Re: [PATCH 2/3] powerpc/book3e: Fix set_memory_x() and set_memory_nx()

[Christophe Leroy]

Le 27/10/2021 à 07:27, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of October 27, 2021 2:55 pm:
> > Le 27/10/2021 à 06:44, Nicholas Piggin a écrit :
> > > Excerpts from Christophe Leroy's message of October 26, 2021 3:39 pm:
> > > > set_memory_x() calls pte_mkexec() which sets _PAGE_EXEC.
> > > > set_memory_nx() calls pte_exprotec() which clears _PAGE_EXEC.
> > > >
> > > > Book3e has 2 bits, UX and SX, which defines the exec rights
> > > > resp. for user (PR=1) and for kernel (PR=0).
> > > >
> > > > _PAGE_EXEC is defined as UX only.
> > > >
> > > > An executable kernel page is set with either _PAGE_KERNEL_RWX
> > > > or _PAGE_KERNEL_ROX, which both have SX set and UX cleared.
> > > >
> > > > So set_memory_nx() call for an executable kernel page does
> > > > nothing because UX is already cleared.
> > > >
> > > > And set_memory_x() on a non-executable kernel page makes it
> > > > executable for the user and keeps it non-executable for kernel.
> > > >
> > > > Also, pte_exec() always returns 'false' on kernel pages, because
> > > > it checks _PAGE_EXEC which doesn't include SX, so for instance
> > > > the W+X check doesn't work.
> > > >
> > > > To fix this:
> > > > - change tlb_low_64e.S to use _PAGE_BAP_UX instead of _PAGE_USER
> > > > - sets both UX and SX in _PAGE_EXEC so that pte_user() returns
> > > > true whenever one of the two bits is set
> > >
> > > I don't understand this change. Which pte_user() returns true after
> > > this change? Or do you mean pte_exec()?
> >
> > Oops, yes, I mean pte_exec()
> >
> > Unless I have to re-spin, can Michael eventually fix that typo while
> > applying ?
> >
> > > Does this filter through in some cases at least for kernel executable
> > > PTEs will get both bits set? Seems cleaner to distinguish user and
> > > kernel exec for that but maybe it's a lot of churn?
> >
> > Didn't understand what you mean.
> >
> > I did it like that to be able to continue using _PAGE_EXEC for checking
> > executability regardless of whether this is user or kernel, and then
> > continue using the generic nohash pte_exec() helper.
> >
> > Other solution would be to get rid of _PAGE_EXEC completely for book3e
> > and implement both pte_exec() and pte_mkexec() with _PAGE_BAP_UX and
> > _PAGE_BAP_SX, but I'm not sure it is worth the churn as you say. It
> > would also mean different helpers for book3s/32 when it is using 32 bits
> > PTE (CONFIG_PTE_64BIT=n)
>
> That's basically what I mean. And _PAGE_KERNEL_ROX etc would then not
> set the UX bit. But at least for now it seems to be an improvement.

That's already the case:

#define _PAGE_KERNEL_RWX	(_PAGE_BAP_SW | _PAGE_BAP_SR | _PAGE_DIRTY | 
_PAGE_BAP_SX)
#define _PAGE_KERNEL_ROX	(_PAGE_BAP_SR | _PAGE_BAP_SX)

Christophe