Re: [RFC PATCH 01/13] list: introduce speculative safe list_for_each_entry()

[Jakob]

> On 18. Feb 2022, at 17:29, Jann Horn <jannh@xxxxxxxxxx> wrote:
> 
> On Thu, Feb 17, 2022 at 7:48 PM Jakob Koschel <jakobkoschel@xxxxxxxxx> wrote:
>> list_for_each_entry() selects either the correct value (pos) or a safe
>> value for the additional mispredicted iteration (NULL) for the list
>> iterator.
>> list_for_each_entry() calls select_nospec(), which performs
>> a branch-less select.
>> 
>> On x86, this select is performed via a cmov. Otherwise, it's performed
>> via various shift/mask/etc. operations.
>> 
>> Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
>> Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
>> Amsterdam.
>> 
>> Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@xxxxxxxxx>
>> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@xxxxxxxxx>
>> Signed-off-by: Jakob Koschel <jakobkoschel@xxxxxxxxx>
> 
> Yeah, I think this is the best way to do this without deeply intrusive
> changes to how lists are represented in memory.
> 
> Some notes on the specific implementation:
> 
>> arch/x86/include/asm/barrier.h | 12 ++++++++++++
>> include/linux/list.h           |  3 ++-
>> include/linux/nospec.h         | 16 ++++++++++++++++
>> 3 files changed, 30 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
>> index 35389b2af88e..722797ad74e2 100644
>> --- a/arch/x86/include/asm/barrier.h
>> +++ b/arch/x86/include/asm/barrier.h
>> @@ -48,6 +48,18 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
>> /* Override the default implementation from linux/nospec.h. */
>> #define array_index_mask_nospec array_index_mask_nospec
>> 
>> +/* Override the default implementation from linux/nospec.h. */
>> +#define select_nospec(cond, exptrue, expfalse)                         \
>> +({                                                                     \
>> +       typeof(exptrue) _out = (exptrue);                               \
>> +                                                                       \
>> +       asm volatile("test %1, %1\n\t"                                  \
> 
> This shouldn't need "volatile", because it is only necessary if _out
> is actually used. Using "volatile" here could prevent optimizing out
> useless code. OPTIMIZER_HIDE_VAR() also doesn't use "volatile".
> 
>> +           "cmove %2, %0"                                              \
>> +           : "+r" (_out)                                               \
>> +           : "r" (cond), "r" (expfalse));                              \
>> +       _out;                                                           \
>> +})
> 
> I guess the idea is probably to also add code like this for other
> important architectures, in particular arm64?

yes indeed, with a fallback of using the shifting/masking mechanism for
other archs.

> 
> 
> It might also be a good idea to rename the arch-overridable macro to
> something like "arch_select_nospec" and then have a wrapper macro in
> include/linux/nospec.h that takes care of type safety issues.
> 
> The current definition of the macro doesn't warn if you pass in
> incompatible pointer types, like this:
> 
> int *bogus_pointer_mix(int cond, int *a, long *b) {
>  return select_nospec(cond, a, b);
> }
> 
> and if you pass in integers of different sizes, it may silently
> truncate to the size of the smaller one - this C code:
> 
> long wrong_int_conversion(int cond, int a, long b) {
>  return select_nospec(cond, a, b);
> }
> 
> generates this assembly:
> 
> wrong_int_conversion:
>  test %edi, %edi
>  cmove %rdx, %esi
>  movslq %esi, %rax
>  ret
> 
> It might be a good idea to add something like a
> static_assert(__same_type(...), ...) to protect against that.

These are good points, thank you for your input. Will be good to incorporate.
> 
>> /* Prevent speculative execution past this barrier. */
>> #define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
>> 
>> diff --git a/include/linux/list.h b/include/linux/list.h
>> index dd6c2041d09c..1a1b39fdd122 100644
>> --- a/include/linux/list.h
>> +++ b/include/linux/list.h
>> @@ -636,7 +636,8 @@ static inline void list_splice_tail_init(struct list_head *list,
>>  */
>> #define list_for_each_entry(pos, head, member)                         \
>>        for (pos = list_first_entry(head, typeof(*pos), member);        \
>> -            !list_entry_is_head(pos, head, member);                    \
>> +           ({ bool _cond = !list_entry_is_head(pos, head, member);     \
>> +            pos = select_nospec(_cond, pos, NULL); _cond; }); \
>>             pos = list_next_entry(pos, member))
> 
> I wonder if it'd look nicer to write it roughly like this:
> 
> #define NOSPEC_TYPE_CHECK(_guarded_var, _cond)                  \
> ({                                                              \
>  bool __cond = (_cond);                                        \
>  typeof(_guarded_var) *__guarded_var = &(_guarded_var);        \
>  *__guarded_var = select_nospec(__cond, *__guarded_var, NULL); \
>  __cond;                                                       \
> })
> 
> #define list_for_each_entry(pos, head, member)                                \
>        for (pos = list_first_entry(head, typeof(*pos), member);              \
>             NOSPEC_TYPE_CHECK(head, !list_entry_is_head(pos, head, member)); \
>             pos = list_next_entry(pos, member))
> 
> I think having a NOSPEC_TYPE_CHECK() like this makes it semantically
> clearer, and easier to add in other places? But I don't know if the
> others agree...

That sounds like a good idea. I wonder if the pointer and dereference in 
NOSPEC_TYPE_CHECK() simply get optimized away. Or why you can't simply
use _guarded_var directly instead of a pointer to it.

> 
>> /**
>> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
>> index c1e79f72cd89..ca8ed81e4f9e 100644
>> --- a/include/linux/nospec.h
>> +++ b/include/linux/nospec.h
>> @@ -67,4 +67,20 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
>> /* Speculation control for seccomp enforced mitigation */
>> void arch_seccomp_spec_mitigate(struct task_struct *task);
>> 
>> +/**
>> + * select_nospec - select a value without using a branch; equivalent to:
>> + * cond ? exptrue : expfalse;
>> + */
>> +#ifndef select_nospec
>> +#define select_nospec(cond, exptrue, expfalse)                         \
>> +({                                                                     \
>> +       unsigned long _t = (unsigned long) (exptrue);                   \
>> +       unsigned long _f = (unsigned long) (expfalse);                  \
>> +       unsigned long _c = (unsigned long) (cond);                      \
>> +       OPTIMIZER_HIDE_VAR(_c);                                         \
>> +       unsigned long _m = -((_c | -_c) >> (BITS_PER_LONG - 1));        \
>> +       (typeof(exptrue)) ((_t & _m) | (_f & ~_m));                     \
>> +})
>> +#endif
> 
> (As a sidenote, it might be easier to implement a conditional zeroing
> primitive than a generic conditional select primitive if that's all
> you need, something like:
> 
> #define cond_nullptr_nospec(_cond, _exp)          \
> ({                                             \
>  unsigned long __exp = (unsigned long)(_exp); \
>  unsigned long _mask = 0UL - !(_cond);       \
>  OPTIMIZER_HIDE_VAR(_mask);                   \
>  (typeof(_exp)) (_mask & __exp);              \
> })
> 
> )

Ah yes, if NULL is actually the value to choose, this might be good enough.