[PATCH 0/5] mm: arch/shstk: Common shadow stack mapping helper and VM_NOHUGEPAGE

[Catalin Marinas <catalin.marinas@arm.com>]

Hi,

arm64, riscv and x86 all implement shadow stack support and use a
similar pattern for mapping the user shadow stack (originally cloned
from x86). Extract this common pattern into a shared helper -
vm_mmap_shadow_stack().

Patch 1 introduces vm_mmap_shadow_stack() in mm/util.c, which wraps
do_mmap() with the flags required for a VM_SHADOW_STACK mapping. The
helper uses PROT_READ|PROT_WRITE prot bits instead of the earlier x86
approach of PROT_READ with an explicit VM_WRITE vm_flag. Functionally
there is no difference. I looked up the history of this flag on the
lists but it wasn't conclusive. My guess is that the original aim was to
mark the vma not writable but that would confuse the kernel, so it ended
up with the VM_WRITE flag instead.

Patches 2-4 update arm64, riscv and x86 respectively to use the new
helper, removing the duplicated mmap logic.

Patch 5 forces VM_NOHUGEPAGE when allocating the shadow stack via the
new helper, mirroring what commit c4608d1bf7c6 ("mm: mmap: map MAP_STACK
to VM_NOHUGEPAGE") did for normal stacks. It will save some memory,
especially when the ulimit -s is high.

Boot-tested on x86, fully tested on arm64. I do not have a compiler
version that supports the -march=rv64ima_zicfiss_zicfilp option for
riscv, so any help with testing is welcome.

Thanks.

Catalin Marinas (5):
  mm: Introduce vm_mmap_shadow_stack() as a helper for VM_SHADOW_STACK
    mappings
  arm64: gcs: Use the new common vm_mmap_shadow_stack() helper
  riscv: shstk: Use the new common vm_mmap_shadow_stack() helper
  x86: shstk: Use the new common vm_mmap_shadow_stack() helper
  mm: Do not map the shadow stack as THP

 arch/arm64/mm/gcs.c         | 14 +-------------
 arch/riscv/kernel/usercfi.c | 12 +-----------
 arch/x86/kernel/shstk.c     | 12 ++----------
 include/linux/mm.h          |  4 ++++
 mm/util.c                   | 29 +++++++++++++++++++++++++++++
 5 files changed, 37 insertions(+), 34 deletions(-)