On Mon, Jun 22, 2020 at 06:56:15PM -0400, Arvind Sankar wrote:
> On Mon, Jun 22, 2020 at 12:31:44PM -0700, Kees Cook wrote:
> > +
> > +#define add_random_kstack_offset() do { \
> > + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
> > + &randomize_kstack_offset)) { \
> > + u32 offset = this_cpu_read(kstack_offset); \
> > + u8 *ptr = __builtin_alloca(offset & 0x3FF); \
> > + asm volatile("" : "=m"(*ptr)); \
> > + } \
> > +} while (0)
>
> This feels a little fragile. ptr doesn't escape the block, so the
> compiler is free to restore the stack immediately after this block. In
> fact, given that all you've said is that the asm modifies *ptr, but
> nothing uses that output, the compiler could eliminate the whole thing,
> no?
>
> https://godbolt.org/z/HT43F5
>
> gcc restores the stack immediately, if no function calls come after it.
>
> clang completely eliminates the code if no function calls come after.
nothing uses the stack in your example. And adding a barrier (which is
what the "=m" is, doesn't change it.
> I'm not sure why function calls should affect it, though, given that
> those functions can't possibly access ptr or the memory it points to.
It seems to work correctly:
https://godbolt.org/z/c3W5BW
--
Kees Cook
