libbpf: Fix using invalidated memory in bpf_linker

add_dst_sec() can invalidate bpf_linker's section index making
dst_symtab pointer pointing into unallocated memory. Reinitialize
dst_symtab pointer on each iteration to make sure it's always valid.

Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-7-andrii@kernel.org

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 3e1b2a15fdc74c90a38298e539f0ad9380c1391a..9aa016fb55aa6a345a4d92cab5b06a0cad184f1c 100644 (file)
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -2000,7 +2000,7 @@ add_sym:
 static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *obj)
 {
        struct src_sec *src_symtab = &obj->secs[obj->symtab_sec_idx];
-       struct dst_sec *dst_symtab = &linker->secs[linker->symtab_sec_idx];
+       struct dst_sec *dst_symtab;
        int i, err;
 
        for (i = 1; i < obj->sec_cnt; i++) {
@@ -2033,6 +2033,9 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
                        return -1;
                }
 
+               /* add_dst_sec() above could have invalidated linker->secs */
+               dst_symtab = &linker->secs[linker->symtab_sec_idx];
+
                /* shdr->sh_link points to SYMTAB */
                dst_sec->shdr->sh_link = linker->symtab_sec_idx;