On Mon, Jun 22, 2020 at 9:31 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> This provides the ability for architectures to enable kernel stack base
> address offset randomization. This feature is controlled by the boot
> param "randomize_kstack_offset=on/off", with its default value set by
> CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT.
[...]
> +#define add_random_kstack_offset() do { \
> + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
> + &randomize_kstack_offset)) { \
> + u32 offset = this_cpu_read(kstack_offset); \
> + u8 *ptr = __builtin_alloca(offset & 0x3FF); \
> + asm volatile("" : "=m"(*ptr)); \
> + } \
> +} while (0)
clang generates better code here if the mask is stack-aligned -
otherwise it needs to round the stack pointer / the offset:
$ cat alloca_align.c
#include <alloca.h>
void callee(void);
void alloca_blah(unsigned long rand) {
asm volatile(""::"r"(alloca(rand & MASK)));
callee();
}
$ clang -O3 -c -o alloca_align.o alloca_align.c -DMASK=0x3ff
$ objdump -d alloca_align.o
[...]
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 81 e7 ff 03 00 00 and $0x3ff,%edi
a: 83 c7 0f add $0xf,%edi
d: 83 e7 f0 and $0xfffffff0,%edi
10: 48 89 e0 mov %rsp,%rax
13: 48 29 f8 sub %rdi,%rax
16: 48 89 c4 mov %rax,%rsp
19: e8 00 00 00 00 callq 1e <alloca_blah+0x1e>
1e: 48 89 ec mov %rbp,%rsp
21: 5d pop %rbp
22: c3 retq
$ clang -O3 -c -o alloca_align.o alloca_align.c -DMASK=0x3f0
$ objdump -d alloca_align.o
[...]
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 89 e0 mov %rsp,%rax
7: 81 e7 f0 03 00 00 and $0x3f0,%edi
d: 48 29 f8 sub %rdi,%rax
10: 48 89 c4 mov %rax,%rsp
13: e8 00 00 00 00 callq 18 <alloca_blah+0x18>
18: 48 89 ec mov %rbp,%rsp
1b: 5d pop %rbp
1c: c3 retq
$
(From a glance at the assembly, gcc seems to always assume that the
length may be misaligned.)
Maybe this should be something along the lines of
__builtin_alloca(offset & (0x3ff & ARCH_STACK_ALIGN_MASK)) (with
appropriate definitions of the stack alignment mask depending on the
architecture's choice of stack alignment for kernel code).
