diff options
| -rw-r--r-- | arch/Kconfig | 1 | ||||
| -rw-r--r-- | include/linux/randomize_kstack.h | 16 |
2 files changed, 15 insertions, 2 deletions
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 diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h index 91f1b990a3c3..1468caf001c0 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -17,8 +17,20 @@ 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 @@ -37,7 +49,7 @@ 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"); \ } \ |