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

[Philippe Mathieu-Daudé]

On 6/14/21 9:16 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(+)
> 
> diff --git a/target/ppc/mmu_helper.c b/target/ppc/mmu_helper.c
> index 9dcdf88597..41c727c690 100644
> --- a/target/ppc/mmu_helper.c
> +++ b/target/ppc/mmu_helper.c
> @@ -2947,6 +2947,29 @@ hwaddr ppc_cpu_get_phys_page_debug(CPUState *cs, vaddr 
> addr)
>                    cpu_mmu_index(&cpu->env, true), false)) {
>          return raddr & TARGET_PAGE_MASK;
>      }
> +
> +    /*
> +     * This is a fallback, in case we're asking for priviledged memory to
> +     * be printed, but the PCU is not executing in a priviledged manner.
> +     *
> +     * The code could be considered a security vulnerability if
> +     * this function can be called in a scenario that does not involve
> +     * debugging.
> +     * Given the name and how useful using real addresses may be for
> +     * actually debugging, however, we decided to include it anyway and
> +     * discuss how to best avoid the possible security concerns.
> +     * The current plan is that, if there is a chance this code is called in
> +     * a production environment, we can surround it with ifdefs so that it
> +     * is only compiled with --enable-debug

Nothing forbid us to use --enable-debug in a production environment.

> +     */
> +        /* attempt to translate first with virtual addresses */
> +    if (ppc_xlate(cpu, addr, MMU_DATA_LOAD, &raddr, &s, &p, 1, false) ||
> +        ppc_xlate(cpu, addr, MMU_INST_FETCH, &raddr, &s, &p, 1, false) ||
> +        /* if didn't work, attempt to translate with real addresses */
> +        ppc_xlate(cpu, addr, MMU_DATA_LOAD, &raddr, &s, &p, 3, false) ||
> +        ppc_xlate(cpu, addr, MMU_INST_FETCH, &raddr, &s, &p, 3, false)) {
> +        return raddr & TARGET_PAGE_MASK;
> +    }
>      return -1;
>  }
>  
>