Messages in this thread | | | Date | Wed, 27 Oct 2021 15:27:21 +1000 | From | Nicholas Piggin <> | Subject | Re: [PATCH 2/3] powerpc/book3e: Fix set_memory_x() and set_memory_nx() |
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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.
Thanks, Nick
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