Messages in this thread | | | Date | Wed, 5 May 2021 14:12:32 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] stack: replace "o" output with "r" input constraint |
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On Thu, Apr 22, 2021 at 06:02:27PM -0700, Nathan Chancellor wrote: > On Mon, Apr 19, 2021 at 04:17:41PM -0700, Kees Cook wrote: > > From: Nick Desaulniers <ndesaulniers@google.com> > > > > "o" isn't a common asm() constraint to use; it triggers an assertion in > > assert-enabled builds of LLVM that it's not recognized when targeting > > aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM > > 13 now, but there isn't really a good reason to be using "o" in particular > > here. To avoid causing build issues for those using assert-enabled builds > > of earlier LLVM versions, the constraint needs changing. > > > > Instead, if the point is to retain the __builtin_alloca(), we can make ptr > > appear to "escape" via being an input to an empty inline asm block. This > > is preferable anyways, since otherwise this looks like a dead store. > > > > While the use of "r" was considered in > > https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/ > > it was only tested as an output (which looks like a dead store, and > > wasn't sufficient). Use "r" as an input constraint instead, which > > behaves correctly across compilers and architectures: > > https://godbolt.org/z/E9cd411ob > > > > Link: https://reviews.llvm.org/D100412 > > Link: https://bugs.llvm.org/show_bug.cgi?id=49956 > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> > > Tested-by: Kees Cook <keescook@chromium.org> > > Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Kees, were you planning on taking this to Linus or someone else? It would be nice to have this in for -rc1 (although I understand it might be too late), if not, by -rc2.
Cheers, Nathan
> > Signed-off-by: Kees Cook <keescook@chromium.org> > > I built arm64 defconfig with and without > CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT with LLVM 12 (which does not have > Nick's LLVM fix) without any issues and did a quick boot test in QEMU, > nothing exploded. > > Reviewed-by: Nathan Chancellor <nathan@kernel.org> > Tested-by: Nathan Chancellor <nathan@kernel.org> > > > --- > > include/linux/randomize_kstack.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h > > index fd80fab663a9..bebc911161b6 100644 > > --- a/include/linux/randomize_kstack.h > > +++ b/include/linux/randomize_kstack.h > > @@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size); > > u32 offset = raw_cpu_read(kstack_offset); \ > > u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ > > /* Keep allocation even after "ptr" loses scope. */ \ > > - asm volatile("" : "=o"(*ptr) :: "memory"); \ > > + asm volatile("" :: "r"(ptr) : "memory"); \ > > } \ > > } while (0) > > > > -- > > 2.25.1 > >
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