Messages in this thread | | | Date | Mon, 22 Jun 2020 16:07:11 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall |
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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
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