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Message-ID: <20171112000036.3597189-2-yhs@fb.com>
Date: Sat, 11 Nov 2017 16:00:34 -0800
From: Yonghong Song <yhs@...com>
To: <ast@...com>, <daniel@...earbox.net>, <netdev@...r.kernel.org>
CC: <kernel-team@...com>
Subject: [PATCH net-next 1/3] bpf: improve verifier ARG_CONST_SIZE_OR_ZERO semantics
For helpers, the argument type ARG_CONST_SIZE_OR_ZERO permits the
access size to be 0 when accessing the previous argument (arg).
Right now, it requires the arg needs to be NULL when size passed
is 0 or could be 0. It also requires a non-NULL arg when the size
is proved to be non-0.
This patch changes verifier ARG_CONST_SIZE_OR_ZERO behavior
such that for size-0 or possible size-0, it is not required
the arg equal to NULL.
There are a couple of reasons for this semantics change, and
all of them intends to simplify user bpf programs which
may improve user experience and/or increase chances of
verifier acceptance. Together with the next patch which
changes bpf_probe_read arg2 type from ARG_CONST_SIZE to
ARG_CONST_SIZE_OR_ZERO, the following two examples, which
fail the verifier currently, are able to get verifier acceptance.
Example 1:
==========
unsigned long len = pend - pstart;
len = len > MAX_PAYLOAD_LEN ? MAX_PAYLOAD_LEN : len;
len &= MAX_PAYLOAD_LEN;
bpf_probe_read(data->payload, len, pstart);
It does not have test for "len > 0" and it failed the verifier.
Users may not be aware that they have to add this test.
Converting the bpf_probe_read helper to have
ARG_CONST_SIZE_OR_ZERO helps the above code get
verifier acceptance.
Example 2:
==========
Here is one example where llvm "messed up" the code and
the verifier fails.
......
unsigned long len = pend - pstart;
if (len > 0 && len <= MAX_PAYLOAD_LEN)
bpf_probe_read(data->payload, len, pstart);
......
The compiler generates the following code and verifier fails:
......
39: (79) r2 = *(u64 *)(r10 -16)
40: (1f) r2 -= r8
41: (bf) r1 = r2
42: (07) r1 += -1
43: (25) if r1 > 0xffe goto pc+3
R0=inv(id=0) R1=inv(id=0,umax_value=4094,var_off=(0x0; 0xfff))
R2=inv(id=0) R6=map_value(id=0,off=0,ks=4,vs=4095,imm=0) R7=inv(id=0)
R8=inv(id=0) R9=inv0 R10=fp0
44: (bf) r1 = r6
45: (bf) r3 = r8
46: (85) call bpf_probe_read#45
R2 min value is negative, either use unsigned or 'var &= const'
......
The compiler optimization is correct. If r1 = 0,
r1 - 1 = 0xffffffffffffffff > 0xffe. If r1 != 0, r1 - 1 will not wrap.
r1 > 0xffe at insn #43 can actually capture
both "r1 > 0" and "len <= MAX_PAYLOAD_LEN".
This however causes an issue in verifier as the value range of arg2
"r2" does not properly get refined and lead to verification failure.
Relaxing bpf_prog_read arg2 from ARG_CONST_SIZE to ARG_CONST_SIZE_OR_ZERO
allows the following simplied code:
unsigned long len = pend - pstart;
if (len <= MAX_PAYLOAD_LEN)
bpf_probe_read(data->payload, len, pstart);
The llvm compiler will generate less complex code and the
verifier is able to verify that the program is okay.
Signed-off-by: Yonghong Song <yhs@...com>
Acked-by: Alexei Starovoitov <ast@...nel.org>
---
kernel/bpf/verifier.c | 40 ++++++++++++++++++++++++----------------
1 file changed, 24 insertions(+), 16 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4a942e2..dd54d20 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -799,12 +799,13 @@ static int check_stack_read(struct bpf_verifier_env *env,
/* check read/write into map element returned by bpf_map_lookup_elem() */
static int __check_map_access(struct bpf_verifier_env *env, u32 regno, int off,
- int size)
+ int size, bool zero_size_allowed)
{
struct bpf_reg_state *regs = cur_regs(env);
struct bpf_map *map = regs[regno].map_ptr;
- if (off < 0 || size <= 0 || off + size > map->value_size) {
+ if (off < 0 || size < 0 || (size == 0 && !zero_size_allowed) ||
+ off + size > map->value_size) {
verbose(env, "invalid access to map value, value_size=%d off=%d size=%d\n",
map->value_size, off, size);
return -EACCES;
@@ -814,7 +815,7 @@ static int __check_map_access(struct bpf_verifier_env *env, u32 regno, int off,
/* check read/write into a map element with possible variable offset */
static int check_map_access(struct bpf_verifier_env *env, u32 regno,
- int off, int size)
+ int off, int size, bool zero_size_allowed)
{
struct bpf_verifier_state *state = env->cur_state;
struct bpf_reg_state *reg = &state->regs[regno];
@@ -837,7 +838,8 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
regno);
return -EACCES;
}
- err = __check_map_access(env, regno, reg->smin_value + off, size);
+ err = __check_map_access(env, regno, reg->smin_value + off, size,
+ zero_size_allowed);
if (err) {
verbose(env, "R%d min value is outside of the array range\n",
regno);
@@ -853,7 +855,8 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
regno);
return -EACCES;
}
- err = __check_map_access(env, regno, reg->umax_value + off, size);
+ err = __check_map_access(env, regno, reg->umax_value + off, size,
+ zero_size_allowed);
if (err)
verbose(env, "R%d max value is outside of the array range\n",
regno);
@@ -889,12 +892,13 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
}
static int __check_packet_access(struct bpf_verifier_env *env, u32 regno,
- int off, int size)
+ int off, int size, bool zero_size_allowed)
{
struct bpf_reg_state *regs = cur_regs(env);
struct bpf_reg_state *reg = ®s[regno];
- if (off < 0 || size <= 0 || (u64)off + size > reg->range) {
+ if (off < 0 || size < 0 || (size == 0 && !zero_size_allowed) ||
+ (u64)off + size > reg->range) {
verbose(env, "invalid access to packet, off=%d size=%d, R%d(id=%d,off=%d,r=%d)\n",
off, size, regno, reg->id, reg->off, reg->range);
return -EACCES;
@@ -903,7 +907,7 @@ static int __check_packet_access(struct bpf_verifier_env *env, u32 regno,
}
static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
- int size)
+ int size, bool zero_size_allowed)
{
struct bpf_reg_state *regs = cur_regs(env);
struct bpf_reg_state *reg = ®s[regno];
@@ -922,7 +926,7 @@ static int check_packet_access(struct bpf_verifier_env *env, u32 regno, int off,
regno);
return -EACCES;
}
- err = __check_packet_access(env, regno, off, size);
+ err = __check_packet_access(env, regno, off, size, zero_size_allowed);
if (err) {
verbose(env, "R%d offset is outside of the packet\n", regno);
return err;
@@ -1097,7 +1101,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
return -EACCES;
}
- err = check_map_access(env, regno, off, size);
+ err = check_map_access(env, regno, off, size, false);
if (!err && t == BPF_READ && value_regno >= 0)
mark_reg_unknown(env, regs, value_regno);
@@ -1184,7 +1188,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
value_regno);
return -EACCES;
}
- err = check_packet_access(env, regno, off, size);
+ err = check_packet_access(env, regno, off, size, false);
if (!err && t == BPF_READ && value_regno >= 0)
mark_reg_unknown(env, regs, value_regno);
} else {
@@ -1281,7 +1285,7 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
}
off = regs[regno].off + regs[regno].var_off.value;
if (off >= 0 || off < -MAX_BPF_STACK || off + access_size > 0 ||
- access_size <= 0) {
+ access_size < 0 || (access_size == 0 && !zero_size_allowed)) {
verbose(env, "invalid stack type R%d off=%d access_size=%d\n",
regno, off, access_size);
return -EACCES;
@@ -1319,9 +1323,11 @@ static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,
switch (reg->type) {
case PTR_TO_PACKET:
case PTR_TO_PACKET_META:
- return check_packet_access(env, regno, reg->off, access_size);
+ return check_packet_access(env, regno, reg->off, access_size,
+ zero_size_allowed);
case PTR_TO_MAP_VALUE:
- return check_map_access(env, regno, reg->off, access_size);
+ return check_map_access(env, regno, reg->off, access_size,
+ zero_size_allowed);
default: /* scalar_value|ptr_to_stack or invalid ptr */
return check_stack_boundary(env, regno, access_size,
zero_size_allowed, meta);
@@ -1415,7 +1421,8 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
}
if (type_is_pkt_pointer(type))
err = check_packet_access(env, regno, reg->off,
- meta->map_ptr->key_size);
+ meta->map_ptr->key_size,
+ false);
else
err = check_stack_boundary(env, regno,
meta->map_ptr->key_size,
@@ -1431,7 +1438,8 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
}
if (type_is_pkt_pointer(type))
err = check_packet_access(env, regno, reg->off,
- meta->map_ptr->value_size);
+ meta->map_ptr->value_size,
+ false);
else
err = check_stack_boundary(env, regno,
meta->map_ptr->value_size,
--
2.9.5
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