Commit d08ab82f authored Mar 28, 2023 by Eduard Zingerman Committed by Andrii Nakryiko Mar 27, 2023

libbpf: Fix double-free when linker processes empty sections



Double-free error in bpf_linker__free() was reported by James Hilliard.
The error is caused by miss-use of realloc() in extend_sec().
The error occurs when two files with empty sections of the same name
are linked:
- when first file is processed:
  - extend_sec() calls realloc(dst->raw_data, dst_align_sz)
    with dst->raw_data == NULL and dst_align_sz == 0;
  - dst->raw_data is set to a special pointer to a memory block of
    size zero;
- when second file is processed:
  - extend_sec() calls realloc(dst->raw_data, dst_align_sz)
    with dst->raw_data == <special pointer> and dst_align_sz == 0;
  - realloc() "frees" dst->raw_data special pointer and returns NULL;
  - extend_sec() exits with -ENOMEM, and the old dst->raw_data value
    is preserved (it is now invalid);
  - eventually, bpf_linker__free() attempts to free dst->raw_data again.

This patch fixes the bug by avoiding -ENOMEM exit for dst_align_sz == 0.
The fix was suggested by Andrii Nakryiko <andrii.nakryiko@gmail.com>.

Reported-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: James Hilliard <james.hilliard1@gmail.com>
Link: https://lore.kernel.org/bpf/CADvTj4o7ZWUikKwNTwFq0O_AaX+46t_+Ca9gvWMYdWdRtTGeHQ@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20230328004738.381898-3-eddyz87@gmail.com

parent 7283137a

tools/lib/bpf/linker.c

+13 −1

Original line number	Diff line number	Diff line
		@@ -1115,7 +1115,19 @@ static int extend_sec(struct bpf_linker linker, struct dst_sec dst, struct src

		if (src->shdr->sh_type != SHT_NOBITS) {
		tmp = realloc(dst->raw_data, dst_final_sz);
		if (!tmp)
		/* If dst_align_sz == 0, realloc() behaves in a special way:
		* 1. When dst->raw_data is NULL it returns:
		* "either NULL or a pointer suitable to be passed to free()" [1].
		* 2. When dst->raw_data is not-NULL it frees dst->raw_data and returns NULL,
		* thus invalidating any "pointer suitable to be passed to free()" obtained
		* at step (1).
		*
		* The dst_align_sz > 0 check avoids error exit after (2), otherwise
		* dst->raw_data would be freed again in bpf_linker__free().
		*
		* [1] man 3 realloc
		*/
		if (!tmp && dst_align_sz > 0)
		return -ENOMEM;
		dst->raw_data = tmp;