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Re: [RFC PATCH v2 2/2] target/ppc: make gdb able to translate priviledge

From: Richard Henderson
Subject: Re: [RFC PATCH v2 2/2] target/ppc: make gdb able to translate priviledged addresses
Date: Mon, 14 Jun 2021 15:37:20 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 6/14/21 12:16 PM, Bruno Larsen (billionai) wrote:
This patch changes ppc_cpu_get_phys_page_debug so that it is now
able to translate both, priviledged and real mode addresses
independently of whether the CPU executing it has those permissions

This was mentioned by Fabiano as something that would be very useful to
help with debugging, but could possibly constitute a security issue if
that debug function can be called in some way by prodution code. the
solution was implemented such that it would be trivial to wrap it around
ifdefs for building only with --enable-debug, for instance, but we are
not sure this is the best approach, hence why it is an RFC.

Suggested-by: Fabiano Rosas<farosas@linux.ibm.com>
Signed-off-by: Bruno Larsen (billionai)<bruno.larsen@eldorado.org.br>
  target/ppc/mmu_helper.c | 23 +++++++++++++++++++++++
  1 file changed, 23 insertions(+)

I think the first part is unnecessary. Either the cpu is in supervisor mode or it isn't, and gdb should use the correct address space. If you really want to force supervisor lookup from a guest that is paused in usermode, I suppose you could force MSR.PR=1 while you're performing the access and set it back afterward.

I think the second part is actively wrong -- real-mode address lookup will (for the most part) always succeed. Moreover, the gdb user will have no idea that you've silently changed addressing methods.


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