bpf: simplify narrower ctx access

This work tries to make the semantics and code around the
narrower ctx access a bit easier to follow. Right now
everything is done inside the .is_valid_access(). Offset
matching is done differently for read/write types, meaning
writes don't support narrower access and thus matching only
on offsetof(struct foo, bar) is enough whereas for read
case that supports narrower access we must check for
offsetof(struct foo, bar) + offsetof(struct foo, bar) +
sizeof(<bar>) - 1 for each of the cases. For read cases of
individual members that don't support narrower access (like
packet pointers or skb->cb[] case which has its own narrow
access logic), we check as usual only offsetof(struct foo,
bar) like in write case. Then, for the case where narrower
access is allowed, we also need to set the aux info for the
access. Meaning, ctx_field_size and converted_op_size have
to be set. First is the original field size e.g. sizeof(<bar>)
as in above example from the user facing ctx, and latter
one is the target size after actual rewrite happened, thus
for the kernel facing ctx. Also here we need the range match
and we need to keep track changing convert_ctx_access() and
converted_op_size from is_valid_access() as both are not at
the same location.

We can simplify the code a bit: check_ctx_access() becomes
simpler in that we only store ctx_field_size as a meta data
and later in convert_ctx_accesses() we fetch the target_size
right from the location where we do convert. Should the verifier
be misconfigured we do reject for BPF_WRITE cases or target_size
that are not provided. For the subsystems, we always work on
ranges in is_valid_access() and add small helpers for ranges
and narrow access, convert_ctx_accesses() sets target_size
for the relevant instruction.

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

1 files changed, 34 insertions, 44 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6ea2adcb233b..6f820a044079 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -546,20 +546,6 @@ static int check_reg_arg(struct bpf_reg_state *regs, u32 regno,
 	return 0;
 }
 
-static int bpf_size_to_bytes(int bpf_size)
-{
-	if (bpf_size == BPF_W)
-		return 4;
-	else if (bpf_size == BPF_H)
-		return 2;
-	else if (bpf_size == BPF_B)
-		return 1;
-	else if (bpf_size == BPF_DW)
-		return 8;
-	else
-		return -EINVAL;
-}
-
 static bool is_spillable_regtype(enum bpf_reg_type type)
 {
 	switch (type) {
@@ -761,7 +747,9 @@ 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)
 {
-	struct bpf_insn_access_aux info = { .reg_type = *reg_type };
+	struct bpf_insn_access_aux info = {
+		.reg_type = *reg_type,
+	};
 
 	/* for analyzer ctx accesses are already validated and converted */
 	if (env->analyzer_ops)
@@ -769,25 +757,14 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off,
 
 	if (env->prog->aux->ops->is_valid_access &&
 	    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.
+		/* A non zero info.ctx_field_size indicates that this field is a
+		 * candidate for later verifier transformation to load the whole
+		 * field and then apply a mask when accessed with a narrower
+		 * access than actual ctx access size. A zero info.ctx_field_size
+		 * will only allow for whole field access and rejects any other
+		 * type of narrower access.
 		 */
-		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;
-		}
+		env->insn_aux_data[insn_idx].ctx_field_size = info.ctx_field_size;
 		*reg_type = info.reg_type;
 
 		/* remember the offset of last byte accessed in ctx */
@@ -3401,11 +3378,13 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
 static int convert_ctx_accesses(struct bpf_verifier_env *env)
 {
 	const struct bpf_verifier_ops *ops = env->prog->aux->ops;
+	int i, cnt, size, ctx_field_size, delta = 0;
 	const int insn_cnt = env->prog->len;
 	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, converted_op_size, is_narrower_load, delta = 0;
+	bool is_narrower_load;
+	u32 target_size;
 
 	if (ops->gen_prologue) {
 		cnt = ops->gen_prologue(insn_buf, env->seen_direct_write,
@@ -3445,39 +3424,50 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		if (env->insn_aux_data[i + delta].ptr_type != PTR_TO_CTX)
 			continue;
 
-		off = insn->off;
-		size = bpf_size_to_bytes(BPF_SIZE(insn->code));
 		ctx_field_size = env->insn_aux_data[i + delta].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;
+		size = BPF_LDST_BYTES(insn);
 
 		/* If the read access is a narrower load of the field,
 		 * convert to a 4/8-byte load, to minimum program type specific
 		 * convert_ctx_access changes. If conversion is successful,
 		 * we will apply proper mask to the result.
 		 */
+		is_narrower_load = size < ctx_field_size;
 		if (is_narrower_load) {
-			int size_code = BPF_H;
+			u32 off = insn->off;
+			u8 size_code;
+
+			if (type == BPF_WRITE) {
+				verbose("bpf verifier narrow ctx access misconfigured\n");
+				return -EINVAL;
+			}
 
+			size_code = BPF_H;
 			if (ctx_field_size == 4)
 				size_code = BPF_W;
 			else if (ctx_field_size == 8)
 				size_code = BPF_DW;
+
 			insn->off = off & ~(ctx_field_size - 1);
 			insn->code = BPF_LDX | BPF_MEM | size_code;
 		}
-		cnt = ops->convert_ctx_access(type, insn, insn_buf, env->prog);
-		if (cnt == 0 || cnt >= ARRAY_SIZE(insn_buf)) {
+
+		target_size = 0;
+		cnt = ops->convert_ctx_access(type, insn, insn_buf, env->prog,
+					      &target_size);
+		if (cnt == 0 || cnt >= ARRAY_SIZE(insn_buf) ||
+		    (ctx_field_size && !target_size)) {
 			verbose("bpf verifier is misconfigured\n");
 			return -EINVAL;
 		}
-		if (is_narrower_load && size < converted_op_size) {
+
+		if (is_narrower_load && size < target_size) {
 			if (ctx_field_size <= 4)
 				insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
-							(1 << size * 8) - 1);
+								(1 << size * 8) - 1);
 			else
 				insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
-							(1 << size * 8) - 1);
+								(1 << size * 8) - 1);
 		}
 
 		new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);