On Mon, 2023-02-20 at 13:57 +0100, David Hildenbrand wrote:

quoted

   
+     /*
+      * 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,

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?


/*
 * 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.
 */



Thanks,
Rick