Re: [pcpu] BUG: KASAN: use-after-scope in pcpu_setup_first_chunk+0x1e3b/0x29e2

[Dmitry Vyukov]

On Wed, Nov 29, 2017 at 6:54 PM, Dennis Zhou <dennisszhou@xxxxxxxxx> wrote:
> Hi everyone,
>
> I spent a bit of time learning more about this problem as Fengguang was
> able to determine the root commit f7dd2507893cc3. I reproduced the bug
> in userspace to make life a bit easier and below the assignment occurs
> before the unpoison. This is fine if we're sequentially proceeding, but
> as in the case in percpu, it's calling the function in a for loop
> causing the assignment to happen after it has been poisoned in the prior
> iteration.
>
> <bb 3> [0.00%]:
>   _1 = (long unsigned int) i_4;
>   _2 = _1 * 16;
>   _3 = p_8 + _2;
>   list_14 = _3;
>   __u = {};
>   ASAN_MARK (UNPOISON, &__u, 8);
>   __u.__val = list_14;
>
> <bb 9> [0.00%]:
>   _24 = __u.__val;
>   ASAN_MARK (POISON, &__u, 8);
>   list_14->prev = list_14;
>   i_13 = i_4 + 1;
>
> <bb 10> [0.00%]:
>   # i_4 = PHI <i_9(2), i_13(9)>
>   if (i_4 <= 9)
>     goto <bb 3>; [0.00%]
>   else
>     goto <bb 11>; [0.00%]
>
> I don't know how to go about fixing this though. The reproducing code is
> below and was compiled with gcc-7 and the structleak_plugin.


Are we sure that structleak plugin is not at fault? If yes, then we
need to report this to https://gcc.gnu.org/bugzilla/ with instructions
on how to build/use the plugin.


> I hope this helps.
>
> Thanks,
> Dennis
>
> ----
> #include <stdint.h>
> #include <stdlib.h>
>
> #define barrier()
>
> #define WRITE_ONCE(x, val) \
> ({                                                      \
>         union { typeof(x) __val; char __c[1]; } __u =   \
>                 { .__val = (typeof(x)) (val) }; \
>         __write_once_size(&(x), __u.__c, sizeof(x));    \
>         __u.__val;                                      \
> })
>
> typedef         uint8_t         __u8;
> typedef         uint16_t        __u16;
> typedef         uint32_t        __u32;
> typedef         uint64_t        __u64;
>
> static inline __attribute__((always_inline)) void __write_once_size(volatile void *p, void *res, int size)
> {
>         switch (size) {
>         case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
>         case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
>         case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
>         case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
>         default:
>                 barrier();
>                 __builtin_memcpy((void *)p, (const void *)res, size);
>                 barrier();
>         }
> }
>
> struct list_head {
>         struct list_head *next, *prev;
> };
>
> static inline __attribute__((always_inline)) void INIT_LIST_HEAD(struct list_head *list)
> {
>         WRITE_ONCE(list->next, list);
>         list->prev = list;
> }
>
> int main(int argc, char *argv[])
> {
>         struct list_head *p = malloc(10 * sizeof(struct list_head));
>         int i;
>
>         for (i = 0; i < 10; i++) {
>                 INIT_LIST_HEAD(&p[i]);
>         }
>
>         free(p);
>
>         return 0;
> }