Messages in this thread | | | Date | Mon, 31 Jan 2022 13:15:29 -0800 | From | Kees Cook <> | Subject | Re: [PATCH v2 2/2] stack: Constrain and fix stack offset randomization with Clang builds |
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On Mon, Jan 31, 2022 at 10:05:21AM +0100, Marco Elver wrote: > All supported versions of Clang perform auto-init of __builtin_alloca() > when stack auto-init is on (CONFIG_INIT_STACK_ALL_{ZERO,PATTERN}). > > add_random_kstack_offset() uses __builtin_alloca() to add a stack > offset. This means, when CONFIG_INIT_STACK_ALL_{ZERO,PATTERN} is > enabled, add_random_kstack_offset() will auto-init that unused portion > of the stack used to add an offset. > > There are several problems with this: > > 1. These offsets can be as large as 1023 bytes. Performing > memset() on them isn't exactly cheap, and this is done on > every syscall entry. > > 2. Architectures adding add_random_kstack_offset() to syscall > entry implemented in C require them to be 'noinstr' (e.g. see > x86 and s390). The potential problem here is that a call to > memset may occur, which is not noinstr. > > A x86_64 defconfig kernel with Clang 11 and CONFIG_VMLINUX_VALIDATION shows: > > | vmlinux.o: warning: objtool: do_syscall_64()+0x9d: call to memset() leaves .noinstr.text section > | vmlinux.o: warning: objtool: do_int80_syscall_32()+0xab: call to memset() leaves .noinstr.text section > | vmlinux.o: warning: objtool: __do_fast_syscall_32()+0xe2: call to memset() leaves .noinstr.text section > | vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section > > Clang 14 (unreleased) will introduce a way to skip alloca initialization > via __builtin_alloca_uninitialized() (https://reviews.llvm.org/D115440). > > Constrain RANDOMIZE_KSTACK_OFFSET to only be enabled if no stack > auto-init is enabled, the compiler is GCC, or Clang is version 14+. Use > __builtin_alloca_uninitialized() if the compiler provides it, as is done > by Clang 14. > > Link: https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com > Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") > Signed-off-by: Marco Elver <elver@google.com>
Thanks for the tweaks; this looks good to me now.
Acked-by: Kees Cook <keescook@chromium.org>
-- Kees Cook
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