On Mon, Jun 12, 2023 at 05:10:28PM -0700, Rick Edgecombe wrote: > The x86 Shadow stack feature includes a new type of memory called shadow > stack. This shadow stack memory has some unusual properties, which requires > some core mm changes to function properly. > > One of these unusual properties is that shadow stack memory is writable, > but only in limited ways. These limits are applied via a specific PTE > bit combination. Nevertheless, the memory is writable, and core mm code > will need to apply the writable permissions in the typical paths that > call pte_mkwrite(). Future patches will make pte_mkwrite() take a VMA, so > that the x86 implementation of it can know whether to create regular > writable memory or shadow stack memory. Nit: ^ mappings? > But there are a couple of challenges to this. Modifying the signatures of > each arch pte_mkwrite() implementation would be error prone because some > are generated with macros and would need to be re-implemented. Also, some > pte_mkwrite() callers operate on kernel memory without a VMA. > > So this can be done in a three step process. First pte_mkwrite() can be > renamed to pte_mkwrite_novma() in each arch, with a generic pte_mkwrite() > added that just calls pte_mkwrite_novma(). Next callers without a VMA can > be moved to pte_mkwrite_novma(). And lastly, pte_mkwrite() and all callers > can be changed to take/pass a VMA. > > Previous patches have done the first step, so next move the callers that > don't have a VMA to pte_mkwrite_novma(). Also do the same for I hear x86 maintainers asking to drop "previous patches" ;-) Maybe This is the second step of the conversion that moves the callers ... > pmd_mkwrite(). This will be ok for the shadow stack feature, as these > callers are on kernel memory which will not need to be made shadow stack, > and the other architectures only currently support one type of memory > in pte_mkwrite() > > Cc: linux-doc@xxxxxxxxxxxxxxx > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > Cc: linux-s390@xxxxxxxxxxxxxxx > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx > Cc: linux-arch@xxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Reviewed-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> -- Sincerely yours, Mike.
