[PATCH] libbpf: reject linker STT_SECTION symbols with out-of-range st_shndx

[Naveed Khan <naveed@digiscrypt.com>]

linker_sanity_check_elf_symtab() only validates a symbol's st_shndx
against the section count when it is below SHN_LORESERVE:

	if (sym->st_shndx < SHN_LORESERVE && sym->st_shndx >= obj->sec_cnt)
		return -EINVAL;

For a STT_SECTION symbol the following check only rejects a non-zero
st_value, so a section symbol carrying a reserved index such as
SHN_ABS (0xfff1) with st_value == 0 passes validation.

When a relocation references such a symbol, linker_append_elf_relos()
indexes the per-object section array with no bound check:

	if (ELF64_ST_TYPE(src_sym->st_info) == STT_SECTION) {
		struct src_sec *sec = &obj->secs[src_sym->st_shndx];
		...
			insn->imm += sec->dst_off;

obj->secs holds only obj->sec_cnt entries, so obj->secs[0xfff1]
dereferences memory tens of thousands of entries past the allocation
(reading sec->dst_off) when linking a crafted relocatable object,
an out-of-bounds read that faults or folds unrelated heap contents
into the patched instruction.

A STT_SECTION symbol must reference a real section, so reject one whose
st_shndx is not a valid section index. Reserved indices are always
>= SHN_LORESERVE, which exceeds sec_cnt, and the in-range case is
already handled above, so a single sec_cnt bound closes the gap without
affecting valid inputs.

Signed-off-by: Naveed Khan <naveed@digiscrypt.com>
---
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 78f92c3929..8a93faa41d 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -973,7 +973,7 @@ static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *s
 			return -EINVAL;
 		}
 		if (sym_type == STT_SECTION) {
-			if (sym->st_value != 0)
+			if (sym->st_value != 0 || sym->st_shndx >= obj->sec_cnt)
 				return -EINVAL;
 			continue;
 		}
-- 
2.52.0