Re: [PATCH v2] x86: consider effective protection attributes in W+X check

[Jan Beulich]

>>> On 23.02.18 at 08:49, <mingo@xxxxxxxxxx> wrote:

> * Jan Beulich <JBeulich@xxxxxxxx> wrote:
> 
>> >>> On 21.02.18 at 17:53, <mingo@xxxxxxxxxx> wrote:
>> 
>> > * Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> > 
>> >> Using just the leaf page table entry flags would cause a false warning
>> >> in case _PAGE_RW is clear or _PAGE_NX is set in a higher level entry.
>> >> Hand through both the current entry's flags as well as the accumulated
>> >> effective value (the latter as pgprotval_t instead of pgprot_t, as it's
>> >> not an actual entry's value).
>> >> 
>> >> This in particular eliminates the false W+X warning when running under
>> >> Xen, as commit 2cc42bac1c ("x86-64/Xen: eliminate W+X mappings") has to
>> >> make the necessary adjustment in L2 rather than L1 (the reason is
>> >> explained there). I.e. _PAGE_RW is clear there in L1, but _PAGE_NX is
>> >> set in L2.
>> >> 
>> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> >> Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
>> >> ---
>> >> v2: Re-base onto tip tree. Add Xen related paragraph to description.
>> >> ---
>> >>  arch/x86/mm/dump_pagetables.c |   92 
>> > ++++++++++++++++++++++++++----------------
>> >>  1 file changed, 57 insertions(+), 35 deletions(-)
>> > 
>> > There's a build failure with CONFIG_KASAN=y enabled:
>> > 
>> >  arch/x86/mm/dump_pagetables.c: In function âkasan_page_tableâ:
>> >  arch/x86/mm/dump_pagetables.c:365:3: error: too few arguments to function ânote_pageâ
>> >  arch/x86/mm/dump_pagetables.c:238:13: note: declared here
>> 
>> Oh, I see. Question though is what to pass as the extra argument:
>> Do I need to pass in the caller's effective rights, or should I take
>> kasan_page_table()'s checking against kasan_zero_p?d as an
>> indication that the effective permission is zero here? I'm sorry for
>> this probably trivial question, but I know nothing about how KASAN
>> works.
> 
> I'm not sure either - but I've Cc:-ed the KASAN gents who might be able to
> help us out here?

Actually, the "zero" in the names of the symbols meanwhile makes
me be pretty sure passing 0 for the effective permissions here is
exactly what is wanted. I'm about to produce v3.

Jan