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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: George Dunlap <george.dunlap@citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH 12/12] SUPPORT.md: write down restriction of 32-bit tool stacks
Date: Fri, 25 Jun 2021 20:45:56 +0100	[thread overview]
Message-ID: <f3a758a2-a8a9-60a3-92c9-1a490084dbb6@citrix.com> (raw)
In-Reply-To: <03512f29-7a4e-70ff-271b-7d65ed471935@suse.com>

On 25/06/2021 14:24, Jan Beulich wrote:
> Let's try to avoid giving the impression that 32-bit tool stacks are as
> capable as 64-bit ones.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/SUPPORT.md
> +++ b/SUPPORT.md
> @@ -131,6 +131,11 @@ ARM only has one guest type at the momen
>  
>  ## Toolstack
>  
> +While 32-bit builds of the tool stack are generally supported, restrictions
> +apply in particular when running on top of a 64-bit hypervisor.

Actually, this isn't true, and in a way which helps us right now.

PV32 isn't security supported, and neither ARM nor x86 support dom0
bitness != Xen bitness.  On x86, it doesn't remotely work because of the
pointer size mismatch, and while this was bodged in a horrifying way in
the ARM ABI, I doubt anyone is in a hurry to turn that into a supported
configuration.

That said, it is my intent with the ABIv2 changes for a 32bit toolstack,
under 64bit guest kernel, under 64bit or 128bit Xen (yes - RISCV-128 is
already a thing being discussed) to function.

>   For example,
> +very large guests aren't supported in this case.

The wording here wants to be careful, because under certain readings,
you've just dropped security support for 32bit toolstacks.

What we actually mean is "a toolstack with bitness < Xen is not expected
to be able to manage very large domains correctly, and don't pester us
with bugs when it doesn't work because we won't fix them".

Whereas we will fix security issues which only happen to manifest in
32bit builds of the toolstack.


>   This includes guests giving
> +the appearance of being large, by altering their own memory layouts.

I'd drop sentence.  Its an internal detail of a corner case which we're
expecting to remove in the future, and "the guest kernel can DoS itself"
isn't interesting.

~Andrew



  reply	other threads:[~2021-06-25 19:46 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-25 13:15 [PATCH 00/12] x86: more or less log-dirty related improvements Jan Beulich
2021-06-25 13:17 ` [PATCH 01/12] libxc: split xc_logdirty_control() from xc_shadow_control() Jan Beulich
2021-06-25 14:51   ` Christian Lindig
2021-06-25 15:49   ` Andrew Cooper
2021-06-28  9:40     ` Jan Beulich
2021-06-25 13:18 ` [PATCH 02/12] libxenguest: deal with log-dirty op stats overflow Jan Beulich
2021-06-25 16:36   ` Andrew Cooper
2021-06-28  7:48     ` Jan Beulich
2021-06-28 11:10       ` Olaf Hering
2021-06-28 11:20         ` Jan Beulich
2021-06-28 11:30           ` Olaf Hering
2021-06-25 13:18 ` [PATCH 03/12] libxenguest: short-circuit "all-dirty" handling Jan Beulich
2021-06-25 17:02   ` Andrew Cooper
2021-06-28  8:26     ` Jan Beulich
2021-09-02 17:11       ` Ian Jackson
2021-06-25 13:19 ` [PATCH 04/12] libxenguest: avoid allocating unused deferred-pages bitmap Jan Beulich
2021-06-25 18:08   ` Andrew Cooper
2021-06-28  8:47     ` Jan Beulich
2021-09-02 17:17       ` Ian Jackson
2021-06-25 13:19 ` [PATCH 05/12] libxenguest: complete loops in xc_map_domain_meminfo() Jan Beulich
2021-06-25 18:30   ` Andrew Cooper
2021-06-28  8:53     ` Jan Beulich
2021-06-25 13:20 ` [PATCH 06/12] libxenguest: guard against overflow from too large p2m when checkpointing Jan Beulich
2021-06-25 19:00   ` Andrew Cooper
2021-06-28  9:05     ` Jan Beulich
2021-06-25 13:20 ` [PATCH 07/12] libxenguest: fix off-by-1 in colo-secondary-bitmap merging Jan Beulich
2021-06-25 19:06   ` Andrew Cooper
2021-06-25 13:21 ` [PATCH 08/12] x86/paging: deal with log-dirty stats overflow Jan Beulich
2021-06-25 19:09   ` Andrew Cooper
2021-06-25 13:21 ` [PATCH 09/12] x86/paging: supply more useful log-dirty page count Jan Beulich
2021-06-25 13:22 ` [PATCH 10/12] x86/mm: update log-dirty bitmap when manipulating P2M Jan Beulich
2021-06-25 13:22 ` [PATCH 11/12] x86/mm: pull a sanity check earlier in xenmem_add_to_physmap_one() Jan Beulich
2021-06-25 19:10   ` Andrew Cooper
2021-06-25 13:24 ` [PATCH 12/12] SUPPORT.md: write down restriction of 32-bit tool stacks Jan Beulich
2021-06-25 19:45   ` Andrew Cooper [this message]
2021-06-28  9:22     ` Jan Beulich

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