From: Marco Elver elver@google.com
[ Upstream commit efa90c11f62e6b7252fb75efe2787056872a627c ]
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 Reviewed-by: Nathan Chancellor nathan@kernel.org Acked-by: Kees Cook keescook@chromium.org Signed-off-by: Kees Cook keescook@chromium.org Link: https://lore.kernel.org/r/20220131090521.1947110-2-elver@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/Kconfig | 1 + include/linux/randomize_kstack.h | 16 ++++++++++++++-- 2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig index 678a80713b21..5e88237f84d2 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1162,6 +1162,7 @@ config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET config RANDOMIZE_KSTACK_OFFSET_DEFAULT bool "Randomize kernel stack offset on syscall entry" 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 diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h index bebc911161b6..d373f1bcbf7c 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -16,8 +16,20 @@ DECLARE_PER_CPU(u32, kstack_offset); * alignment. Also, since this use is being explicitly masked to a max of * 10 bits, stack-clash style attacks are unlikely. For more details see * "VLAs" in Documentation/process/deprecated.rst + * + * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently + * only with Clang and not GCC). Initializing the unused area on each syscall + * entry is expensive, and generating an implicit call to memset() may also be + * problematic (such as in noinstr functions). Therefore, if the compiler + * supports it (which it should if it initializes allocas), always use the + * "uninitialized" variant of the builtin. */ -void *__builtin_alloca(size_t size); +#if __has_builtin(__builtin_alloca_uninitialized) +#define __kstack_alloca __builtin_alloca_uninitialized +#else +#define __kstack_alloca __builtin_alloca +#endif + /* * Use, at most, 10 bits of entropy. We explicitly cap this to keep the * "VLA" from being unbounded (see above). 10 bits leaves enough room for @@ -36,7 +48,7 @@ void *__builtin_alloca(size_t size); if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ &randomize_kstack_offset)) { \ u32 offset = raw_cpu_read(kstack_offset); \ - u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ + u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ /* Keep allocation even after "ptr" loses scope. */ \ asm volatile("" :: "r"(ptr) : "memory"); \ } \