Re: [PATCH 0/2] UefiCpuPkg: Add RSB stuffing before rsm instruction

[Laszlo Ersek <lersek@redhat.com>]

On 08/10/18 03:43, Hao Wu wrote:
> The series will add RSB stuffing logics to avoid RSB underflow on return
> from SMM (rsm instruction).
> 
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> 
> Hao Wu (2):
>   UefiCpuPkg/SmmCpuFeaturesLib: Add RSB stuffing before rsm instruction
>   UefiCpuPkg/PiSmmCpuDxeSmm: Add RSB stuffing before rsm instruction
> 
>  UefiCpuPkg/Library/SmmCpuFeaturesLib/Ia32/SmiEntry.nasm     | 20 +++++++++
>  UefiCpuPkg/Library/SmmCpuFeaturesLib/Ia32/SmiException.nasm | 44 ++++++++++++++++++--
>  UefiCpuPkg/Library/SmmCpuFeaturesLib/X64/SmiEntry.nasm      | 20 +++++++++
>  UefiCpuPkg/Library/SmmCpuFeaturesLib/X64/SmiException.nasm  | 42 ++++++++++++++++++-
>  UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmiEntry.nasm                | 20 +++++++++
>  UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmmInit.nasm                 | 21 ++++++++++
>  UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmiEntry.nasm                 | 20 +++++++++
>  UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmInit.nasm                  | 20 +++++++++
>  8 files changed, 202 insertions(+), 5 deletions(-)
> 

I haven't tested this patch set yet; first I'd like to make some comments:

(1) I think the commit messages are very lacking. Please explain
*precisely* why the Return Stack Buffer has to be stuffed before RSM.

(1a) To my understanding, speculation is micro-architectural (and not
architectural) state, therefore it makes no sense to say that "RSB is
left in a state that application program or operating-system does
not expect". Applications and operating systems can only have
expectations for architectural state, and not for micro-architectural state.

(1b) Furthermore, to my understanding, speculation can be abused by
training the predictor in a non-privileged context, then calling into a
higher privilege level, where the previous (unprivileged) training will
lead to the speculative execution of privileged code, for example
bypassing range checks. In turn, the result of those (invalid and
speculative) privileged operations can be sniffed from
micro-architectural state, such as timing memory accesses (to see
whether something has been cached or not by the speculative privileged
execution).

Is this correct more or less? If so, then why are we stuffing the RSB
just before we *leave* the privileged mode (=SMM) for the less
privileged mode (=ring 0, IIUC)? Shouldn't we kill the "external
training" of the predictor right after we *enter* SMM?

(1c) Or, perhaps, in this kind of attack, the RSB is not used for
triggering speculative execution in the more privileged mode, but to
*leak* information from the more privileged mode to the less privileged
mode. IOW, the RSB is what is used by the attacker as the "read end" of
the side-channel; perhaps by timing returns (in non-privileged code)
that reflect the training that the predictor picked up while in SMM.

Now, if that's the case, then the current commit messages are even more
confusing; they should state, "System Management Interrupt (SMI)
handlers can leave the Return Stack Buffer (RSB) in a state that leaks
information to malicious code that runs with lesser privileges".
Because, the point is not whether the OS or the app find the state
"unexpected" (a benign OS or app won't care at all); the point is that a
malicious OS or app will *definitely* expect some leaked information,
and we must prevent that.


I imagine that I'm pretty confused about this. Please document the exact
threat that the RSB stuffing is supposed to mitigate. I know I can find
long articles and blogs about this. The commit messages should
nonetheless provide a good concise summary.


(2) If I understand correctly, the same pattern is used everywhere -- a
loop body is executed 32 times, and in the loop body, we jump (via
subroutine calls) twice, and each call is followed by a "trap" for
speculative execution. At the end of the loop, we forcefully unwind the
stack, and then we proceed to RSM.

I think this should be implemented with a lot less code duplication.
NASM supports macros with labels that are local to macro *invocation*
(not macro *definition*); please see the %%skip example here:

  https://www.nasm.us/doc/nasmdoc4.html
  4.3.2 Macro-Local Labels

In addition, it should be possible to pass parameters to macros, such as:
- the register to use as counter (eax vs. rax),
- the stack pointer to restore (esp vs. rsp),
- the size of a stack frame (4 vs. 8)

Using all those tools, it should be possible to define the macro only
once, in a UefiCpuPkg-level ".inc" file (for example,
"UefiCpuPkg/Include/StuffRsb.inc"), and then only invoke the macro near
all 10 RSM instructions:

-------------
%define RSB_STUFF_ENTRIES 0x20

; @param 1: register to use as counter (eax vs. rax)
; @param 2: stack pointer to restore (esp vs. rsp)
; @param 3: the size of a stack frame (4 vs. 8)
%macro StuffRsb 3
      mov     %1, RSB_STUFF_ENTRIES / 2
  %%Unroll1:
      call    %%Unroll2
  %%SpecTrap1:
      pause
      lfence
      jmp     %%SpecTrap1
  %%Unroll2:
      call    %%StuffLoop
  %%SpecTrap2:
      pause
      lfence
      jmp     %%SpecTrap2
  %%StuffLoop:
      dec     %1
      jnz     %%Unroll1
      add     %2, RSB_STUFF_ENTRIES * %3 ; Restore the stack pointer
%endmacro

%define StuffRsb32 StuffRsb (eax, esp, 4)
%define StuffRsb64 StuffRsb (rax, rsp, 8)
-------------

Thanks,
Laszlo