[PATCH v2 net-next 1/2] bpf: allow extended BPF programs access skb fields

From: Alexei Starovoitov
Date: Fri Mar 13 2015 - 14:58:53 EST

introduce user accessible mirror of in-kernel 'struct sk_buff':
struct __sk_buff {
__u32 len;
__u32 pkt_type;
__u32 mark;
__u32 queue_mapping;
};

bpf programs can do:

int bpf_prog(struct __sk_buff *skb)
{
__u32 var = skb->pkt_type;

which will be compiled to bpf assembler as:

dst_reg = *(u32 *)(src_reg + 4) // 4 == offsetof(struct __sk_buff, pkt_type)

bpf verifier will check validity of access and will convert it to:

dst_reg = *(u8 *)(src_reg + offsetof(struct sk_buff, __pkt_type_offset))
dst_reg &= 7

since skb->pkt_type is a bitfield.

Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxxxx>
---
include/linux/bpf.h | 5 +-
include/uapi/linux/bpf.h | 10 +++
kernel/bpf/syscall.c | 2 +-
kernel/bpf/verifier.c | 152 +++++++++++++++++++++++++++++++++++++++++-----
net/core/filter.c | 100 ++++++++++++++++++++++++------
5 files changed, 234 insertions(+), 35 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 80f2e0fc3d02..2c17ebdfb5ae 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -103,6 +103,9 @@ struct bpf_verifier_ops {
* with 'type' (read or write) is allowed
*/
bool (*is_valid_access)(int off, int size, enum bpf_access_type type);
+
+ u32 (*convert_ctx_access)(int dst_reg, int src_reg, int ctx_off,
+ struct bpf_insn *insn);
};

struct bpf_prog_type_list {
@@ -133,7 +136,7 @@ struct bpf_map *bpf_map_get(struct fd f);
void bpf_map_put(struct bpf_map *map);

/* verify correctness of eBPF program */
-int bpf_check(struct bpf_prog *fp, union bpf_attr *attr);
+int bpf_check(struct bpf_prog **fp, union bpf_attr *attr);
#else
static inline void bpf_register_prog_type(struct bpf_prog_type_list *tl)
{
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 3fa1af8a58d7..936e9257da95 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -168,4 +168,14 @@ enum bpf_func_id {
__BPF_FUNC_MAX_ID,
};

+/* user accessible mirror of in-kernel sk_buff.
+ * new fields can only be added to the end of this structure
+ */
+struct __sk_buff {
+ __u32 len;
+ __u32 pkt_type;
+ __u32 mark;
+ __u32 queue_mapping;
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 669719ccc9ee..ea75c654af1b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -519,7 +519,7 @@ static int bpf_prog_load(union bpf_attr *attr)
goto free_prog;

/* run eBPF verifier */
- err = bpf_check(prog, attr);
+ err = bpf_check(&prog, attr);
if (err < 0)
goto free_used_maps;

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e6b522496250..c22ebd36fa4b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1620,11 +1620,10 @@ static int do_check(struct verifier_env *env)
return err;

} else if (class == BPF_LDX) {
- if (BPF_MODE(insn->code) != BPF_MEM ||
- insn->imm != 0) {
- verbose("BPF_LDX uses reserved fields\n");
- return -EINVAL;
- }
+ enum bpf_reg_type src_reg_type;
+
+ /* check for reserved fields is already done */
+
/* check src operand */
err = check_reg_arg(regs, insn->src_reg, SRC_OP);
if (err)
@@ -1643,6 +1642,29 @@ static int do_check(struct verifier_env *env)
if (err)
return err;

+ src_reg_type = regs[insn->src_reg].type;
+
+ if (insn->imm == 0 && BPF_SIZE(insn->code) == BPF_W) {
+ /* saw a valid insn
+ * dst_reg = *(u32 *)(src_reg + off)
+ * use reserved 'imm' field to mark this insn
+ */
+ insn->imm = src_reg_type;
+
+ } else if (src_reg_type != insn->imm &&
+ (src_reg_type == PTR_TO_CTX ||
+ insn->imm == PTR_TO_CTX)) {
+ /* ABuser program is trying to use the same insn
+ * dst_reg = *(u32*) (src_reg + off)
+ * with different pointer types:
+ * src_reg == ctx in one branch and
+ * src_reg == stack|map in some other branch.
+ * Reject it.
+ */
+ verbose("same insn cannot be used with different pointers\n");
+ return -EINVAL;
+ }
+
} else if (class == BPF_STX) {
if (BPF_MODE(insn->code) == BPF_XADD) {
err = check_xadd(env, insn);
@@ -1790,6 +1812,13 @@ static int replace_map_fd_with_map_ptr(struct verifier_env *env)
int i, j;

for (i = 0; i < insn_cnt; i++, insn++) {
+ if (BPF_CLASS(insn->code) == BPF_LDX &&
+ (BPF_MODE(insn->code) != BPF_MEM ||
+ insn->imm != 0)) {
+ verbose("BPF_LDX uses reserved fields\n");
+ return -EINVAL;
+ }
+
if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) {
struct bpf_map *map;
struct fd f;
@@ -1881,6 +1910,92 @@ static void convert_pseudo_ld_imm64(struct verifier_env *env)
insn->src_reg = 0;
}

+static void adjust_branches(struct bpf_prog *prog, int pos, int delta)
+{
+ struct bpf_insn *insn = prog->insnsi;
+ int insn_cnt = prog->len;
+ int i;
+
+ for (i = 0; i < insn_cnt; i++, insn++) {
+ if (BPF_CLASS(insn->code) != BPF_JMP ||
+ BPF_OP(insn->code) == BPF_CALL ||
+ BPF_OP(insn->code) == BPF_EXIT)
+ continue;
+
+ /* adjust offset of jmps if necessary */
+ if (i < pos && i + insn->off + 1 > pos)
+ insn->off += delta;
+ else if (i > pos && i + insn->off + 1 < pos)
+ insn->off -= delta;
+ }
+}
+
+/* convert load instructions that access fields of 'struct __sk_buff'
+ * into sequence of instructions that access fields of 'struct sk_buff'
+ */
+static int convert_ctx_accesses(struct verifier_env *env)
+{
+ struct bpf_insn *insn = env->prog->insnsi;
+ int insn_cnt = env->prog->len;
+ struct bpf_insn insn_buf[16];
+ struct bpf_prog *new_prog;
+ u32 cnt;
+ int i;
+
+ if (!env->prog->aux->ops->convert_ctx_access)
+ return 0;
+
+ for (i = 0; i < insn_cnt; i++, insn++) {
+ if (insn->code != (BPF_LDX | BPF_MEM | BPF_W))
+ continue;
+
+ if (insn->imm != PTR_TO_CTX) {
+ /* clear internal mark */
+ insn->imm = 0;
+ continue;
+ }
+
+ cnt = env->prog->aux->ops->
+ convert_ctx_access(insn->dst_reg, insn->src_reg,
+ insn->off, insn_buf);
+ if (cnt == 0 || cnt >= ARRAY_SIZE(insn_buf)) {
+ verbose("bpf verifier is misconfigured\n");
+ return -EINVAL;
+ }
+
+ if (cnt == 1) {
+ memcpy(insn, insn_buf, sizeof(*insn));
+ continue;
+ }
+
+ /* several new insns need to be inserted. Make room for them */
+ insn_cnt += cnt - 1;
+ new_prog = bpf_prog_realloc(env->prog,
+ bpf_prog_size(insn_cnt),
+ GFP_USER);
+ if (!new_prog)
+ return -ENOMEM;
+
+ new_prog->len = insn_cnt;
+
+ memmove(new_prog->insnsi + i + cnt, new_prog->insns + i + 1,
+ sizeof(*insn) * (insn_cnt - i - cnt));
+
+ /* copy substitute insns in place of load instruction */
+ memcpy(new_prog->insnsi + i, insn_buf, sizeof(*insn) * cnt);
+
+ /* adjust branches in the whole program */
+ adjust_branches(new_prog, i, cnt - 1);
+
+ /* keep walking new program and skip insns we just inserted */
+ env->prog = new_prog;
+ insn = new_prog->insnsi + i + cnt - 1;
+ i += cnt - 1;
+ }
+
+ return 0;
+}
+
static void free_states(struct verifier_env *env)
{
struct verifier_state_list *sl, *sln;
@@ -1903,13 +2018,13 @@ static void free_states(struct verifier_env *env)
kfree(env->explored_states);
}

-int bpf_check(struct bpf_prog *prog, union bpf_attr *attr)
+int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
{
char __user *log_ubuf = NULL;
struct verifier_env *env;
int ret = -EINVAL;

- if (prog->len <= 0 || prog->len > BPF_MAXINSNS)
+ if ((*prog)->len <= 0 || (*prog)->len > BPF_MAXINSNS)
return -E2BIG;

/* 'struct verifier_env' can be global, but since it's not small,
@@ -1919,7 +2034,7 @@ int bpf_check(struct bpf_prog *prog, union bpf_attr *attr)
if (!env)
return -ENOMEM;

- env->prog = prog;
+ env->prog = *prog;

/* grab the mutex to protect few globals used by verifier */
mutex_lock(&bpf_verifier_lock);
@@ -1951,7 +2066,7 @@ int bpf_check(struct bpf_prog *prog, union bpf_attr *attr)
if (ret < 0)
goto skip_full_check;

- env->explored_states = kcalloc(prog->len,
+ env->explored_states = kcalloc(env->prog->len,
sizeof(struct verifier_state_list *),
GFP_USER);
ret = -ENOMEM;
@@ -1968,6 +2083,10 @@ skip_full_check:
while (pop_stack(env, NULL) >= 0);
free_states(env);

+ if (ret == 0)
+ /* program is valid, convert *(u32*)(ctx + off) accesses */
+ ret = convert_ctx_accesses(env);
+
if (log_level && log_len >= log_size - 1) {
BUG_ON(log_len >= log_size);
/* verifier log exceeded user supplied buffer */
@@ -1983,18 +2102,18 @@ skip_full_check:

if (ret == 0 && env->used_map_cnt) {
/* if program passed verifier, update used_maps in bpf_prog_info */
- prog->aux->used_maps = kmalloc_array(env->used_map_cnt,
- sizeof(env->used_maps[0]),
- GFP_KERNEL);
+ env->prog->aux->used_maps = kmalloc_array(env->used_map_cnt,
+ sizeof(env->used_maps[0]),
+ GFP_KERNEL);

- if (!prog->aux->used_maps) {
+ if (!env->prog->aux->used_maps) {
ret = -ENOMEM;
goto free_log_buf;
}

- memcpy(prog->aux->used_maps, env->used_maps,
+ memcpy(env->prog->aux->used_maps, env->used_maps,
sizeof(env->used_maps[0]) * env->used_map_cnt);
- prog->aux->used_map_cnt = env->used_map_cnt;
+ env->prog->aux->used_map_cnt = env->used_map_cnt;

/* program is valid. Convert pseudo bpf_ld_imm64 into generic
* bpf_ld_imm64 instructions
@@ -2006,11 +2125,12 @@ free_log_buf:
if (log_level)
vfree(log_buf);
free_env:
- if (!prog->aux->used_maps)
+ if (!env->prog->aux->used_maps)
/* if we didn't copy map pointers into bpf_prog_info, release
* them now. Otherwise free_bpf_prog_info() will release them.
*/
release_maps(env);
+ *prog = env->prog;
kfree(env);
mutex_unlock(&bpf_verifier_lock);
return ret;
diff --git a/net/core/filter.c b/net/core/filter.c
index 7a4eb7030dba..23b7c087cece 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -150,10 +150,43 @@ static u64 __get_random_u32(u64 ctx, u64 a, u64 x, u64 r4, u64 r5)
return prandom_u32();
}

+static u32 convert_skb_access(int skb_field, int dst_reg, int src_reg,
+ struct bpf_insn *insn_buf)
+{
+ struct bpf_insn *insn = insn_buf;
+
+ switch (skb_field) {
+ case SKF_AD_MARK:
+ BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, mark) != 4);
+
+ *insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+ offsetof(struct sk_buff, mark));
+ break;
+
+ case SKF_AD_PKTTYPE:
+ *insn++ = BPF_LDX_MEM(BPF_B, dst_reg, src_reg, PKT_TYPE_OFFSET());
+ *insn++ = BPF_ALU32_IMM(BPF_AND, dst_reg, PKT_TYPE_MAX);
+#ifdef __BIG_ENDIAN_BITFIELD
+ *insn++ = BPF_ALU32_IMM(BPF_RSH, dst_reg, 5);
+#endif
+ break;
+
+ case SKF_AD_QUEUE:
+ BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, queue_mapping) != 2);
+
+ *insn++ = BPF_LDX_MEM(BPF_H, dst_reg, src_reg,
+ offsetof(struct sk_buff, queue_mapping));
+ break;
+ }
+
+ return insn - insn_buf;
+}
+
static bool convert_bpf_extensions(struct sock_filter *fp,
struct bpf_insn **insnp)
{
struct bpf_insn *insn = *insnp;
+ u32 cnt;

switch (fp->k) {
case SKF_AD_OFF + SKF_AD_PROTOCOL:
@@ -167,13 +200,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
break;

case SKF_AD_OFF + SKF_AD_PKTTYPE:
- *insn++ = BPF_LDX_MEM(BPF_B, BPF_REG_A, BPF_REG_CTX,
- PKT_TYPE_OFFSET());
- *insn = BPF_ALU32_IMM(BPF_AND, BPF_REG_A, PKT_TYPE_MAX);
-#ifdef __BIG_ENDIAN_BITFIELD
- insn++;
- *insn = BPF_ALU32_IMM(BPF_RSH, BPF_REG_A, 5);
-#endif
+ cnt = convert_skb_access(SKF_AD_PKTTYPE, BPF_REG_A, BPF_REG_CTX, insn);
+ insn += cnt - 1;
break;

case SKF_AD_OFF + SKF_AD_IFINDEX:
@@ -197,10 +225,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
break;

case SKF_AD_OFF + SKF_AD_MARK:
- BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, mark) != 4);
-
- *insn = BPF_LDX_MEM(BPF_W, BPF_REG_A, BPF_REG_CTX,
- offsetof(struct sk_buff, mark));
+ cnt = convert_skb_access(SKF_AD_MARK, BPF_REG_A, BPF_REG_CTX, insn);
+ insn += cnt - 1;
break;

case SKF_AD_OFF + SKF_AD_RXHASH:
@@ -211,10 +237,8 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
break;

case SKF_AD_OFF + SKF_AD_QUEUE:
- BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, queue_mapping) != 2);
-
- *insn = BPF_LDX_MEM(BPF_H, BPF_REG_A, BPF_REG_CTX,
- offsetof(struct sk_buff, queue_mapping));
+ cnt = convert_skb_access(SKF_AD_QUEUE, BPF_REG_A, BPF_REG_CTX, insn);
+ insn += cnt - 1;
break;

case SKF_AD_OFF + SKF_AD_VLAN_TAG:
@@ -1147,13 +1171,55 @@ sk_filter_func_proto(enum bpf_func_id func_id)
static bool sk_filter_is_valid_access(int off, int size,
enum bpf_access_type type)
{
- /* skb fields cannot be accessed yet */
- return false;
+ /* only read is allowed */
+ if (type != BPF_READ)
+ return false;
+
+ /* check bounds */
+ if (off < 0 || off >= sizeof(struct __sk_buff))
+ return false;
+
+ /* disallow misaligned access */
+ if (off % size != 0)
+ return false;
+
+ /* all __sk_buff fields are __u32 */
+ if (size != 4)
+ return false;
+
+ return true;
+}
+
+static u32 sk_filter_convert_ctx_access(int dst_reg, int src_reg, int ctx_off,
+ struct bpf_insn *insn_buf)
+{
+ struct bpf_insn *insn = insn_buf;
+
+ switch (ctx_off) {
+ case offsetof(struct __sk_buff, len):
+ BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, len) != 4);
+
+ *insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+ offsetof(struct sk_buff, len));
+ break;
+
+ case offsetof(struct __sk_buff, mark):
+ return convert_skb_access(SKF_AD_MARK, dst_reg, src_reg, insn);
+
+ case offsetof(struct __sk_buff, pkt_type):
+ return convert_skb_access(SKF_AD_PKTTYPE, dst_reg, src_reg, insn);
+
+ case offsetof(struct __sk_buff, queue_mapping):
+ return convert_skb_access(SKF_AD_QUEUE, dst_reg, src_reg, insn);
+ }
+
+ return insn - insn_buf;
}

static const struct bpf_verifier_ops sk_filter_ops = {
.get_func_proto = sk_filter_func_proto,
.is_valid_access = sk_filter_is_valid_access,
+ .convert_ctx_access = sk_filter_convert_ctx_access,
};

static struct bpf_prog_type_list sk_filter_type __read_mostly = {
--
1.7.9.5

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/