Re: [PATCH v3 4/6] x86/mm/KASLR: Fix the wrong calculation of memory region initial size

[Baoquan He]

On 02/17/19 at 08:53am, Kees Cook wrote:
> On Sat, Feb 16, 2019 at 6:03 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
> >
> > In memory region KASLR, __PHYSICAL_MASK_SHIFT is taken to calculate
> > the initial size of the direct mapping region. This is correct in
> > the old code where __PHYSICAL_MASK_SHIFT was equal to MAX_PHYSMEM_BITS,
> > 46 bits, and only 4-level mode was supported.
> >
> > Later, in commit b83ce5ee91471d ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT
> > always 52"), __PHYSICAL_MASK_SHIFT was changed to be always 52 bits, no
> > matter it's 5-level or 4-level. This is wrong for 4-level paging. Then
> > when we adapt physical memory region size based on available memory, it
> > will overflow if the amount of system RAM and the padding is bigger
> > than 64 TB.
> >
> > In fact, here MAX_PHYSMEM_BITS should be used instead. Fix it by
> > replacing __PHYSICAL_MASK_SHIFT with MAX_PHYSMEM_BITS.
> >
> > Fixes: b83ce5ee9147 ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52")
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Reviewed-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> 
> Nice catch! I wish I had a system with >64TB RAM. ;)
> 
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks for reviewing and ack-ing. I don't have system with 64 TB RAM
either. This fix is from code reading. In patch 0006, the UV system
issue is a serious regression when I introduced KASLR into RHEL, now
even though a RHEL-only fix has been merged in our distros, the tracker
bug which tracks upstream fix will go to me during planning stage of
each RHEL version. After Kirill pushed 5-level code, SGI UV dev said the
old bug can't be reproduced any more in upstream kernel, I read code
and found that the code bug fixed in this patch will hide the SGI UV
issue :-).

> 
> > ---
> >  arch/x86/mm/kaslr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> > index bf680929fe26..97768df923e3 100644
> > --- a/arch/x86/mm/kaslr.c
> > +++ b/arch/x86/mm/kaslr.c
> > @@ -137,7 +137,7 @@ void __init kernel_randomize_memory(void)
> >         if (!kaslr_memory_enabled())
> >                 return;
> >
> > -       kaslr_regions[0].size_tb = 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT);
> > +       kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
> >         kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
> >
> >         /*
> > --
> > 2.17.2
> >
> 
> 
> -- 
> Kees Cook