On Wed, 29 Aug 2018 01:11:43 -0700 Nadav Amit <namit@xxxxxxxxxx> wrote: > From: Andy Lutomirski <luto@xxxxxxxxxx> > > Sometimes we want to set a temporary page-table entries (PTEs) in one of > the cores, without allowing other cores to use - even speculatively - > these mappings. There are two benefits for doing so: > > (1) Security: if sensitive PTEs are set, temporary mm prevents their use > in other cores. This hardens the security as it prevents exploding a > dangling pointer to overwrite sensitive data using the sensitive PTE. > > (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in > remote page-tables. > > To do so a temporary mm_struct can be used. Mappings which are private > for this mm can be set in the userspace part of the address-space. > During the whole time in which the temporary mm is loaded, interrupts > must be disabled. > > The first use-case for temporary PTEs, which will follow, is for poking > the kernel text. > > [ Commit message was written by Nadav ] > > Cc: Andy Lutomirski <luto@xxxxxxxxxx> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> > --- > arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h > index eeeb9289c764..96afc8c0cf15 100644 > --- a/arch/x86/include/asm/mmu_context.h > +++ b/arch/x86/include/asm/mmu_context.h > @@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void) > return cr3; > } > > +typedef struct { > + struct mm_struct *prev; > +} temporary_mm_state_t; > + > +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm) > +{ > + temporary_mm_state_t state; > + > + lockdep_assert_irqs_disabled(); > + state.prev = this_cpu_read(cpu_tlbstate.loaded_mm); > + switch_mm_irqs_off(NULL, mm, current); > + return state; > +} Hmm, why don't we return mm_struct *prev directly? Thank you, > + > +static inline void unuse_temporary_mm(temporary_mm_state_t prev) > +{ > + lockdep_assert_irqs_disabled(); > + switch_mm_irqs_off(NULL, prev.prev, current); > +} > + > #endif /* _ASM_X86_MMU_CONTEXT_H */ > -- > 2.17.1 > -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>