On Thu, Apr 01, 2021 at 03:10:51PM -0700, Yu-cheng Yu wrote: > INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the > first and the last elements in the range, effectively touches those memory > areas. > > The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and > 255 * 4 = 1020 bytes by INCSSPD. Both ranges are far from PAGE_SIZE. > Thus, putting a gap page on both ends of a shadow stack prevents INCSSP, > CALL, and RET from going beyond. > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > --- > v24: > - Instead changing vm_*_gap(), create x86-specific versions. > > arch/x86/include/asm/page_types.h | 17 +++++++++++++++ > arch/x86/mm/mmap.c | 36 +++++++++++++++++++++++++++++++ > include/linux/mm.h | 4 ++++ > 3 files changed, 57 insertions(+) > > diff --git a/arch/x86/include/asm/page_types.h b/arch/x86/include/asm/page_types.h > index a506a411474d..3a5529bcfd76 100644 > --- a/arch/x86/include/asm/page_types.h > +++ b/arch/x86/include/asm/page_types.h > @@ -73,6 +73,23 @@ bool pfn_range_is_mapped(unsigned long start_pfn, unsigned long end_pfn); > > extern void initmem_init(void); > > +/* > + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D). INCSSPQ > + * moves shadow stack pointer up to 255 * 8 = ~2 KB (~1KB for INCSSPD) and > + * touches the first and the last element in the range, which triggers a > + * page fault if the range is not in a shadow stack. Because of this, > + * creating 4-KB guard pages around a shadow stack prevents these > + * instructions from going beyond. > + */ > +#define SHADOW_STACK_GUARD_GAP PAGE_SIZE The define is only used within arch/x86/mm/mmap.c. Maybe move it there? Otherwise: Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> -- Kirill A. Shutemov
