From: Lorenzo Stoakes <ljs@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>,
"Liam R. Howlett" <liam@infradead.org>,
Vlastimil Babka <vbabka@kernel.org>,
Mike Rapoport <rppt@kernel.org>,
Suren Baghdasaryan <surenb@google.com>,
Michal Hocko <mhocko@suse.com>, Toshi Kani <toshi.kani@hpe.com>,
Catalin Marinas <catalin.marinas@arm.com>,
David Carlier <devnexen@gmail.com>, Dev Jain <dev.jain@arm.com>,
Ryan Roberts <ryan.roberts@arm.com>,
Shakeel Butt <shakeel.butt@linux.dev>,
Will Deacon <will@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org,
Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH] mm/ptdump: always stabilise against page table freeing using init_mm
Date: Sat, 11 Jul 2026 08:57:11 +0100 [thread overview]
Message-ID: <alH27K6kvUVxJ672@lucifer> (raw)
In-Reply-To: <20260710132954.49c85be140500c59c1fcb773@linux-foundation.org>
(+cc Dave as we were discussing on another thread)
On Fri, Jul 10, 2026 at 01:29:54PM -0700, Andrew Morton wrote:
> On Fri, 10 Jul 2026 14:29:21 +0100 Lorenzo Stoakes <ljs@kernel.org> wrote:
>
> > x86 and arm64 invokes ptdump_walk_pgd() with non-init_mm mm whilst still
> > walking kernel page table ranges.
> >
> > For x86 this is done in ptdump_curknl_show() and ptdump_efi_show(), the
> > first passing current->mm, and the second passing efi_mm (we reach kernel
> > mappings that init_mm protects for current->mm due to x86 cloning shared
> > kernel page tables for arbitrary mm's).
> >
> > arm64 does so via ptdump_debugfs_register(), configured by efi_ptdump_info
> > for efi ranges against efi_mm.
> >
> > The init_mm mmap lock is used to stabilise page table freeing against
> > ptdump, so take a nested lock on init_mm to ensure that we are correctly
> > stabilised.
> >
> > We take this after mmap write locking the non-init_mm mm. Nothing acquires
> > the init_mm lock first before locking an arbitrary mm, so no deadlock is
> > possible.
> >
> > Other fixes have been sent which update the two cases which can cause races
> > with ptdump in init_mm ranges to acquire the init_mm mmap write lock - vmap
> > and x86 CPA huge page promotion.
> >
> > For arm64, commit fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump")
> > already provides exclusion against init_mm for the vmap case, which this
> > patch also pairs with.
> >
> > The first point at which ptdump can race kernel page table freeing is
> > commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page
> > table"), so we target this in the Fixes tag.
>
> Does this have any known userspace impact?
Taken in conjunction with the other fixes mentioned it resolves a UAF, so yes :)
>
> > Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
>
> Since 2018, so I won't make this a hotfix, OK?
>
No, this has to be a hotfix, it is part of solving a live UAF issue.
Cheers, Lorenzo
next prev parent reply other threads:[~2026-07-11 7:57 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-10 13:29 [PATCH] mm/ptdump: always stabilise against page table freeing using init_mm Lorenzo Stoakes
2026-07-10 20:29 ` Andrew Morton
2026-07-11 7:57 ` Lorenzo Stoakes [this message]
2026-07-12 10:47 ` Lorenzo Stoakes
2026-07-11 10:08 ` Mike Rapoport
2026-07-12 8:53 ` Lorenzo Stoakes
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