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

[Christophe Leroy]

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)

Christophe

> Thanks,
> Nick
>
> > and pte_exprotect()
> > clears both bits.
> > - Define a book3e specific version of pte_mkexec() which sets
> > either SX or UX based on UR.
> >
> > Fixes: 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> > v3: Removed pte_mkexec() from nohash/64/pgtable.h
> > v2: New
> > ---