Commit 51081a3f authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Alexei Starovoitov
Browse files

bpf: track changes_pkt_data property for global functions 



When processing calls to certain helpers, verifier invalidates all
packet pointers in a current state. For example, consider the
following program:

    __attribute__((__noinline__))
    long skb_pull_data(struct __sk_buff *sk, __u32 len)
    {
        return bpf_skb_pull_data(sk, len);
    }

    SEC("tc")
    int test_invalidate_checks(struct __sk_buff *sk)
    {
        int *p = (void *)(long)sk->data;
        if ((void *)(p + 1) > (void *)(long)sk->data_end) return TCX_DROP;
        skb_pull_data(sk, 0);
        *p = 42;
        return TCX_PASS;
    }

After a call to bpf_skb_pull_data() the pointer 'p' can't be used
safely. See function filter.c:bpf_helper_changes_pkt_data() for a list
of such helpers.

At the moment verifier invalidates packet pointers when processing
helper function calls, and does not traverse global sub-programs when
processing calls to global sub-programs. This means that calls to
helpers done from global sub-programs do not invalidate pointers in
the caller state. E.g. the program above is unsafe, but is not
rejected by verifier.

This commit fixes the omission by computing field
bpf_subprog_info->changes_pkt_data for each sub-program before main
verification pass.
changes_pkt_data should be set if:
- subprogram calls helper for which bpf_helper_changes_pkt_data
  returns true;
- subprogram calls a global function,
  for which bpf_subprog_info->changes_pkt_data should be set.

The verifier.c:check_cfg() pass is modified to compute this
information. The commit relies on depth first instruction traversal
done by check_cfg() and absence of recursive function calls:
- check_cfg() would eventually visit every call to subprogram S in a
  state when S is fully explored;
- when S is fully explored:
  - every direct helper call within S is explored
    (and thus changes_pkt_data is set if needed);
  - every call to subprogram S1 called by S was visited with S1 fully
    explored (and thus S inherits changes_pkt_data from S1).

The downside of such approach is that dead code elimination is not
taken into account: if a helper call inside global function is dead
because of current configuration, verifier would conservatively assume
that the call occurs for the purpose of the changes_pkt_data
computation.

Reported-by: default avatarNick Zavaritsky <mejedi@gmail.com>
Closes: https://lore.kernel.org/bpf/0498CA22-5779-4767-9C0C-A9515CEA711F@gmail.com/


Signed-off-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20241210041100.1898468-4-eddyz87@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent b238e187

include/linux/bpf_verifier.h

+1 −0
Original line number Diff line number Diff line
@@ -659,6 +659,7 @@ struct bpf_subprog_info { 
	bool args_cached: 1;

	/* true if bpf_fastcall stack region is used by functions that can't be inlined */

	bool keep_fastcall_stack: 1;

	bool changes_pkt_data: 1;



	enum priv_stack_mode priv_stack_mode;

	u8 arg_cnt;

kernel/bpf/verifier.c

+31 −1
Original line number Diff line number Diff line
@@ -10042,6 +10042,8 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn, 








		verbose(env, "Func#%d ('%s') is global and assumed valid.\n",









			subprog, sub_name);









		if (env->subprog_info[subprog].changes_pkt_data)









			clear_all_pkt_pointers(env);









		/* mark global subprog for verifying after main prog */









		subprog_aux(env, subprog)->called = true;









		clear_caller_saved_regs(env, caller->regs);








@@ -16246,6 +16248,29 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char







	return 0;









}

















static void mark_subprog_changes_pkt_data(struct bpf_verifier_env *env, int off)









{









	struct bpf_subprog_info *subprog;

















	subprog = find_containing_subprog(env, off);









	subprog->changes_pkt_data = true;









}

















/* 't' is an index of a call-site.









 * 'w' is a callee entry point.









 * Eventually this function would be called when env->cfg.insn_state[w] == EXPLORED.









 * Rely on DFS traversal order and absence of recursive calls to guarantee that









 * callee's change_pkt_data marks would be correct at that moment.









 */









static void merge_callee_effects(struct bpf_verifier_env *env, int t, int w)









{









	struct bpf_subprog_info *caller, *callee;

















	caller = find_containing_subprog(env, t);









	callee = find_containing_subprog(env, w);









	caller->changes_pkt_data |= callee->changes_pkt_data;









}

















/* non-recursive DFS pseudo code









 * 1  procedure DFS-iterative(G,v):









 * 2      label v as discovered








@@ -16379,6 +16404,7 @@ static int visit_func_call_insn(int t, struct bpf_insn *insns,







				bool visit_callee)









{









	int ret, insn_sz;









	int w;

















	insn_sz = bpf_is_ldimm64(&insns[t]) ? 2 : 1;









	ret = push_insn(t, t + insn_sz, FALLTHROUGH, env);








@@ -16390,8 +16416,10 @@ static int visit_func_call_insn(int t, struct bpf_insn *insns,







	mark_jmp_point(env, t + insn_sz);

















	if (visit_callee) {









		w = t + insns[t].imm + 1;









		mark_prune_point(env, t);









		ret = push_insn(t, t + insns[t].imm + 1, BRANCH, env);









		merge_callee_effects(env, t, w);









		ret = push_insn(t, w, BRANCH, env);









	}









	return ret;









}








@@ -16708,6 +16736,8 @@ static int visit_insn(int t, struct bpf_verifier_env *env)







			mark_prune_point(env, t);









			mark_jmp_point(env, t);









		}









		if (bpf_helper_call(insn) && bpf_helper_changes_pkt_data(insn->imm))









			mark_subprog_changes_pkt_data(env, t);









		if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {









			struct bpf_kfunc_call_arg_meta meta;