From efa90c11f62e6b7252fb75efe2787056872a627c Mon Sep 17 00:00:00 2001 From: Marco Elver Date: Mon, 31 Jan 2022 10:05:21 +0100 Subject: stack: Constrain and fix stack offset randomization with Clang builds 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 Reviewed-by: Nathan Chancellor Acked-by: Kees Cook Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220131090521.1947110-2-elver@google.com --- arch/Kconfig | 1 + 1 file changed, 1 insertion(+) (limited to 'arch/Kconfig') diff --git a/arch/Kconfig b/arch/Kconfig index 2cde48d9b77c..c5b50bfe31c1 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1163,6 +1163,7 @@ config RANDOMIZE_KSTACK_OFFSET bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT default y depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET + depends on INIT_STACK_NONE || !CC_IS_CLANG || CLANG_VERSION >= 140000 help The kernel stack offset can be randomized (after pt_regs) by roughly 5 bits of entropy, frustrating memory corruption -- cgit