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
