Date: Mon, 7 Aug 2017 15:12:36 -0400 From: Jerome Glisse <jglisse@...hat.com> To: Igor Stoppa <igor.stoppa@...wei.com> Cc: Michal Hocko <mhocko@...nel.org>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, linux-security-module@...r.kernel.org, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Kees Cook <keescook@...gle.com> Subject: Re: [RFC] Tagging of vmalloc pages for supporting the pmalloc allocator On Mon, Aug 07, 2017 at 05:13:00PM +0300, Igor Stoppa wrote: > > > On 07/08/17 16:31, Jerome Glisse wrote: > > On Mon, Aug 07, 2017 at 02:26:21PM +0300, Igor Stoppa wrote: > > [...] > > >> I'll add a vm_area field as you advised. > >> > >> Is this something I could send as standalone patch? > > > > Note that vmalloc() is not the only thing that use vmalloc address > > space. There is also vmap() and i know one set of drivers that use > > vmap() and also use the mapping field of struct page namely GPU > > drivers. > > Ah, yes, you mentioned this. > > > So like i said previously i would store a flag inside vm_struct to > > know if page you are looking at are pmalloc or not. > > And I was planning to follow your advice, using one of the flags. > But ... > > > Again do you > > need to store something per page ? Would storing it per vm_struct > > not be enough ? > > ... there was this further comment, about speeding up the access to > vm_area, which seemed good from performance perspective. > > ---8<--------------8<--------------8<--------------8<--------------8<--- > On 03/08/17 14:48, Michal Hocko wrote: > > On Thu 03-08-17 13:11:45, Igor Stoppa wrote: > > [...] > > >> But, to reply more specifically to your advice, yes, I think I could > >> add a flag to vm_struct and then retrieve its value, for the address > >> being processed, by passing through find_vm_area(). > > > > ... and you can store vm_struct pointer to the struct page there and > > you won't need to do the slow find_vm_area. I haven't checked very > > closely but this should be possible in principle. I guess other > > callers might benefit from this as well. > ---8<--------------8<--------------8<--------------8<--------------8<--- > > I do not strictly need to modify the page struct, but it seems it might > harm performance, if it is added on the path of hardened usercopy. > > I have an updated version of the old proposal: > > * put a magic number in the private field, during initialization of > pmalloc pages > > * during hardened usercopy verification, when I have to assess if a page > is of pmalloc type, compare the private field against the magic number > > * if and only if the private field matches the magic number, then invoke > find_vm_area(), so that the slowness affects only a possibly limited > amount of false positives. This all sounds good to me. Jérôme
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