Messages in this thread | | | From | Marco Elver <> | Date | Thu, 9 Dec 2021 21:54:30 +0100 | Subject | Re: randomize_kstack: To init or not to init? |
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On Thu, 9 Dec 2021 at 21:48, Kees Cook <keescook@chromium.org> wrote: > > On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote: > > Clang supports CONFIG_INIT_STACK_ALL_ZERO, which appears to be the > > default since dcb7c0b9461c2, which is why this came on my radar. And > > Clang also performs auto-init of allocas when auto-init is on > > (https://reviews.llvm.org/D60548), with no way to skip. As far as I'm > > aware, GCC 12's upcoming -ftrivial-auto-var-init= doesn't yet auto-init > > allocas. > > > > 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 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 > > > > Switching to INIT_STACK_ALL_NONE resolves the warnings as expected. > > > > To figure out what the right solution is, the first thing to figure out > > is, do we actually want that offset portion of the stack to be > > auto-init'd? > > > > There are several options: > > > > A. Make memset (and probably all other mem-transfer functions) > > noinstr compatible, if that is even possible. This only solves > > problem #2. > > I'd agree: "A" isn't going to work well here. > > > > > B. A workaround could be using a VLA with > > __attribute__((uninitialized)), but requires some restructuring > > to make sure the VLA remains in scope and other trickery to > > convince the compiler to not give up that stack space. > > I was hoping the existing trickery would work for a VLA, but it seems > not. It'd be nice if it could work with a VLA, which could just gain the > attribute and we'd be done. > > > C. Introduce a new __builtin_alloca_uninitialized(). > > Hrm, this means conditional logic between compilers, too. :(
And as Segher just pointed out, I think Clang has a "bug" because explicit alloca() calls aren't "automatic storage". I think Clang needs a new -mllvm param.
Because I think making #B work is quite ugly and also brittle. :-/
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