On Mon, Jun 12, 2023 at 05:10:27PM -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: ^ mapping? > 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. > > Start the process by renaming pte_mkwrite() to pte_mkwrite_novma() and > adding the pte_mkwrite() wrapper in linux/pgtable.h. Apply the same > pattern for pmd_mkwrite(). Since not all archs have a pmd_mkwrite_novma(), > create a new arch config HAS_HUGE_PAGE that can be used to tell if > pmd_mkwrite() should be defined. Otherwise in the !HAS_HUGE_PAGE cases the > compiler would not be able to find pmd_mkwrite_novma(). > > No functional change. > > Cc: linux-doc@xxxxxxxxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx > Cc: linux-alpha@xxxxxxxxxxxxxxx > Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > Cc: linux-csky@xxxxxxxxxxxxxxx > Cc: linux-hexagon@xxxxxxxxxxxxxxx > Cc: linux-ia64@xxxxxxxxxxxxxxx > Cc: loongarch@xxxxxxxxxxxxxxx > Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx > Cc: Michal Simek <monstr@xxxxxxxxx> > Cc: Dinh Nguyen <dinguyen@xxxxxxxxxx> > Cc: linux-mips@xxxxxxxxxxxxxxx > Cc: openrisc@xxxxxxxxxxxxxxxxxxxx > Cc: linux-parisc@xxxxxxxxxxxxxxx > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx > Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx > Cc: linux-s390@xxxxxxxxxxxxxxx > Cc: linux-sh@xxxxxxxxxxxxxxx > Cc: sparclinux@xxxxxxxxxxxxxxx > Cc: linux-um@xxxxxxxxxxxxxxxxxxx > Cc: linux-arch@xxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> > Link: https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@xxxxxxxxxxxxxx/ Reviewed-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> -- Sincerely yours, Mike.