_ext *old_ext = lookup_page_ext(&old->page);
>> - struct page_ext *new_ext = lookup_page_ext(&newfolio->page);
>> + struct page_ext *old_ext;
>> + struct page_ext *new_ext;
>> struct page_owner *old_page_owner, *new_page_owner;
>> - if (unlikely(!old_ext || !new_ext))
>> + old_ext = page_ext_get(&old->page);
>> + if (unlikely(!old_ext))
>> + return;
>> +
>> + new_ext = page_ext_get(&newfolio->page);
>
> The second one can keep using just lookup_page_ext() and we can have a
> single page_ext_put()? I don't think it would be dangerous in case the
> internals change, as page_ext_put() doesn't have a page parameter anyway
> so it can't be specific to a page.
Actually we did hide the lookup_page_ext() while exposing only a single
interface i.e. page_ext_get/put(). And this suggestion requires to
expose the lookup_page_ext as well which leaves two interfaces to get
the page_ext which seems not look good, IMO. Please let me know If you
think otherwise here.
>
>> + if (unlikely(!new_ext)) {
>> + page_ext_put();
>> return;
>> + }
>> old_page_owner = get_page_owner(old_ext);
>> new_page_owner = get_page_owner(new_ext);
>> @@ -254,6 +271,8 @@ void __folio_copy_owner(struct folio *newfolio,
>> struct folio *old)
>> */
>> __set_bit(PAGE_EXT_OWNER, &new_ext->flags);
>> __set_bit(PAGE_EXT_OWNER_ALLOCATED, &new_ext->flags);
>> + page_ext_put();
>> + page_ext_put();
>> }
>> void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>> @@ -307,12 +326,12 @@ void pagetypeinfo_showmixedcount_print(struct
>> seq_file *m,
>> if (PageReserved(page))
>> continue;
>> - page_ext = lookup_page_ext(page);
>> + page_ext = page_ext_get(page);
>> if (unlikely(!page_ext))
>> continue;
>> if (!test_bit(PAGE_EXT_OWNER_ALLOCATED,
>> &page_ext->flags))
>> - continue;
>> + goto loop;
>> page_owner = get_page_owner(page_ext);
>> page_mt = gfp_migratetype(page_owner->gfp_mask);
>> @@ -323,9 +342,12 @@ void pagetypeinfo_showmixedcount_print(struct
>> seq_file *m,
>> count[pageblock_mt]++;
>> pfn = block_end_pfn;
>> + page_ext_put();
>> break;
>> }
>> pfn += (1UL << page_owner->order) - 1;
>> +loop:
>> + page_ext_put();
>> }
>> }
>> @@ -435,7 +457,7 @@ print_page_owner(char __user *buf, size_t count,
>> unsigned long pfn,
>> void __dump_page_owner(const struct page *page)
>> {
>> - struct page_ext *page_ext = lookup_page_ext(page);
>> + struct page_ext *page_ext = page_ext_get((void *)page);
>> struct page_owner *page_owner;
>> depot_stack_handle_t handle;
>> gfp_t gfp_mask;
>> @@ -452,6 +474,7 @@ void __dump_page_owner(const struct page *page)
>> if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags)) {
>> pr_alert("page_owner info is not present (never set?)\n");
>> + page_ext_put();
>> return;
>> }
>> @@ -482,6 +505,7 @@ void __dump_page_owner(const struct page *page)
>> if (page_owner->last_migrate_reason != -1)
>> pr_alert("page has been migrated, last migrate reason: %s\n",
>> migrate_reason_names[page_owner->last_migrate_reason]);
>> + page_ext_put();
>> }
>> static ssize_t
>> @@ -508,6 +532,14 @@ read_page_owner(struct file *file, char __user
>> *buf, size_t count, loff_t *ppos)
>> /* Find an allocated page */
>> for (; pfn < max_pfn; pfn++) {
>> /*
>> + * This temporary page_owner is required so
>> + * that we can avoid the context switches while holding
>> + * the rcu lock and copying the page owner information to
>> + * user through copy_to_user() or GFP_KERNEL allocations.
>> + */
>> + struct page_owner page_owner_tmp;
>> +
>> + /*
>> * If the new page is in a new MAX_ORDER_NR_PAGES area,
>> * validate the area as existing, skip it if not
>> */
>> @@ -525,7 +557,7 @@ read_page_owner(struct file *file, char __user
>> *buf, size_t count, loff_t *ppos)
>> continue;
>> }
>> - page_ext = lookup_page_ext(page);
>> + page_ext = page_ext_get(page);
>> if (unlikely(!page_ext))
>> continue;
>> @@ -534,14 +566,14 @@ read_page_owner(struct file *file, char __user
>> *buf, size_t count, loff_t *ppos)
>> * because we don't hold the zone lock.
>> */
>> if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
>> - continue;
>> + goto loop;
>> /*
>> * Although we do have the info about past allocation of free
>> * pages, it's not relevant for current memory usage.
>> */
>> if (!test_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags))
>> - continue;
>> + goto loop;
>> page_owner = get_page_owner(page_ext);
>> @@ -550,7 +582,7 @@ read_page_owner(struct file *file, char __user
>> *buf, size_t count, loff_t *ppos)
>> * would inflate the stats.
>> */
>> if (!IS_ALIGNED(pfn, 1 << page_owner->order))
>> - continue;
>> + goto loop;
>> /*
>> * Access to page_ext->handle isn't synchronous so we should
>> @@ -558,13 +590,17 @@ read_page_owner(struct file *file, char __user
>> *buf, size_t count, loff_t *ppos)
>> */
>> handle = READ_ONCE(page_owner->handle);
>> if (!handle)
>> - continue;
>> + goto loop;
>> /* Record the next PFN to read in the file offset */
>> *ppos = (pfn - min_low_pfn) + 1;
>> + memcpy(&page_owner_tmp, page_owner, sizeof(struct
>> page_owner));
>> + page_ext_put();
>> return print_page_owner(buf, count, pfn, page,
>> - page_owner, handle);
>> + &page_owner_tmp, handle);
>> +loop:
>> + page_ext_put();
>> }
>> return 0;
>> @@ -617,18 +653,20 @@ static void init_pages_in_zone(pg_data_t *pgdat,
>> struct zone *zone)
>> if (PageReserved(page))
>> continue;
>> - page_ext = lookup_page_ext(page);
>> + page_ext = page_ext_get(page);
>> if (unlikely(!page_ext))
>> continue;
>> /* Maybe overlapping zone */
>> if (test_bit(PAGE_EXT_OWNER, &page_ext->flags))
>> - continue;
>> + goto loop;
>> /* Found early allocated page */
>> __set_page_owner_handle(page_ext, early_handle,
>> 0, 0);
>> count++;
>> +loop:
>> + page_ext_put();
>> }
>> cond_resched();
>
> This is called from init_page_owner() where races with offline are
> impossible, so it's unnecessary. Although it won't hurt.
Totally agree. Infact in V2, this change is not there. And there are
some other places too that it is not required to go for
page_ext_get/put, eg: page_owner_migration, but these changes are done
as we have exposed a single interface to get the page_ext.
>
Thanks,
Chararan
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