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

[Dennis Zhou]

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.

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;
}