Messages in this thread | | | Date | Thu, 23 Feb 2023 13:55:47 +0100 | From | David Hildenbrand <> | Subject | Re: [PATCH v6 19/41] x86/mm: Check shadow stack page fault errors |
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On 23.02.23 00:07, Edgecombe, Rick P wrote: > On Mon, 2023-02-20 at 13:57 +0100, David Hildenbrand wrote: >>> >>> + /* >>> + * When a page becomes COW it changes from a shadow stack >>> permission >>> + * page (Write=0,Dirty=1) to (Write=0,Dirty=0,SavedDirty=1), >>> which is simply >>> + * read-only to the CPU. When shadow stack is enabled, a RET >>> would >>> + * normally pop the shadow stack by reading it with a "shadow >>> stack >>> + * read" access. However, in the COW case the shadow stack >>> memory does >>> + * not have shadow stack permissions, it is read-only. So it >>> will >>> + * generate a fault. >>> + * >>> + * For conventionally writable pages, a read can be serviced >>> with a >>> + * read only PTE, and COW would not have to happen. But for >>> shadow >>> + * stack, there isn't the concept of read-only shadow stack >>> memory. >>> + * If it is shadow stack permission, it can be modified via >>> CALL and >>> + * RET instructions. So COW needs to happen before any memory >>> can be >>> + * mapped with shadow stack permissions. >>> + * >>> + * Shadow stack accesses (read or write) need to be serviced >>> with >>> + * shadow stack permission memory, so in the case of a shadow >>> stack >>> + * read access, treat it as a WRITE fault so both COW will >>> happen and >>> + * the write fault path will tickle maybe_mkwrite() and map >>> the memory >>> + * shadow stack. >>> + */ >> >> Again, I suggest dropping all details about COW from this comment >> and >> from the patch description. It's just one such case that can happen. > > Hi David,
Hi Rick,
> > I was just trying to edit this one to drop COW details, but I think in > this case, one of the major reasons for the code *is* actually COW. We > are not working around the whole inadvertent shadow stack memory piece > here, but something else: Making sure shadow stack memory is faulted in > and doing COW if required to make this possible. I came up with this, > does it seem better?
Regarding the fault handling I completely agree. We have to treat a read like a write event. And as read-only shadow stack PTEs don't exist, we have to tell the MM to create a writable one for us.
> > > /* > * For conventionally writable pages, a read can be serviced with a > * > read only PTE. But for shadow stack, there isn't a concept of > * read- > only shadow stack memory. If it a PTE has the shadow stack > * > permission, it can be modified via CALL and RET instructions. So > * core > MM needs to fault in a writable PTE and do things it already > * does for > write faults. > * > * Shadow stack accesses (read or write) need to be > serviced with > * shadow stack permission memory, so in the case of a > shadow stack > * read access, treat it as a WRITE fault so both any > required COW will > * happen and the write fault path will tickle > maybe_mkwrite() and map > * the memory shadow stack. > */
That sounds good! I'd rewrite the last part slightly.
" Shadow stack accesses (read or write) need to be serviced with shadow stack permission memory, which always include write permissions. So in the case of a shadow stack read access, treat it as a WRITE fault. This will make sure that MM will prepare everything (e.g., break COW) such that maybe_mkwrite() can create a proper shadow stack PTE. "
-- Thanks,
David / dhildenb
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