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Tue, 07 Apr 2020 01:27:26 -0700 (PDT) MIME-Version: 1.0 References: <0000000000002b25f105a2a3434d@google.com> <20200407004745.GA48345@xz-x1> <20200406183941.38a2e52026e42dbfde239a56@linux-foundation.org> <20200407015535.GC48345@xz-x1> <20200406191534.aafd8f74406c242ba1a42549@linux-foundation.org> <20200407024254.GD48345@xz-x1> In-Reply-To: <20200407024254.GD48345@xz-x1> From: Dmitry Vyukov Date: Tue, 7 Apr 2020 10:27:15 +0200 Message-ID: Subject: Re: BUG: unable to handle kernel paging request in kernel_get_mempolicy To: Peter Xu Cc: Andrew Morton , syzbot , Brian Geffon , LKML , Linux-MM , syzkaller-bugs , Linus Torvalds , Andrey Konovalov Content-Type: text/plain; charset="UTF-8" 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 Tue, Apr 7, 2020 at 4:43 AM Peter Xu wrote: > > On Mon, Apr 06, 2020 at 07:15:34PM -0700, Andrew Morton wrote: > > On Mon, 6 Apr 2020 21:55:35 -0400 Peter Xu wrote: > > > > > On Mon, Apr 06, 2020 at 06:39:41PM -0700, Andrew Morton wrote: > > > > On Mon, 6 Apr 2020 20:47:45 -0400 Peter Xu wrote: > > > > > > > > > >From 23800bff6fa346a4e9b3806dc0cfeb74498df757 Mon Sep 17 00:00:00 2001 > > > > > From: Peter Xu > > > > > Date: Mon, 6 Apr 2020 20:40:13 -0400 > > > > > Subject: [PATCH] mm/mempolicy: Allow lookup_node() to handle fatal signal > > > > > > > > > > lookup_node() uses gup to pin the page and get node information. It > > > > > checks against ret>=0 assuming the page will be filled in. However > > > > > it's also possible that gup will return zero, for example, when the > > > > > thread is quickly killed with a fatal signal. Teach lookup_node() to > > > > > gracefully return an error -EFAULT if it happens. > > > > > > > > > > ... > > > > > > > > > > --- a/mm/mempolicy.c > > > > > +++ b/mm/mempolicy.c > > > > > @@ -902,7 +902,10 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr) > > > > > > > > > > int locked = 1; > > > > > err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); > > > > > - if (err >= 0) { > > > > > + if (err == 0) { > > > > > + /* E.g. GUP interupted by fatal signal */ > > > > > + err = -EFAULT; > > > > > + } else if (err > 0) { > > > > > err = page_to_nid(p); > > > > > put_page(p); > > > > > } > > > > > > > > Doh. Thanks. > > > > > > > > Should it have been -EINTR? > > > > > > It looks ok to me too. I was returning -EFAULT to follow the same > > > value as get_vaddr_frames() (which is the other caller of > > > get_user_pages_locked()). So far the only path that I found can > > > trigger this is when there's a fatal signal pending right after the > > > gup. If so, the userspace won't have a chance to see the -EINTR (or > > > whatever we return) anyways. > > > > Yup. I guess we're a victim of get_user_pages()'s screwy return value > > conventions - the caller cannot distinguish between invalid-addr and > > fatal-signal. > > Indeed. > > > > > Which makes one wonder why lookup_node() ever worked. What happens if > > get_mempolicy(MPOL_F_NODE) is passed a wild userspace address? > > > > I'm not familiar with mempolicy at all, but do you mean MPOL_F_NODE > with MPOL_F_ADDR? Asked since iiuc if only MPOL_F_NODE is specified, > the kernel should not use the userspace addr at all (which seems to be > the thing we do now). get_mempolicy(MPOL_F_NODE|MPOL_F_ADDR) seems to > return -EFAULT as expected, though I agree maybe it would still be > nicer to differentiate the two cases. Am I reading this correctly that we put an initialized struct page* in this case? If so, with stack spraying this looks like an "interesting" bug.