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bpf: Detect identical PTR_TO_MAP_VALUE_OR_NULL registers
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A BPF program is required to check the return register of a
map_elem_lookup() call before accessing memory. The verifier keeps
track of this by converting the type of the result register from
PTR_TO_MAP_VALUE_OR_NULL to PTR_TO_MAP_VALUE after a conditional
jump ensures safety. This check is currently exclusively performed
for the result register 0.

In the event the compiler reorders instructions, BPF_MOV64_REG
instructions may be moved before the conditional jump which causes
them to keep their type PTR_TO_MAP_VALUE_OR_NULL to which the
verifier objects when the register is accessed:

0: (b7) r1 = 10
1: (7b) *(u64 *)(r10 -8) = r1
2: (bf) r2 = r10
3: (07) r2 += -8
4: (18) r1 = 0x59c00000
6: (85) call 1
7: (bf) r4 = r0
8: (15) if r0 == 0x0 goto pc+1
 R0=map_value(ks=8,vs=8) R4=map_value_or_null(ks=8,vs=8) R10=fp
9: (7a) *(u64 *)(r4 +0) = 0
R4 invalid mem access 'map_value_or_null'

This commit extends the verifier to keep track of all identical
PTR_TO_MAP_VALUE_OR_NULL registers after a map_elem_lookup() by
assigning them an ID and then marking them all when the conditional
jump is observed.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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Thomas Graf authored and David S. Miller committed Oct 19, 2016
1 parent 3134e9b commit 57a09bf
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Showing 3 changed files with 118 additions and 17 deletions.
2 changes: 1 addition & 1 deletion include/linux/bpf_verifier.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -23,13 +23,13 @@ struct bpf_reg_state {
* result in a bad access.
*/
u64 min_value, max_value;
u32 id;
union {
/* valid when type == CONST_IMM | PTR_TO_STACK | UNKNOWN_VALUE */
s64 imm;

/* valid when type == PTR_TO_PACKET* */
struct {
u32 id;
u16 off;
u16 range;
};
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61 changes: 45 additions & 16 deletions kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -212,9 +212,10 @@ static void print_verifier_state(struct bpf_verifier_state *state)
else if (t == CONST_PTR_TO_MAP || t == PTR_TO_MAP_VALUE ||
t == PTR_TO_MAP_VALUE_OR_NULL ||
t == PTR_TO_MAP_VALUE_ADJ)
verbose("(ks=%d,vs=%d)",
verbose("(ks=%d,vs=%d,id=%u)",
reg->map_ptr->key_size,
reg->map_ptr->value_size);
reg->map_ptr->value_size,
reg->id);
if (reg->min_value != BPF_REGISTER_MIN_RANGE)
verbose(",min_value=%llu",
(unsigned long long)reg->min_value);
Expand Down Expand Up @@ -447,6 +448,7 @@ static void mark_reg_unknown_value(struct bpf_reg_state *regs, u32 regno)
{
BUG_ON(regno >= MAX_BPF_REG);
regs[regno].type = UNKNOWN_VALUE;
regs[regno].id = 0;
regs[regno].imm = 0;
}

Expand Down Expand Up @@ -1252,6 +1254,7 @@ static int check_call(struct bpf_verifier_env *env, int func_id)
return -EINVAL;
}
regs[BPF_REG_0].map_ptr = meta.map_ptr;
regs[BPF_REG_0].id = ++env->id_gen;
} else {
verbose("unknown return type %d of func %d\n",
fn->ret_type, func_id);
Expand Down Expand Up @@ -1644,8 +1647,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
insn->src_reg);
return -EACCES;
}
regs[insn->dst_reg].type = UNKNOWN_VALUE;
regs[insn->dst_reg].map_ptr = NULL;
mark_reg_unknown_value(regs, insn->dst_reg);
}
} else {
/* case: R = imm
Expand Down Expand Up @@ -1907,6 +1909,38 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
check_reg_overflow(true_reg);
}

static void mark_map_reg(struct bpf_reg_state *regs, u32 regno, u32 id,
enum bpf_reg_type type)
{
struct bpf_reg_state *reg = &regs[regno];

if (reg->type == PTR_TO_MAP_VALUE_OR_NULL && reg->id == id) {
reg->type = type;
if (type == UNKNOWN_VALUE)
mark_reg_unknown_value(regs, regno);
}
}

/* The logic is similar to find_good_pkt_pointers(), both could eventually
* be folded together at some point.
*/
static void mark_map_regs(struct bpf_verifier_state *state, u32 regno,
enum bpf_reg_type type)
{
struct bpf_reg_state *regs = state->regs;
int i;

for (i = 0; i < MAX_BPF_REG; i++)
mark_map_reg(regs, i, regs[regno].id, type);

for (i = 0; i < MAX_BPF_STACK; i += BPF_REG_SIZE) {
if (state->stack_slot_type[i] != STACK_SPILL)
continue;
mark_map_reg(state->spilled_regs, i / BPF_REG_SIZE,
regs[regno].id, type);
}
}

static int check_cond_jmp_op(struct bpf_verifier_env *env,
struct bpf_insn *insn, int *insn_idx)
{
Expand Down Expand Up @@ -1994,18 +2028,13 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
if (BPF_SRC(insn->code) == BPF_K &&
insn->imm == 0 && (opcode == BPF_JEQ || opcode == BPF_JNE) &&
dst_reg->type == PTR_TO_MAP_VALUE_OR_NULL) {
if (opcode == BPF_JEQ) {
/* next fallthrough insn can access memory via
* this register
*/
regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
/* branch targer cannot access it, since reg == 0 */
mark_reg_unknown_value(other_branch->regs,
insn->dst_reg);
} else {
other_branch->regs[insn->dst_reg].type = PTR_TO_MAP_VALUE;
mark_reg_unknown_value(regs, insn->dst_reg);
}
/* Mark all identical map registers in each branch as either
* safe or unknown depending R == 0 or R != 0 conditional.
*/
mark_map_regs(this_branch, insn->dst_reg,
opcode == BPF_JEQ ? PTR_TO_MAP_VALUE : UNKNOWN_VALUE);
mark_map_regs(other_branch, insn->dst_reg,
opcode == BPF_JEQ ? UNKNOWN_VALUE : PTR_TO_MAP_VALUE);
} else if (BPF_SRC(insn->code) == BPF_X && opcode == BPF_JGT &&
dst_reg->type == PTR_TO_PACKET &&
regs[insn->src_reg].type == PTR_TO_PACKET_END) {
Expand Down
72 changes: 72 additions & 0 deletions tools/testing/selftests/bpf/test_verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2588,6 +2588,78 @@ static struct bpf_test tests[] = {
.result_unpriv = REJECT,
.result = REJECT,
},
{
"multiple registers share map_lookup_elem result",
.insns = {
BPF_MOV64_IMM(BPF_REG_1, 10),
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
BPF_FUNC_map_lookup_elem),
BPF_MOV64_REG(BPF_REG_4, BPF_REG_0),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
BPF_ST_MEM(BPF_DW, BPF_REG_4, 0, 0),
BPF_EXIT_INSN(),
},
.fixup_map1 = { 4 },
.result = ACCEPT,
.prog_type = BPF_PROG_TYPE_SCHED_CLS
},
{
"invalid memory access with multiple map_lookup_elem calls",
.insns = {
BPF_MOV64_IMM(BPF_REG_1, 10),
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_MOV64_REG(BPF_REG_8, BPF_REG_1),
BPF_MOV64_REG(BPF_REG_7, BPF_REG_2),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
BPF_FUNC_map_lookup_elem),
BPF_MOV64_REG(BPF_REG_4, BPF_REG_0),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
BPF_ST_MEM(BPF_DW, BPF_REG_4, 0, 0),
BPF_EXIT_INSN(),
},
.fixup_map1 = { 4 },
.result = REJECT,
.errstr = "R4 !read_ok",
.prog_type = BPF_PROG_TYPE_SCHED_CLS
},
{
"valid indirect map_lookup_elem access with 2nd lookup in branch",
.insns = {
BPF_MOV64_IMM(BPF_REG_1, 10),
BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_MOV64_REG(BPF_REG_8, BPF_REG_1),
BPF_MOV64_REG(BPF_REG_7, BPF_REG_2),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
BPF_FUNC_map_lookup_elem),
BPF_MOV64_IMM(BPF_REG_2, 10),
BPF_JMP_IMM(BPF_JNE, BPF_REG_2, 0, 3),
BPF_MOV64_REG(BPF_REG_1, BPF_REG_8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
BPF_FUNC_map_lookup_elem),
BPF_MOV64_REG(BPF_REG_4, BPF_REG_0),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
BPF_ST_MEM(BPF_DW, BPF_REG_4, 0, 0),
BPF_EXIT_INSN(),
},
.fixup_map1 = { 4 },
.result = ACCEPT,
.prog_type = BPF_PROG_TYPE_SCHED_CLS
},
};

static int probe_filter_length(const struct bpf_insn *fp)
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