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Fri, 26 Aug 2022 07:40:52 +0000 Date: Fri, 26 Aug 2022 09:40:52 +0200 From: Michal Hocko To: Charan Teja Kalla Cc: akpm@linux-foundation.org, david@redhat.com, vbabka@suse.cz, pasha.tatashin@soleen.com, shakeelb@google.com, sieberf@amazon.com, sjpark@amazon.de, william.kucharski@oracle.com, willy@infradead.org, quic_pkondeti@quicinc.com, minchan@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH V5] mm: fix use-after free of page_ext after race with memory-offline Message-ID: References: <1661496993-11473-1-git-send-email-quic_charante@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1661496993-11473-1-git-send-email-quic_charante@quicinc.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 26-08-22 12:26:33, 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())__. > > Fix all the 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 > Suggested-by: Michal Hocko > Signed-off-by: Charan Teja Kalla > --- > Changes in V5: > o Used the loop variable name as ext_put_continue -- David If this is the only change since the last version (I didn't have much time to read through the whole patch) then my ack still applies. -- Michal Hocko SUSE Labs