From 9fd29c08e52023252f0480ab8f6906a1ecc9a8d5 Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Sun, 12 Nov 2017 14:49:09 -0800 Subject: [PATCH] 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 Acked-by: Alexei Starovoitov Acked-by: Daniel Borkmann Signed-off-by: David S. Miller --- 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 4a942e2e753d71..dd54d20ace2ff7 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,