Re: [PATCH v4 03/23] x86/mm: Introduce temporary mm structs

[Andy Lutomirski <luto@kernel.org>]

On Thu, Apr 25, 2019 at 10:37 AM Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > On Apr 25, 2019, at 9:26 AM, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Mon, Apr 22, 2019 at 11:57:45AM -0700, Rick Edgecombe wrote:
> >> From: Andy Lutomirski <luto@kernel.org>
> >>
> >> Using a dedicated page-table for temporary PTEs prevents other cores
> >> from using - even speculatively - these PTEs, thereby providing two
> >> benefits:
> >>
> >> (1) Security hardening: an attacker that gains kernel memory writing
> >> abilities cannot easily overwrite sensitive data.
> >>
> >> (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 mm struct, which will follow, is for
> >> poking the kernel text.
> >>
> >> [ Commit message was written by Nadav Amit ]
> >>
> >> Cc: Kees Cook <keescook@chromium.org>
> >> Cc: Dave Hansen <dave.hansen@intel.com>
> >> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> >> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
> >> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> >> Signed-off-by: Nadav Amit <namit@vmware.com>
> >> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> >> ---
> >> arch/x86/include/asm/mmu_context.h | 33 ++++++++++++++++++++++++++++++
> >> 1 file changed, 33 insertions(+)
> >>
> >> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> >> index 19d18fae6ec6..d684b954f3c0 100644
> >> --- a/arch/x86/include/asm/mmu_context.h
> >> +++ b/arch/x86/include/asm/mmu_context.h
> >> @@ -356,4 +356,37 @@ static inline unsigned long __get_current_cr3_fast(void)
> >>      return cr3;
> >> }
> >>
> >> +typedef struct {
> >> +    struct mm_struct *prev;
> >> +} temp_mm_state_t;
> >> +
> >> +/*
> >> + * Using a temporary mm allows to set temporary mappings that are not accessible
> >> + * by other cores. Such mappings are needed to perform sensitive memory writes
> >
> > s/cores/CPUs/g
> >
> > Yeah, the concept of a thread of execution we call a CPU in the kernel,
> > I'd say. No matter if it is one of the hyperthreads or a single thread
> > in core.
> >
> >> + * that override the kernel memory protections (e.g., W^X), without exposing the
> >> + * temporary page-table mappings that are required for these write operations to
> >> + * other cores.
> >
> > Ditto.
> >
> >> Using temporary mm also allows to avoid TLB shootdowns when the
> >
> > Using a ..
> >
> >> + * mapping is torn down.
> >> + *
> >
> > Nice commenting.
> >
> >> + * Context: The temporary mm needs to be used exclusively by a single core. To
> >> + *          harden security IRQs must be disabled while the temporary mm is
> >                             ^
> >                             ,
> >
> >> + *          loaded, thereby preventing interrupt handler bugs from overriding
> >> + *          the kernel memory protection.
> >> + */
> >> +static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
> >> +{
> >> +    temp_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;
> >> +}
> >> +
> >> +static inline void unuse_temporary_mm(temp_mm_state_t prev)
> >> +{
> >> +    lockdep_assert_irqs_disabled();
> >> +    switch_mm_irqs_off(NULL, prev.prev, current);
> >
> > I think this code would be more readable if you call that
> > temp_mm_state_t variable "temp_state" and the mm_struct pointer "mm" and
> > then you have:
> >
> >       switch_mm_irqs_off(NULL, temp_state.mm, current);
> >
> > And above you'll have:
> >
> >       temp_state.mm = ...
>
> Andy, please let me know whether you are fine with this change and I’ll
> incorporate it.


I'm okay with it.