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[37.188.180.223]) by smtp.gmail.com with ESMTPSA id c83sm15732559wmd.23.2020.04.14.04.04.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Apr 2020 04:04:30 -0700 (PDT) Date: Tue, 14 Apr 2020 13:04:29 +0200 From: Michal Hocko To: Linus Torvalds Cc: Peter Xu , Linux Kernel Mailing List , Linux-MM , Andrew Morton , syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Message-ID: <20200414110429.GF4629@dhcp22.suse.cz> References: <20200408014010.80428-1-peterx@redhat.com> <20200408014010.80428-2-peterx@redhat.com> <20200409070253.GB18386@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 09-04-20 09:42:20, Linus Torvalds wrote: > On Thu, Apr 9, 2020 at 12:03 AM Michal Hocko wrote: > > > > This patch however doesn't go all the way to revert it because 0 return > > value is impossible. > > I'm not convinced it's impossible. __get_user_pages is documented as * -- If nr_pages is 0, returns 0. * -- If nr_pages is >0, but no pages were pinned, returns -errno. * -- If nr_pages is >0, and some pages were pinned, returns the number of * pages pinned. Again, this may be less than nr_pages. but let me double check the actual code... There seem to be only one exception the above rule AFAICS. faultin_page returning EBUSY will be overriden to either 0 for the first page or return the number of already pinned pages. So nr_pages > 0 && ret = 0 is indeed possible from __get_user_pages :/ That will be the case only for VM_FAULT_RETRY, thoug. Now __get_user_pages_locked behaves differently. It keeps retrying the fault until it succeeds unless FOLL_NOWAIT is specified. Then it would return 0. Why we need to return 0 is not really clear to me but it seem to be a long term behavior. I believe we need to document it. > And if it is, then the current code is harmless. Yes from the above it seems that the check is indeed harmless becasue this path doesn't use FOLL_NOWAIT and so it will never see 0 return. I find a reference to EINTR confusing so I would still love to change that. > Now, I do agree that we probably should go through and clarify the > whole range of different get_user_pages() cases of returning zero (or > not doing so), but right now it's so confusing that I'd prefer to keep > that (possibly unnecessary) belt-and-suspenders check for zero in > there. > > If/when somebody actually does a real audit and the result is "these > functions cannot return zero" and it's documented, then we can remove > those checks. Would you mind this patch instead? commit bc6c0fa7c7fb5eb54963dca65ae4a62ba04c9efa Author: Michal Hocko Date: Thu Apr 9 08:26:57 2020 +0200 mm, mempolicy: fix up gup usage in lookup_node ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal") has added a special casing for 0 return value because that was a possible gup return value when interrupted by fatal signal. This has been fixed by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR for fatal signal") in the mean time so ba841078cd05 can be reverted. This patch however doesn't go all the way to revert it because the check for 0 is wrong and confusing here. Firstly it is inherently unsafe to access the page when get_user_pages_locked returns 0 (aka no page returned). Fortunatelly this will not happen because get_user_pages_locked will not return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not the case here. Document this potential error code in gup code while we are at it. Signed-off-by: Michal Hocko diff --git a/mm/gup.c b/mm/gup.c index 50681f0286de..a8575b880baf 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -980,6 +980,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) * -- If nr_pages is >0, but no pages were pinned, returns -errno. * -- If nr_pages is >0, and some pages were pinned, returns the number of * pages pinned. Again, this may be less than nr_pages. + * -- 0 return value is possible when the fault would need to be retried. * * The caller is responsible for releasing returned @pages, via put_page(). * @@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, } EXPORT_SYMBOL_GPL(fixup_user_fault); +/* + * Please note that this function, unlike __get_user_pages will not + * return 0 for nr_pages > 0 without FOLL_NOWAIT + */ static __always_inline long __get_user_pages_locked(struct task_struct *tsk, struct mm_struct *mm, unsigned long start, diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 48ba9729062e..1965e2681877 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -927,10 +927,7 @@ 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) { - /* E.g. GUP interrupted by fatal signal */ - err = -EFAULT; - } else if (err > 0) { + if (err > 0) { err = page_to_nid(p); put_page(p); } -- Michal Hocko SUSE Labs