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d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=6ECjPK11wsuEVryWmqpR3k3ZiIuxweVEaECaw5mE+94=; b=Wi3Sbx3T1+sLT23gwmMuu7Rb5u+znjwYm1zr4s7R4FA49fFCt1PBgvEG4hXwGdns4p C7KOmPT4fi7YtleP4/ltd8u9Q/cW9rUzQhOA0hbjosof0hipHAr5WQ+ZDLcZ9r9+YugQ AO+BgW1atSaKLANQdXLCZkwaJiuJBQ8QvgEsCjZvgzaVepMQqvG/Zo0IjsatUbPEtGlD F7AXBy1M9ttdBMvrdce4etYJ6sJ8VhhAonpQznLfQlxJBTX9yjJS9fLqVg8R6t97ecfj kYI2yXdu/25G9V2msalYsfopXMvM6FaK5lsVp1WILNvXJk9jwxpHsEOnSufniSmY71oi S+ng== X-Gm-Message-State: AOAM532h4cPSCfehnV4El1Van+1DaGo4uZsQz1KEsZTPCG/mmVOhjJTh 7j9UbXS/IVN1RRF/V4pAQM6Nw3v/KhdcVbOz3PZv3Q== X-Google-Smtp-Source: ABdhPJz7BxmSJCbeCL22yKsXMI9ADDC7i5QDGRPLPozbu4jocqeJRgEtOOnO2/+RU2i2pfuuSrPHsi7OTj5OCoKwwJc= X-Received: by 2002:a0d:c847:0:b0:2d6:9010:5721 with SMTP id k68-20020a0dc847000000b002d690105721mr6251548ywd.380.1646955075261; Thu, 10 Mar 2022 15:31:15 -0800 (PST) MIME-Version: 1.0 References: <20220215201922.1908156-1-surenb@google.com> <20220224201859.a38299b6c9d52cb51e6738ea@linux-foundation.org> <20220310155454.g6lt54yxel3ixnp3@revolver> <20220310222206.dttvvlgfqysrcl2s@revolver> In-Reply-To: <20220310222206.dttvvlgfqysrcl2s@revolver> From: Suren Baghdasaryan Date: Thu, 10 Mar 2022 15:31:04 -0800 Message-ID: Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused after being freed To: Liam Howlett Cc: Matthew Wilcox , Andrew Morton , "mhocko@kernel.org" , "mhocko@suse.com" , "shy828301@gmail.com" , "rientjes@google.com" , "hannes@cmpxchg.org" , "guro@fb.com" , "riel@surriel.com" , "minchan@kernel.org" , "kirill@shutemov.name" , "aarcange@redhat.com" , "brauner@kernel.org" , "christian@brauner.io" , "hch@infradead.org" , "oleg@redhat.com" , "david@redhat.com" , "jannh@google.com" , "shakeelb@google.com" , "luto@kernel.org" , "christian.brauner@ubuntu.com" , "fweimer@redhat.com" , "jengelh@inai.de" , "timmurray@google.com" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "kernel-team@android.com" , "syzbot+2ccf63a4bd07cf39cab0@syzkaller.appspotmail.com" Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 5283D140019 X-Stat-Signature: f11fbba5deno9sgdxo4tuuexyqrr7xrb Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=GmLy3PEJ; spf=pass (imf23.hostedemail.com: domain of surenb@google.com designates 209.85.128.173 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1646955076-564520 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 10, 2022 at 2:22 PM Liam Howlett wrote: > > * Suren Baghdasaryan [220310 11:28]: > > On Thu, Mar 10, 2022 at 7:55 AM Liam Howlett wrote: > > > > > > * Suren Baghdasaryan [220225 00:51]: > > > > On Thu, Feb 24, 2022 at 8:23 PM Matthew Wilcox wrote: > > > > > > > > > > On Thu, Feb 24, 2022 at 08:18:59PM -0800, Andrew Morton wrote: > > > > > > On Tue, 15 Feb 2022 12:19:22 -0800 Suren Baghdasaryan wrote: > > > > > > > > > > > > > After exit_mmap frees all vmas in the mm, mm->mmap needs to be reset, > > > > > > > otherwise it points to a vma that was freed and when reused leads to > > > > > > > a use-after-free bug. > > > > > > > > > > > > > > ... > > > > > > > > > > > > > > --- a/mm/mmap.c > > > > > > > +++ b/mm/mmap.c > > > > > > > @@ -3186,6 +3186,7 @@ void exit_mmap(struct mm_struct *mm) > > > > > > > vma = remove_vma(vma); > > > > > > > cond_resched(); > > > > > > > } > > > > > > > + mm->mmap = NULL; > > > > > > > mmap_write_unlock(mm); > > > > > > > vm_unacct_memory(nr_accounted); > > > > > > > } > > > > > > > > > > > > After the Maple tree patches, mm_struct.mmap doesn't exist. So I'll > > > > > > revert this fix as part of merging the maple-tree parts of linux-next. > > > > > > I'll be sending this fix to Linus this week. > > > > > > > > > > > > All of which means that the thusly-resolved Maple tree patches might > > > > > > reintroduce this use-after-free bug. > > > > > > > > > > I don't think so? The problem is that VMAs are (currently) part of > > > > > two data structures -- the rbtree and the linked list. remove_vma() > > > > > only removes VMAs from the rbtree; it doesn't set mm->mmap to NULL. > > > > > > > > > > With maple tree, the linked list goes away. remove_vma() removes VMAs > > > > > from the maple tree. So anyone looking to iterate over all VMAs has to > > > > > go and look in the maple tree for them ... and there's nothing there. > > > > > > > > Yes, I think you are right. With maple trees we don't need this fix. > > > > > > > > > Yes, this is correct. The maple tree removes the entire linked list... > > > but since the mm is unstable in the exit_mmap(), I had added the > > > destruction of the maple tree there. Maybe this is the wrong place to > > > be destroying the tree tracking the VMAs (althought this patch partially > > > destroys the VMA tracking linked list), but it brought my attention to > > > the race that this patch solves and the process_mrelease() function. > > > Couldn't this be avoided by using mmget_not_zero() instead of mmgrab() > > > in process_mrelease()? > > > > That's what we were doing before [1]. That unfortunately has a problem > > of process_mrelease possibly calling the last mmput and being blocked > > on IO completion in exit_aio. > > Oh, I see. Thanks. > > > > The race between exit_mmap and > > process_mrelease is solved by using mmap_lock. > > I think an important part of the race fix isn't just the lock holding > but the setting of the start of the linked list to NULL above. That > means the code in __oom_reap_task_mm() via process_mrelease() will > continue to execute but iterate for zero VMAs. > > > I think by destroying the maple tree in exit_mmap before the > > mmap_write_unlock call, you keep things working and functionality > > intact. Is there any reason this can't be done? > > Yes, unfortunately. If MMF_OOM_SKIP is not set, then process_mrelease() > will call __oom_reap_task_mm() which will get a null pointer dereference > or a use after free in the vma iterator as it tries to iterate the maple > tree. I think the best plan is to set MMF_OOM_SKIP unconditionally > when the mmap_write_lock() is acquired. Doing so will ensure nothing > will try to gain memory by reaping a task that no longer has memory to > yield - or at least won't shortly. If we do use MMF_OOM_SKIP in such a > way, then I think it is safe to quickly drop the lock? That technically would work but it changes the semantics of MMF_OOM_SKIP flag from "mm is of no interest for the OOM killer" to something like "mm is empty" akin to mm->mmap == NULL. So, there is no way for maple tree to indicate that it is empty? > > Also, should process_mrelease() be setting MMF_OOM_VICTIM on this mm? > It would enable the fast path on a race with exit_mmap() - thought that > may not be desirable? Michal does not like that approach because again, process_mrelease is not oom-killer to set MMF_OOM_VICTIM flag. Besides, we want to get rid of that special mm_is_oom_victim(mm) branch inside exit_mmap. Which reminds me to look into it again. > > > > > [1] ba535c1caf3ee78a ("mm/oom_kill: allow process_mrelease to run > > under mmap_lock protection") > > > > > That would ensure we aren't stepping on an > > > exit_mmap() and potentially the locking change in exit_mmap() wouldn't > > > be needed either? Logically, I view this as process_mrelease() having > > > issue with the fact that the mmaps are no longer stable in tear down > > > regardless of the data structure that is used. > > > > > > Thanks, > > > Liam > > > > > > -- > > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. > > > > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >