linux-kernel - Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZzTWQqr-zFQz0HHY@x1n>
Date: Wed, 13 Nov 2024 11:39:30 -0500
From: Peter Xu <peterx@...hat.com>
To: Jann Horn <jannh@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Nicholas Piggin <npiggin@...il.com>,
	David Hildenbrand <david@...hat.com>,
	Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Houghton <jthoughton@...gle.com>,
	Huang Ying <ying.huang@...el.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Rick P Edgecombe <rick.p.edgecombe@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Borislav Petkov <bp@...en8.de>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Michael Ellerman <mpe@...erman.id.au>,
	Rik van Riel <riel@...riel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Mel Gorman <mgorman@...hsingularity.net>, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Oscar Salvador <osalvador@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

On Mon, Nov 11, 2024 at 10:20:59PM +0100, Jann Horn wrote:
> On Mon, Aug 12, 2024 at 8:12 PM Peter Xu <peterx@...hat.com> wrote:
> > Dax supports pud pages for a while, but mprotect on puds was missing since
> > the start.  This series tries to fix that by providing pud handling in
> > mprotect().  The goal is to add more types of pud mappings like hugetlb or
> > pfnmaps.  This series paves way for it by fixing known pud entries.
> 
> Do people actually use hardware where they can use PUD THP mappings
> for DAX? I thought that was just some esoteric feature that isn't
> actually usable on almost any system.
> Was I wrong about that?

I did run it with a qemu emulated nvdimm device.  Though in reality I've no
idea on how many people are using it..

> 
> I think another example that probably doesn't play entirely nice with
> PUD THP mappings is mremap()'s move_page_tables(). If
> dax_get_unmapped_area() allows creating a VMA at an unaligned start
> address (which I think it does?), move_page_tables() can probably end
> up copying from an aligned address mapped with a huge PUD entry to an
> unaligned address that needs to be mapped at the PTE level, and I
> think that will probably cause it to call into get_old_pmd() while a
> huge PUD entry is still present, which will probably get us a
> pud_bad() error or such?

I think you're probably right, that we have other places that may not work
well with pud mappings.

I also wonder whether dax_get_unmapped_area() needs to properly handle
MAP_FIXED, even for PMD mappings.

It looks like it always fallbacks to the default mm_get_unmapped_area()
with FIXED, which have no idea on dax->alignment so it'll always allow
it.. The issue is I'm not sure dax pmd can be split at all, while I think
split-able is needed when mremap from a pmd-aligned address to a
!pmd-aligned address.

Thanks,

-- 
Peter Xu