[PATCH bpf-next v7 02/24] bpf/verifier: allow kfunc to read user provided context

[Benjamin Tissoires]

When a kfunc was trying to access data from context in a syscall eBPF
program, the verifier was rejecting the call.
This is because the syscall context is not known at compile time, and
so we need to check this when actually accessing it.

Check for the valid memory access and allow such situation to happen.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>

---

changes in v7:
- renamed access_t into atype
- allow zero-byte read
- check_mem_access() to the correct offset/size

new in v6
---
 kernel/bpf/verifier.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7c1e056624f9..d5fe7e618c52 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -248,6 +248,7 @@ struct bpf_call_arg_meta {
 	struct bpf_map *map_ptr;
 	bool raw_mode;
 	bool pkt_access;
+	bool is_kfunc;
 	u8 release_regno;
 	int regno;
 	int access_size;
@@ -5170,6 +5171,7 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 				   struct bpf_call_arg_meta *meta)
 {
 	struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
+	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	u32 *max_access;
 
 	switch (base_type(reg->type)) {
@@ -5223,6 +5225,24 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
 				env,
 				regno, reg->off, access_size,
 				zero_size_allowed, ACCESS_HELPER, meta);
+	case PTR_TO_CTX:
+		/* in case of a kfunc called in a program of type SYSCALL, the context is
+		 * user supplied, so not computed statically.
+		 * Dynamically check it now
+		 */
+		if (prog_type == BPF_PROG_TYPE_SYSCALL && meta && meta->is_kfunc) {
+			enum bpf_access_type atype = meta->raw_mode ? BPF_WRITE : BPF_READ;
+			int offset = access_size - 1;
+
+			/* Allow zero-byte read from NULL or PTR_TO_CTX */
+			if (access_size == 0)
+				return zero_size_allowed ? 0 : -EINVAL;
+
+			return check_mem_access(env, env->insn_idx, regno, offset, BPF_B,
+						atype, -1, false);
+		}
+
+		fallthrough;
 	default: /* scalar_value or invalid ptr */
 		/* Allow zero-byte read from NULL, regardless of pointer type */
 		if (zero_size_allowed && access_size == 0 &&
@@ -5335,6 +5355,7 @@ int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state
 	WARN_ON_ONCE(regno < BPF_REG_2 || regno > BPF_REG_5);
 
 	memset(&meta, 0, sizeof(meta));
+	meta.is_kfunc = true;
 
 	if (may_be_null) {
 		saved_reg = *mem_reg;
-- 
2.36.1