Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 10 Aug 2022 13:40:09 +0200 | Subject | Re: [PATCH V3] mm: fix use-after free of page_ext after race with memory-offline | From | Vlastimil Babka <> |
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On 8/9/22 16:46, Charan Teja Kalla wrote: > The below is one path where race between page_ext and offline of the > respective memory blocks will cause use-after-free on the access of > page_ext structure. > > process1 process2 > --------- --------- > a)doing /proc/page_owner doing memory offline > through offline_pages. > > b)PageBuddy check is failed > thus proceed to get the > page_owner information > through page_ext access. > page_ext = lookup_page_ext(page); > > migrate_pages(); > ................. > Since all pages are successfully > migrated as part of the offline > operation,send MEM_OFFLINE notification > where for page_ext it calls: > offline_page_ext()--> > __free_page_ext()--> > free_page_ext()--> > vfree(ms->page_ext) > mem_section->page_ext = NULL > > c) Check for the PAGE_EXT flags > in the page_ext->flags access > results into the use-after-free(leading > to the translation faults). > > As mentioned above, there is really no synchronization between page_ext > access and its freeing in the memory_offline. > > The memory offline steps(roughly) on a memory block is as below: > 1) Isolate all the pages > 2) while(1) > try free the pages to buddy.(->free_list[MIGRATE_ISOLATE]) > 3) delete the pages from this buddy list. > 4) Then free page_ext.(Note: The struct page is still alive as it is > freed only during hot remove of the memory which frees the memmap, which > steps the user might not perform). > > This design leads to the state where struct page is alive but the struct > page_ext is freed, where the later is ideally part of the former which > just representing the page_flags (check [3] for why this design is > chosen). > > The above mentioned race is just one example __but the problem persists > in the other paths too involving page_ext->flags access(eg: > page_is_idle())__. Since offline waits till the last reference on the > page goes down i.e. any path that took the refcount on the page can make > the memory offline operation to wait. Eg: In the migrate_pages() > operation, we do take the extra refcount on the pages that are under > migration and then we do copy page_owner by accessing page_ext. > > Fix those paths where offline races with page_ext access by maintaining > synchronization with rcu lock and is achieved in 3 steps: > 1) Invalidate all the page_ext's of the sections of a memory block by > storing a flag in the LSB of mem_section->page_ext. > > 2) Wait till all the existing readers to finish working with the > ->page_ext's with synchronize_rcu(). Any parallel process that starts > after this call will not get page_ext, through lookup_page_ext(), for > the block parallel offline operation is being performed. > > 3) Now safely free all sections ->page_ext's of the block on which > offline operation is being performed. > > Note: If synchronize_rcu() takes time then optimizations can be done in > this path through call_rcu()[2]. > > Thanks to David Hildenbrand for his views/suggestions on the initial > discussion[1] and Pavan kondeti for various inputs on this patch. > > [1] https://lore.kernel.org/linux-mm/59edde13-4167-8550-86f0-11fc67882107@quicinc.com/ > [2] https://lore.kernel.org/all/a26ce299-aed1-b8ad-711e-a49e82bdd180@quicinc.com/T/#u > [3] https://lore.kernel.org/all/6fa6b7aa-731e-891c-3efb-a03d6a700efa@redhat.com/ > > Suggested-by: David Hildenbrand <david@redhat.com> > Suggested-by: Michal Hocko <mhocko@suse.com> > Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
<snip>
> --- 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.
> + 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.
> + 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.
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