From: Will Deacon <will@kernel.org>
To: Mike Rapoport <rppt@kernel.org>
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Subject: Re: [PATCH 0/4] arch, mm: improve robustness of direct map manipulation
Date: Wed, 28 Oct 2020 11:20:12 +0000 [thread overview]
Message-ID: <20201028112011.GB27927@willie-the-truck> (raw)
In-Reply-To: <20201027083816.GG1154158@kernel.org>
On Tue, Oct 27, 2020 at 10:38:16AM +0200, Mike Rapoport wrote:
> On Mon, Oct 26, 2020 at 06:05:30PM +0000, Edgecombe, Rick P wrote:
> > On Mon, 2020-10-26 at 11:05 +0200, Mike Rapoport wrote:
> > > On Mon, Oct 26, 2020 at 01:13:52AM +0000, Edgecombe, Rick P wrote:
> > > > On Sun, 2020-10-25 at 12:15 +0200, Mike Rapoport wrote:
> > > > > Indeed, for architectures that define
> > > > > CONFIG_ARCH_HAS_SET_DIRECT_MAP
> > > > > it is
> > > > > possible that __kernel_map_pages() would fail, but since this
> > > > > function is
> > > > > void, the failure will go unnoticed.
> > > >
> > > > Could you elaborate on how this could happen? Do you mean during
> > > > runtime today or if something new was introduced?
> > >
> > > A failure in__kernel_map_pages() may happen today. For instance, on
> > > x86
> > > if the kernel is built with DEBUG_PAGEALLOC.
> > >
> > > __kernel_map_pages(page, 1, 0);
> > >
> > > will need to split, say, 2M page and during the split an allocation
> > > of
> > > page table could fail.
> >
> > On x86 at least, DEBUG_PAGEALLOC expects to never have to break a page
> > on the direct map and even disables locking in cpa because it assumes
> > this. If this is happening somehow anyway then we should probably fix
> > that. Even if it's a debug feature, it will not be as useful if it is
> > causing its own crashes.
> >
> > I'm still wondering if there is something I'm missing here. It seems
> > like you are saying there is a bug in some arch's, so let's add a WARN
> > in cross-arch code to log it as it crashes. A warn and making things
> > clearer seem like good ideas, but if there is a bug we should fix it.
> > The code around the callers still functionally assume re-mapping can't
> > fail.
>
> Oh, I've meant x86 kernel *without* DEBUG_PAGEALLOC, and indeed the call
> that unmaps pages back in safe_copy_page will just reset a 4K page to
> NP because whatever made it NP at the first place already did the split.
>
> Still, on arm64 with DEBUG_PAGEALLOC=n there is a possibility of a race
> between map/unmap dance in __vunmap() and safe_copy_page() that may
> cause access to unmapped memory:
>
> __vunmap()
> vm_remove_mappings()
> set_direct_map_invalid()
> safe_copy_page()
> __kernel_map_pages()
> return
> do_copy_page() -> fault
>
> This is a theoretical bug, but it is still not nice :)
Just to clarify: this patch series fixes this problem, right?
Will
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next prev parent reply other threads:[~2020-10-28 11:20 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-25 10:15 [PATCH 0/4] arch, mm: improve robustness of direct map manipulation Mike Rapoport
2020-10-25 10:15 ` [PATCH 1/4] mm: introduce debug_pagealloc_map_pages() helper Mike Rapoport
2020-10-26 11:05 ` David Hildenbrand
2020-10-26 11:54 ` Mike Rapoport
2020-10-26 11:55 ` David Hildenbrand
2020-10-25 10:15 ` [PATCH 2/4] PM: hibernate: improve robustness of mapping pages in the direct map Mike Rapoport
2020-10-26 0:38 ` Edgecombe, Rick P
2020-10-26 9:15 ` Mike Rapoport
2020-10-26 18:57 ` Edgecombe, Rick P
2020-10-27 8:49 ` Mike Rapoport
2020-10-27 22:44 ` Edgecombe, Rick P
2020-10-28 9:41 ` Mike Rapoport
2020-10-27 1:10 ` Edgecombe, Rick P
2020-10-28 21:15 ` Edgecombe, Rick P
2020-10-29 7:54 ` Mike Rapoport
2020-10-29 23:19 ` Edgecombe, Rick P
2020-11-01 17:02 ` Mike Rapoport
2020-10-25 10:15 ` [PATCH 3/4] arch, mm: restore dependency of __kernel_map_pages() of DEBUG_PAGEALLOC Mike Rapoport
2020-10-25 10:15 ` [PATCH 4/4] arch, mm: make kernel_page_present() always available Mike Rapoport
2020-10-26 0:54 ` Edgecombe, Rick P
2020-10-26 9:31 ` Mike Rapoport
2020-10-26 1:13 ` [PATCH 0/4] arch, mm: improve robustness of direct map manipulation Edgecombe, Rick P
2020-10-26 9:05 ` Mike Rapoport
2020-10-26 18:05 ` Edgecombe, Rick P
2020-10-27 8:38 ` Mike Rapoport
2020-10-27 8:46 ` David Hildenbrand
2020-10-27 9:47 ` Mike Rapoport
2020-10-27 10:34 ` David Hildenbrand
2020-10-28 11:09 ` Mike Rapoport
2020-10-28 11:17 ` David Hildenbrand
2020-10-28 12:22 ` Mike Rapoport
2020-10-28 18:31 ` Edgecombe, Rick P
2020-10-28 11:20 ` Will Deacon [this message]
2020-10-28 11:30 ` Mike Rapoport
2020-10-28 21:03 ` Edgecombe, Rick P
2020-10-29 8:12 ` Mike Rapoport
2020-10-29 23:19 ` Edgecombe, Rick P
2020-10-29 8:15 ` David Hildenbrand
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