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

[Nadav Amit <nadav.amit@gmail.com>]

> 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.