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Re: [PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it

Hi Stefano,

On 12/05/2021 23:00, Stefano Stabellini wrote:
On Sun, 25 Apr 2021, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

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

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)




I thought about it but I didn't want to define to a random value... I felt not define it was better.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

     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);
+    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.


Julien Grall



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