Date: Mon, 7 Aug 2017 17:13:00 +0300 From: Igor Stoppa <igor.stoppa@...wei.com> To: Jerome Glisse <jglisse@...hat.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 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. -- igor
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