bpf: permits narrower load from bpf program context fields

Currently, verifier will reject a program if it contains an
narrower load from the bpf context structure. For example,
        __u8 h = __sk_buff->hash, or
        __u16 p = __sk_buff->protocol
        __u32 sample_period = bpf_perf_event_data->sample_period
which are narrower loads of 4-byte or 8-byte field.

This patch solves the issue by:
  . Introduce a new parameter ctx_field_size to carry the
    field size of narrower load from prog type
    specific *__is_valid_access validator back to verifier.
  . The non-zero ctx_field_size for a memory access indicates
    (1). underlying prog type specific convert_ctx_accesses
         supporting non-whole-field access
    (2). the current insn is a narrower or whole field access.
  . In verifier, for such loads where load memory size is
    less than ctx_field_size, verifier transforms it
    to a full field load followed by proper masking.
  . Currently, __sk_buff and bpf_perf_event_data->sample_period
    are supporting narrowing loads.
  . Narrower stores are still not allowed as typical ctx stores
    are just normal stores.

Because of this change, some tests in verifier will fail and
these tests are removed. As a bonus, rename some out of bound
__sk_buff->cb access to proper field name and remove two
redundant "skb cb oob" tests.

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, 15 insertions, 6 deletions

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 051d7fc..9d3ec82 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -479,7 +479,7 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
 
 /* bpf+kprobe programs can access fields of 'struct pt_regs' */
 static bool kprobe_prog_is_valid_access(int off, int size, enum bpf_access_type type,
-					enum bpf_reg_type *reg_type)
+					enum bpf_reg_type *reg_type, int *ctx_field_size)
 {
 	if (off < 0 || off >= sizeof(struct pt_regs))
 		return false;
@@ -562,7 +562,7 @@ static const struct bpf_func_proto *tp_prog_func_proto(enum bpf_func_id func_id)
 }
 
 static bool tp_prog_is_valid_access(int off, int size, enum bpf_access_type type,
-				    enum bpf_reg_type *reg_type)
+				    enum bpf_reg_type *reg_type, int *ctx_field_size)
 {
 	if (off < sizeof(void *) || off >= PERF_MAX_TRACE_SIZE)
 		return false;
@@ -581,17 +581,26 @@ const struct bpf_verifier_ops tracepoint_prog_ops = {
 };
 
 static bool pe_prog_is_valid_access(int off, int size, enum bpf_access_type type,
-				    enum bpf_reg_type *reg_type)
+				    enum bpf_reg_type *reg_type, int *ctx_field_size)
 {
+	int sample_period_off;
+
 	if (off < 0 || off >= sizeof(struct bpf_perf_event_data))
 		return false;
 	if (type != BPF_READ)
 		return false;
 	if (off % size != 0)
 		return false;
-	if (off == offsetof(struct bpf_perf_event_data, sample_period)) {
-		if (size != sizeof(u64))
-			return false;
+
+	/* permit 1, 2, 4 byte narrower and 8 normal read access to sample_period */
+	sample_period_off = offsetof(struct bpf_perf_event_data, sample_period);
+	if (off >= sample_period_off && off < sample_period_off + sizeof(__u64)) {
+		*ctx_field_size = 8;
+#ifdef __LITTLE_ENDIAN
+		return (off & 0x7) == 0 && size <= 8 && (size & (size - 1)) == 0;
+#else
+		return ((off & 0x7) + size) == 8 && size <= 8 && (size & (size - 1)) == 0;
+#endif
 	} else {
 		if (size != sizeof(long))
 			return false;