On 13.06.23 02:10, 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. 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/ --- Hi Non-x86 Arch’s, x86 has a feature that allows for the creation of a special type of writable memory (shadow stack) that is only writable in limited specific ways. Previously, changes were proposed to core MM code to teach it to decide when to create normally writable memory or the special shadow stack writable memory, but David Hildenbrand suggested[0] to change pXX_mkwrite() to take a VMA, so awareness of shadow stack memory can be moved into x86 code. Later Linus suggested a less error-prone way[1] to go about this after the first attempt had a bug. Since pXX_mkwrite() is defined in every arch, it requires some tree-wide changes. So that is why you are seeing some patches out of a big x86 series pop up in your arch mailing list. There is no functional change. After this refactor, the shadow stack series goes on to use the arch helpers to push arch memory details inside arch/x86 and other arch's with upcoming shadow stack features. Testing was just 0-day build testing. Hopefully that is enough context. Thanks! [0] https://lore.kernel.org/lkml/0e29a2d0-08d8-bcd6-ff26-4bea0e4037b0@xxxxxxxxxx/ [1] https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@xxxxxxxxxxxxxx/ ---
Acked-by: David Hildenbrand <david@xxxxxxxxxx> -- Cheers, David / dhildenb
