From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien.grall@arm.com>
Cc: xen-devel@lists.xenproject.org,
Stefano Stabellini <sstabellini@kernel.org>,
Andrii Anisov <Andrii_Anisov@epam.com>,
Oleksandr_Tyshchenko@epam.com
Subject: Re: [Xen-devel] [PATCH MM-PART3 v2 11/12] xen/arm: mm: Don't open-code Xen PT update in {set, clear}_fixmap()
Date: Thu, 13 Jun 2019 11:51:21 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.21.1906131140290.13737@sstabellini-ThinkPad-T480s> (raw)
In-Reply-To: <a7874b6c-08b4-28a0-cb5f-ea7f7b23e2e7@arm.com>
On Thu, 13 Jun 2019, Julien Grall wrote:
> Hi Stefano,
>
> On 6/12/19 11:33 PM, Stefano Stabellini wrote:
> > On Tue, 14 May 2019, Julien Grall wrote:
> > > {set, clear}_fixmap() are currently open-coding update to the Xen
> > > page-tables. This can be avoided by using the generic helpers
> > > map_pages_to_xen() and destroy_xen_mappings().
> > >
> > > Both function are not meant to fail for fixmap, hence the BUG_ON()
> > > checking the return.
> >
> > BUG_ON crashes the hypervisor even in non-DEBUG builds. Would an ASSERT
> > be a better choice?
> The ASSERT() would disappear in non-debug potentially leading to unknown
> consequence.
>
> If we imagine that map_pages_to_xen() fails, then it likely means that mapping
> has not been done/removed.
>
> As set_fixmap() does not return an error, this means that the user may try to
> access an invalid mapping and therefore crash the hypervisor.
>
> As clear_fixmap() does not return an error, this means that subsequent
> set_fixmap() may fail because map_pages_to_xen() does not allow to replace
> valid mapping.
>
> Ideally we would want to propagate the error, however all the call to the
> functions happen during boot. So most likely the user will panic/BUG_ON as you
> this hint something has gone really wrong and we don't want to continue
> further.
I think the basic principle is that with BUG_ON is "easy" for a guest to
be able to trigger it, potentially causing a DOS. Without the BUG_ON,
the guest is unlikely to be able to trigger a crash. However, if all the
calls happen during boot in regards to operations that have nothing to
do with guests behavior, then it is fine.
I checked all the call sites and I agree that in this case they are all
done during boot only. So in this case it is OK to have the
panic/BUG_ON.
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next prev parent reply other threads:[~2019-06-13 18:51 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-14 12:31 [PATCH MM-PART3 v2 00/12] xen/arm: Provide a generic function to update Xen PT Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-05-14 12:31 ` [PATCH MM-PART3 v2 01/12] xen/arm: lpae: Add a macro to generate offsets from an address Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 18:21 ` Stefano Stabellini
2019-06-11 18:27 ` Julien Grall
2019-05-14 12:31 ` [PATCH MM-PART3 v2 02/12] xen/arm: mm: Rename create_xen_entries() to xen_pt_update() Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 18:23 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 03/12] xen/arm: mm: Move out of xen_pt_update() the logic to update an entry Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 18:29 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 04/12] xen/arm: mm: Only increment mfn when valid in xen_pt_update Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 18:37 ` Stefano Stabellini
2019-06-11 19:56 ` [Xen-devel] Checking INVALID_MFN in mfn_add (WAS: Re: [PATCH MM-PART3 v2 04/12] xen/arm: mm: Only increment mfn when valid in xen_pt_update) Julien Grall
2019-06-11 20:24 ` Andrew Cooper
2019-06-12 12:47 ` Julien Grall
2019-06-12 15:57 ` Stefano Stabellini
2019-06-12 7:53 ` Jan Beulich
2019-05-14 12:31 ` [PATCH MM-PART3 v2 05/12] xen/arm: mm: Introduce _PAGE_PRESENT and _PAGE_POPULATE Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 22:35 ` Stefano Stabellini
2019-06-12 13:00 ` Julien Grall
2019-05-14 12:31 ` [PATCH MM-PART3 v2 06/12] xen/arm: mm: Sanity check any update of Xen page tables Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 0:10 ` Stefano Stabellini
2019-06-12 14:48 ` Julien Grall
2019-06-12 15:54 ` Stefano Stabellini
2019-06-12 15:58 ` Julien Grall
2019-05-14 12:31 ` [PATCH MM-PART3 v2 07/12] xen/arm: mm: Rework xen_pt_update_entry to avoid use xenmap_operation Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 22:22 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 08/12] xen/arm: mm: Remove enum xenmap_operation Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-11 22:38 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 09/12] xen/arm: mm: Use {, un}map_domain_page() to map/unmap Xen page-tables Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 22:25 ` Stefano Stabellini
2019-06-13 8:07 ` Julien Grall
2019-06-13 17:55 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 10/12] xen/arm: mm: Rework Xen page-tables walk during update Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 22:52 ` Stefano Stabellini
2019-06-13 8:20 ` Julien Grall
2019-06-13 17:59 ` Stefano Stabellini
2019-06-13 21:32 ` Julien Grall
2019-06-13 22:57 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 11/12] xen/arm: mm: Don't open-code Xen PT update in {set, clear}_fixmap() Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 22:33 ` Stefano Stabellini
2019-06-13 8:31 ` Julien Grall
2019-06-13 18:51 ` Stefano Stabellini [this message]
2019-06-13 21:21 ` Julien Grall
2019-06-13 22:55 ` Stefano Stabellini
2019-05-14 12:31 ` [PATCH MM-PART3 v2 12/12] xen/arm: mm: Remove set_pte_flags_on_range() Julien Grall
2019-05-14 12:31 ` [Xen-devel] " Julien Grall
2019-06-12 22:41 ` Stefano Stabellini
2019-06-13 8:51 ` Julien Grall
2019-06-13 18:04 ` Stefano Stabellini
2019-06-13 21:22 ` Julien Grall
2019-05-29 17:23 ` [PATCH MM-PART3 v2 00/12] xen/arm: Provide a generic function to update Xen PT Julien Grall
2019-05-29 17:23 ` [Xen-devel] " Julien Grall
2019-06-10 10:08 ` Julien Grall
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