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From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien@xen.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	 xen-devel@lists.xenproject.org, Wei.Chen@arm.com,
	Henry.Wang@arm.com,  Penny.Zheng@arm.com,
	Bertrand.Marquis@arm.com,  Julien Grall <jgrall@amazon.com>,
	 Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it
Date: Thu, 13 May 2021 15:32:49 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.21.2105131528230.5018@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <caec9741-8c0e-b80a-1020-c985beb1e100@xen.org>

On Wed, 12 May 2021, Julien Grall wrote:
> Hi Stefano,
> 
> On 12/05/2021 23:00, Stefano Stabellini wrote:
> > On Sun, 25 Apr 2021, Julien Grall wrote:
> > > From: Julien Grall <jgrall@amazon.com>
> > > 
> > > Only the first 2GB of the virtual address space is shared between all
> > > the page-tables on Arm32.
> > > 
> > > There is a long outstanding TODO in xen_pt_update() stating that the
> > > function is can only work with shared mapping. Nobody has ever called
> >             ^ remove
> > 
> > > the function with private mapping, however as we add more callers
> > > there is a risk to mess things up.
> > > 
> > > Introduce a new define to mark the ened of the shared mappings and use
> >                                       ^end
> > 
> > > it in xen_pt_update() to verify if the address is correct.
> > > 
> > > Note that on Arm64, all the mappings are shared. Some compiler may
> > > complain about an always true check, so the new define is not introduced
> > > for arm64 and the code is protected with an #ifdef.
> >   On arm64 we could maybe define SHARED_VIRT_END to an arbitrarely large
> > value, such as:
> > 
> > #define SHARED_VIRT_END (1UL<<48)
> > 
> > or:
> > 
> > #define SHARED_VIRT_END DIRECTMAP_VIRT_END
> > 
> > ?
> 
> I thought about it but I didn't want to define to a random value... I felt not
> define it was better.

Yeah, I see your point: any restrictions in addressing (e.g. 48bits)
are physical address restrictions. Here we are talking about virtual
address restriction, and I don't think there are actually any
restrictions there?  We could validly map something at
0xffff_ffff_ffff_ffff. So even (1<<48) which makes sense at the physical
level, doesn't make sense in terms of virtual addresses.


> > > Signed-off-by: Julien Grall <jgrall@amazon.com>
> > > 
> > > ---
> > >      Changes in v2:
> > >          - New patch
> > > ---
> > >   xen/arch/arm/mm.c            | 11 +++++++++--
> > >   xen/include/asm-arm/config.h |  4 ++++
> > >   2 files changed, 13 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > > index 8fac24d80086..5c17cafff847 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -1275,11 +1275,18 @@ static int xen_pt_update(unsigned long virt,
> > >        * For arm32, page-tables are different on each CPUs. Yet, they
> > > share
> > >        * some common mappings. It is assumed that only common mappings
> > >        * will be modified with this function.
> > > -     *
> > > -     * XXX: Add a check.
> > >        */
> > >       const mfn_t root = virt_to_mfn(THIS_CPU_PGTABLE);
> > >   +#ifdef SHARED_VIRT_END
> > > +    if ( virt > SHARED_VIRT_END ||
> > > +         (SHARED_VIRT_END - virt) < nr_mfns )
> > 
> > The following would be sufficient, right?
> > 
> >      if ( virt + nr_mfns > SHARED_VIRT_END )
> 
> This would not protect against an overflow. So I think it is best if we keep
> my version.

But there can be no overflow with the way SHARED_VIRT_END is defined.
Even if SHARED_VIRT_END was defined at (1<<48) there can be no overflow.
Only if we defined SHARED_VIRT_END as 0xffff_ffff_ffff_ffff we would
have an overflow, but you wrote above that your preference is not to do
that.


  reply	other threads:[~2021-05-13 22:33 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-25 20:13 [PATCH RFCv2 00/15] xen/arm: mm: Remove open-coding mappings Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 01/15] xen/arm: lpae: Rename LPAE_ENTRIES_MASK_GS to LPAE_ENTRY_MASK_GS Julien Grall
2021-05-11 22:12   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 02/15] xen/arm: lpae: Use the generic helpers to defined the Xen PT helpers Julien Grall
2021-05-11 22:26   ` Stefano Stabellini
2021-05-12 14:26     ` Julien Grall
2021-05-12 21:30       ` Stefano Stabellini
2021-05-12 22:16         ` Julien Grall
2021-05-13 22:44           ` Stefano Stabellini
2021-07-03 17:32             ` Julien Grall
2021-07-13 20:53               ` Stefano Stabellini
2021-07-14 17:40                 ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 03/15] xen/arm: p2m: Replace level_{orders, masks} arrays with LEVEL_{ORDER, MASK} Julien Grall
2021-05-11 22:33   ` Stefano Stabellini
2021-05-12 14:39     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 04/15] xen/arm: mm: Allow other mapping size in xen_pt_update_entry() Julien Grall
2021-05-11 22:42   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 05/15] xen/arm: mm: Avoid flushing the TLBs when mapping are inserted Julien Grall
2021-05-11 22:51   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 06/15] xen/arm: mm: Don't open-code Xen PT update in remove_early_mappings() Julien Grall
2021-05-11 22:58   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 07/15] xen/arm: mm: Re-implement early_fdt_map() using map_pages_to_xen() Julien Grall
2021-05-12 21:41   ` Stefano Stabellini
2021-05-12 22:18     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it Julien Grall
2021-05-12 22:00   ` Stefano Stabellini
2021-05-12 22:23     ` Julien Grall
2021-05-13 22:32       ` Stefano Stabellini [this message]
2021-05-13 22:59         ` Julien Grall
2021-05-14  1:04           ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 09/15] xen/arm32: mm: Re-implement setup_xenheap_mappings() using map_pages_to_xen() Julien Grall
2021-05-12 22:07   ` Stefano Stabellini
2021-05-13 17:55     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 10/15] xen/arm: mm: Allocate xen page tables in domheap rather than xenheap Julien Grall
2021-05-12 22:44   ` Stefano Stabellini
2021-05-13 18:09     ` Julien Grall
2021-05-13 22:27       ` Stefano Stabellini
2021-05-15  8:48         ` Julien Grall
2021-05-18  0:37           ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 11/15] xen/arm: mm: Allow page-table allocation from the boot allocator Julien Grall
2021-05-13 22:58   ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 12/15] xen/arm: add Persistent Map (PMAP) infrastructure Julien Grall
2021-04-26  9:41   ` Xia, Hongyan
2021-07-03 17:57     ` Julien Grall
2021-04-28 21:47   ` Wei Liu
2021-05-14 23:25   ` Stefano Stabellini
2021-05-15  8:54     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 13/15] xen/arm: mm: Use the PMAP helpers in xen_{,un}map_table() Julien Grall
2021-05-14 23:35   ` Stefano Stabellini
2021-05-15  9:03     ` Julien Grall
2021-04-25 20:13 ` [PATCH RFCv2 14/15] xen/arm: mm: Rework setup_xenheap_mappings() Julien Grall
2021-05-14 23:51   ` Stefano Stabellini
2021-05-15  9:21     ` Julien Grall
2021-05-18  0:50       ` Stefano Stabellini
2021-04-25 20:13 ` [PATCH RFCv2 15/15] xen/arm: mm: Re-implement setup_frame_table_mappings() with map_pages_to_xen() Julien Grall
2021-05-15  0:02   ` Stefano Stabellini
2021-05-15  9:25     ` Julien Grall
2021-05-18  0:51       ` Stefano Stabellini

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