bpf: fix uninitialized value usage

it was reported by clang with the option -fsanitize=memory:

Uninitialized bytes in MemcmpInterceptorCommon at offset 0 inside [0x7070000002a0, 56)
==3791089==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x482a2c in memcmp (fuzzer+0x482a2c)
    seccomp#1 0x7fed2f120ebb in _hsh_add src/libseccomp/src/gen_bpf.c:598:9
    seccomp#2 0x7fed2f121715 in _gen_bpf_action_hsh src/libseccomp/src/gen_bpf.c:796:6
    seccomp#3 0x7fed2f121a53 in _gen_bpf_node src/libseccomp/src/gen_bpf.c:831:11
    seccomp#4 0x7fed2f121a53 in _gen_bpf_chain.isra.0 src/libseccomp/src/gen_bpf.c:1072:13
    seccomp#5 0x7fed2f121f16 in _gen_bpf_chain_lvl_res src/libseccomp/src/gen_bpf.c:977:12
    seccomp#6 0x7fed2f121c74 in _gen_bpf_chain.isra.0 src/libseccomp/src/gen_bpf.c:1124:12
    seccomp#7 0x7fed2f12253c in _gen_bpf_syscall src/libseccomp/src/gen_bpf.c:1520:10
    seccomp#8 0x7fed2f12253c in _gen_bpf_syscalls src/libseccomp/src/gen_bpf.c:1615:18
    seccomp#9 0x7fed2f12253c in _gen_bpf_arch src/libseccomp/src/gen_bpf.c:1683:7
    seccomp#10 0x7fed2f12253c in _gen_bpf_build_bpf src/libseccomp/src/gen_bpf.c:2056:11
    seccomp#11 0x7fed2f12253c in gen_bpf_generate src/libseccomp/src/gen_bpf.c:2321:7
    seccomp#12 0x7fed2f11f41c in seccomp_export_bpf src/libseccomp/src/api.c:724:7

  Uninitialized value was created by a heap allocation
    #0 0x4547ef in realloc (fuzzer+0x4547ef)
    seccomp#1 0x7fed2f121244 in _blk_resize src/libseccomp/src/gen_bpf.c:362:8
    seccomp#2 0x7fed2f121244 in _blk_append src/libseccomp/src/gen_bpf.c:394:6

Signed-off-by: Giuseppe Scrivano <[email protected]>

src/gen_bpf.c

@@ -351,15 +351,18 @@ static struct bpf_blk *_blk_resize(struct bpf_state *state,
 {
  unsigned int size_adj = (AINC_BLK > size_add ? AINC_BLK : size_add);
  struct bpf_instr *new;
+ size_t old_size, new_size;
 
  if (blk == NULL)
  return NULL;
 
  if ((blk->blk_cnt + size_adj) <= blk->blk_alloc)
  return blk;
 
+ old_size = blk->blk_alloc * sizeof(*new);
  blk->blk_alloc += size_adj;
- new = realloc(blk->blks, blk->blk_alloc * sizeof(*(blk->blks)));
+ new_size = blk->blk_alloc * sizeof(*new);
+ new = zrealloc(blk->blks, old_size, new_size);
  if (new == NULL) {
  _blk_free(state, blk);
  return NULL;
@@ -443,10 +446,13 @@ static int _bpf_append_blk(struct bpf_program *prg, const struct bpf_blk *blk)
  bpf_instr_raw *i_iter;
  unsigned int old_cnt = prg->blk_cnt;
  unsigned int iter;
+ size_t old_size, new_size;
 
  /* (re)allocate the program memory */
+ old_size = BPF_PGM_SIZE(prg);
  prg->blk_cnt += blk->blk_cnt;
- i_new = realloc(prg->blks, BPF_PGM_SIZE(prg));
+ new_size = BPF_PGM_SIZE(prg);
+ i_new = zrealloc(prg->blks, old_size, new_size);
  if (i_new == NULL) {
  rc = -ENOMEM;
  goto bpf_append_blk_failure;

src/helper.c

@@ -47,3 +47,28 @@ void *zmalloc(size_t size)
 
  return ptr;
 }
+
+/**
+ * Change the size of an allocated buffer
+ * @param ptr pointer to the allocated buffer. If NULL it is equivalent to zmalloc.
+ * @param old_size the current size of the allocated buffer
+ * @param size the new size of the buffer
+ *
+ * This function changes the size of an allocated memory buffer and return a pointer
+ * to the buffer on success, the new buffer portion is initialized to zero. NULL is
+ * returned on failure. The returned buffer could be different than the specified
+ * ptr param.
+ *
+ */
+void *zrealloc(void *ptr, size_t old_size, size_t size)
+{
+ /* NOTE: unlike malloc() zero size allocations always return NULL */
+ if (size == 0)
+ return NULL;
+
+ ptr = realloc(ptr, size);
+ if (!ptr)
+ return NULL;
+ memset(ptr + old_size, 0, size - old_size);
+ return ptr;
+}

src/helper.h

@@ -23,5 +23,6 @@
 #define _FILTER_HELPER_H
 
 void *zmalloc(size_t size);
+void *zrealloc(void *ptr, size_t old_size, size_t size);
 
 #endif