bpf: Stop caching subprog index in the bpf_pseudo_func insn

This patch is to fix an out-of-bound access issue when jit-ing the
bpf_pseudo_func insn (i.e. ld_imm64 with src_reg == BPF_PSEUDO_FUNC)

In jit_subprog(), it currently reuses the subprog index cached in
insn[1].imm. This subprog index is an index into a few array related
to subprogs. For example, in jit_subprog(), it is an index to the newly
allocated 'struct bpf_prog **func' array.

The subprog index was cached in insn[1].imm after add_subprog(). However,
this could become outdated (and too big in this case) if some subprogs
are completely removed during dead code elimination (in
adjust_subprog_starts_after_remove). The cached index in insn[1].imm
is not updated accordingly and causing out-of-bound issue in the later
jit_subprog().

Unlike bpf_pseudo_'func' insn, the current bpf_pseudo_'call' insn
is handling the DCE properly by calling find_subprog(insn->imm) to
figure out the index instead of caching the subprog index.
The existing bpf_adj_branches() will adjust the insn->imm
whenever insn is added or removed.

Instead of having two ways handling subprog index,
this patch is to make bpf_pseudo_func works more like
bpf_pseudo_call.

First change is to stop caching the subprog index result
in insn[1].imm after add_subprog(). The verification
process will use find_subprog(insn->imm) to figure
out the subprog index.

Second change is in bpf_adj_branches() and have it to
adjust the insn->imm for the bpf_pseudo_func insn also
whenever insn is added or removed.

Third change is in jit_subprog(). Like the bpf_pseudo_call handling,
bpf_pseudo_func temporarily stores the find_subprog() result
in insn->off. It is fine because the prog's insn has been finalized
at this point. insn->off will be reset back to 0 later to avoid
confusing the userspace prog dump tool.

Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Signed-off-by: Martin KaFai Lau <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/bpf.h

···
 	aux->ctx_field_size = size;
 }
 
+static inline bool bpf_pseudo_func(const struct bpf_insn *insn)
+{
+	return insn->code == (BPF_LD | BPF_IMM | BPF_DW) &&
+	       insn->src_reg == BPF_PSEUDO_FUNC;
+}
+
 struct bpf_prog_ops {
 	int (*test_run)(struct bpf_prog *prog, const union bpf_attr *kattr,
 			union bpf_attr __user *uattr);

kernel/bpf/core.c

···
 			i = end_new;
 			insn = prog->insnsi + end_old;
 		}
+		if (bpf_pseudo_func(insn)) {
+			ret = bpf_adj_delta_to_imm(insn, pos, end_old,
+						   end_new, i, probe_pass);
+			if (ret)
+				return ret;
+			continue;
+		}
 		code = insn->code;
 		if ((BPF_CLASS(code) != BPF_JMP &&
 		     BPF_CLASS(code) != BPF_JMP32) ||

kernel/bpf/verifier.c

···
 	       insn->src_reg == BPF_PSEUDO_KFUNC_CALL;
 }
 
-static bool bpf_pseudo_func(const struct bpf_insn *insn)
-{
-	return insn->code == (BPF_LD | BPF_IMM | BPF_DW) &&
-	       insn->src_reg == BPF_PSEUDO_FUNC;
-}
-
 struct bpf_call_arg_meta {
 	struct bpf_map *map_ptr;
 	bool raw_mode;
···
 			return -EPERM;
 		}
 
-		if (bpf_pseudo_func(insn)) {
+		if (bpf_pseudo_func(insn) || bpf_pseudo_call(insn))
 			ret = add_subprog(env, i + insn->imm + 1);
-			if (ret >= 0)
-				/* remember subprog */
-				insn[1].imm = ret;
-		} else if (bpf_pseudo_call(insn)) {
-			ret = add_subprog(env, i + insn->imm + 1);
-		} else {
+		else
 			ret = add_kfunc_call(env, insn->imm, insn->off);
-		}
 
 		if (ret < 0)
 			return ret;
···
 
 	if (insn->src_reg == BPF_PSEUDO_FUNC) {
 		struct bpf_prog_aux *aux = env->prog->aux;
-		u32 subprogno = insn[1].imm;
+		u32 subprogno = find_subprog(env,
+					     env->insn_idx + insn->imm + 1);
 
 		if (!aux->func_info) {
 			verbose(env, "missing btf func_info\n");
···
 		return 0;
 
 	for (i = 0, insn = prog->insnsi; i < prog->len; i++, insn++) {
-		if (bpf_pseudo_func(insn)) {
-			env->insn_aux_data[i].call_imm = insn->imm;
-			/* subprog is encoded in insn[1].imm */
+		if (!bpf_pseudo_func(insn) && !bpf_pseudo_call(insn))
 			continue;
-		}
 
-		if (!bpf_pseudo_call(insn))
-			continue;
 		/* Upon error here we cannot fall back to interpreter but
 		 * need a hard reject of the program. Thus -EFAULT is
 		 * propagated in any case.
···
 		env->insn_aux_data[i].call_imm = insn->imm;
 		/* point imm to __bpf_call_base+1 from JITs point of view */
 		insn->imm = 1;
+		if (bpf_pseudo_func(insn))
+			/* jit (e.g. x86_64) may emit fewer instructions
+			 * if it learns a u32 imm is the same as a u64 imm.
+			 * Force a non zero here.
+			 */
+			insn[1].imm = 1;
 	}
 
 	err = bpf_prog_alloc_jited_linfo(prog);
···
 		insn = func[i]->insnsi;
 		for (j = 0; j < func[i]->len; j++, insn++) {
 			if (bpf_pseudo_func(insn)) {
-				subprog = insn[1].imm;
+				subprog = insn->off;
 				insn[0].imm = (u32)(long)func[subprog]->bpf_func;
 				insn[1].imm = ((u64)(long)func[subprog]->bpf_func) >> 32;
 				continue;
···
 	for (i = 0, insn = prog->insnsi; i < prog->len; i++, insn++) {
 		if (bpf_pseudo_func(insn)) {
 			insn[0].imm = env->insn_aux_data[i].call_imm;
-			insn[1].imm = find_subprog(env, i + insn[0].imm + 1);
+			insn[1].imm = insn->off;
+			insn->off = 0;
 			continue;
 		}
 		if (!bpf_pseudo_call(insn))