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@xxxxxxxxx>.
Reported-by: James Hilliard <james.hilliard1@xxxxxxxxx>
Link: https://lore.kernel.org/bpf/CADvTj4o7ZWUikKwNTwFq0O_AaX+46t_+Ca9gvWMYdWdRtTGeHQ@xxxxxxxxxxxxxx/
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
tools/lib/bpf/linker.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index d7069780984a..5ced96d99f8c 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -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;
--
2.40.0
