[PATCH v2 0/3] mm: Care about shadow stack guard gap when getting an unmapped area

[Mark Brown]

As covered in the commit log for c44357c2e76b ("x86/mm: care about shadow
stack guard gap during placement") our current mmap() implementation does
not take care to ensure that a new mapping isn't placed with existing
mappings inside it's own guard gaps. This is particularly important for
shadow stacks since if two shadow stacks end up getting placed adjacent to
each other then they can overflow into each other which weakens the
protection offered by the feature.

On x86 there is a custom arch_get_unmapped_area() which was updated by the
above commit to cover this case by specifying a start_gap for allocations
with VM_SHADOW_STACK. Both arm64 and RISC-V have equivalent features and
use the generic implementation of arch_get_unmapped_area() so let's make
the equivalent change there so they also don't get shadow stack pages
placed without guard pages. The arm64 and RISC-V shadow stack
implementations are currently on the list:

   https://lore.kernel.org/r/20240829-arm64-gcs-v12-0-42fec94743
   https://lore.kernel.org/lkml/20240403234054.2020347-1-debug@xxxxxxxxxxxx/

Given the addition of the use of vm_flags in the generic implementation
we also simplify the set of possibilities that have to be dealt with in
the core code by making arch_get_unmapped_area() take vm_flags as
standard. This is a bit invasive since the prototype change touches
quite a few architectures but since the parameter is ignored the change
is straightforward, the simplification for the generic code seems worth
it.

Changes in v2:
- Add comment to stack_guard_placement()
- Build fixes for xtensa and MIPS.
- Link to v1: https://lore.kernel.org/r/20240902-mm-generic-shadow-stack-guard-v1-0-9acda38b3dd3@xxxxxxxxxx

---
Mark Brown (3):
      mm: Make arch_get_unmapped_area() take vm_flags by default
      mm: Pass vm_flags to generic_get_unmapped_area()
      mm: Care about shadow stack guard gap when getting an unmapped area

 arch/alpha/kernel/osf_sys.c       |  2 +-
 arch/arc/mm/mmap.c                |  3 ++-
 arch/arm/mm/mmap.c                |  7 ++---
 arch/csky/abiv1/mmap.c            |  3 ++-
 arch/loongarch/mm/mmap.c          |  5 ++--
 arch/mips/mm/mmap.c               |  5 ++--
 arch/parisc/kernel/sys_parisc.c   |  5 ++--
 arch/parisc/mm/hugetlbpage.c      |  2 +-
 arch/powerpc/mm/book3s64/slice.c  | 10 ++++---
 arch/s390/mm/mmap.c               |  4 +--
 arch/sh/mm/mmap.c                 |  5 ++--
 arch/sparc/kernel/sys_sparc_32.c  |  2 +-
 arch/sparc/kernel/sys_sparc_64.c  |  4 +--
 arch/x86/include/asm/pgtable_64.h |  1 -
 arch/x86/kernel/sys_x86_64.c      | 21 +++------------
 arch/xtensa/kernel/syscall.c      |  3 ++-
 include/linux/sched/mm.h          | 27 +++++++------------
 mm/mmap.c                         | 55 +++++++++++++++++++--------------------
 18 files changed, 75 insertions(+), 89 deletions(-)
---
base-commit: 7c626ce4bae1ac14f60076d00eafe71af30450ba
change-id: 20240830-mm-generic-shadow-stack-guard-5bc5b8d0e95d

Best regards,
-- 
Mark Brown <broonie@xxxxxxxxxx>