Messages in this thread | | | Date | Thu, 9 Dec 2021 12:48:45 -0800 | From | Kees Cook <> | Subject | Re: randomize_kstack: To init or not to init? |
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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. :(
-- Kees Cook
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