bpf: possibly avoid extra masking for narrower load in verifier

Commit 31fd85816dbe ("bpf: permits narrower load from bpf program
context fields") permits narrower load for certain ctx fields.
The commit however will already generate a masking even if
the prog-specific ctx conversion produces the result with
narrower size.

For example, for __sk_buff->protocol, the ctx conversion
loads the data into register with 2-byte load.
A narrower 2-byte load should not generate masking.
For __sk_buff->vlan_present, the conversion function
set the result as either 0 or 1, essentially a byte.
The narrower 2-byte or 1-byte load should not generate masking.

To avoid unnecessary masking, prog-specific *_is_valid_access
now passes converted_op_size back to verifier, which indicates
the valid data width after perceived future conversion.
Based on this information, verifier is able to avoid
unnecessary marking.

Since we want more information back from prog-specific
*_is_valid_access checking, all of them are packed into
one data structure for more clarity.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 8 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 44b97d958fb7..74ea96ea391b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -761,22 +761,34 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
 static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off, int size,
 			    enum bpf_access_type t, enum bpf_reg_type *reg_type)
 {
-	int ctx_field_size = 0;
+	struct bpf_insn_access_aux info = { .reg_type = *reg_type };
 
 	/* for analyzer ctx accesses are already validated and converted */
 	if (env->analyzer_ops)
 		return 0;
 
 	if (env->prog->aux->ops->is_valid_access &&
-	    env->prog->aux->ops->is_valid_access(off, size, t, reg_type, &ctx_field_size)) {
-		/* a non zero ctx_field_size indicates:
+	    env->prog->aux->ops->is_valid_access(off, size, t, &info)) {
+		/* a non zero info.ctx_field_size indicates:
 		 * . For this field, the prog type specific ctx conversion algorithm
 		 *   only supports whole field access.
 		 * . This ctx access is a candiate for later verifier transformation
 		 *   to load the whole field and then apply a mask to get correct result.
+		 * a non zero info.converted_op_size indicates perceived actual converted
+		 * value width in convert_ctx_access.
 		 */
-		if (ctx_field_size)
-			env->insn_aux_data[insn_idx].ctx_field_size = ctx_field_size;
+		if ((info.ctx_field_size && !info.converted_op_size) ||
+		    (!info.ctx_field_size &&  info.converted_op_size)) {
+			verbose("verifier bug in is_valid_access prog type=%u off=%d size=%d\n",
+				env->prog->type, off, size);
+			return -EACCES;
+		}
+
+		if (info.ctx_field_size) {
+			env->insn_aux_data[insn_idx].ctx_field_size = info.ctx_field_size;
+			env->insn_aux_data[insn_idx].converted_op_size = info.converted_op_size;
+		}
+		*reg_type = info.reg_type;
 
 		/* remember the offset of last byte accessed in ctx */
 		if (env->prog->aux->max_ctx_offset < off + size)
@@ -3388,7 +3400,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 	struct bpf_insn insn_buf[16], *insn;
 	struct bpf_prog *new_prog;
 	enum bpf_access_type type;
-	int i, cnt, off, size, ctx_field_size, is_narrower_load, delta = 0;
+	int i, cnt, off, size, ctx_field_size, converted_op_size, is_narrower_load, delta = 0;
 
 	if (ops->gen_prologue) {
 		cnt = ops->gen_prologue(insn_buf, env->seen_direct_write,
@@ -3431,7 +3443,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		off = insn->off;
 		size = bpf_size_to_bytes(BPF_SIZE(insn->code));
 		ctx_field_size = env->insn_aux_data[i + delta].ctx_field_size;
-		is_narrower_load = (type == BPF_READ && size < ctx_field_size);
+		converted_op_size = env->insn_aux_data[i + delta].converted_op_size;
+		is_narrower_load = type == BPF_READ && size < ctx_field_size;
 
 		/* If the read access is a narrower load of the field,
 		 * convert to a 4/8-byte load, to minimum program type specific
@@ -3453,7 +3466,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 			verbose("bpf verifier is misconfigured\n");
 			return -EINVAL;
 		}
-		if (is_narrower_load) {
+		if (is_narrower_load && size < converted_op_size) {
 			if (ctx_field_size <= 4)
 				insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
 							(1 << size * 8) - 1);