Re: [kernel-hardening] [PATCH v4 02/10] asm/nospec, array_ptr: sanitize speculative array de-references

[Adam Sampson <ats@offog.org>]

Jann Horn <jannh@google.com> writes:

>> +/*
>> + * If idx is negative or if idx > size then bit 63 is set in the mask,
>> + * and the value of ~(-1L) is zero. When the mask is zero, bounds check
>> + * failed, array_ptr will return NULL.
>> + */
>> +#ifndef array_ptr_mask
>> +static inline unsigned long array_ptr_mask(unsigned long idx,
>> unsigned long sz)
>> +{
>> +       return ~(long)(idx | (sz - 1 - idx)) >> (BITS_PER_LONG - 1);
>> +}
>> +#endif
>
> Nit: Maybe add a comment saying that this is equivalent to
> "return ((long)idx >= 0 && idx < sz) ? ULONG_MAX : 0"?

That's only true when sz < LONG_MAX, which is documented below but not
here; it's also different from the asm version, which doesn't do the idx
<= LONG_MAX check. So making the constraint explicit would be a good idea.

From a bit of experimentation, when the top bit of sz is set, this
expression, the C version and the assembler version all have different
behaviour. For example, with 32-bit unsigned long:

index=00000000 size=80000001: expr=ffffffff c=00000000 asm=ffffffff
index=80000000 size=80000001: expr=00000000 c=00000000 asm=ffffffff
index=00000000 size=a0000000: expr=ffffffff c=00000000 asm=ffffffff
index=00000001 size=a0000000: expr=ffffffff c=00000000 asm=ffffffff
index=fffffffe size=ffffffff: expr=00000000 c=00000000 asm=ffffffff

It may be worth noting that:

     return 0 - ((long) (idx < sz));

causes GCC, on ia32 and amd64, to generate exactly the same cmp/sbb
sequence as in Linus's asm. Are there architectures where this form
would allow speculation?

Thanks,

-- 
Adam Sampson <ats@offog.org>                         <http://offog.org/>