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Subject[PATCH 4.9 38/93] bpf: Detect identical PTR_TO_MAP_VALUE_OR_NULL registers
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Graf <tgraf@suug.ch>


[ Upstream commit 57a09bf0a416700676e77102c28f9cfcb48267e0 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/bpf_verifier.h | 2 -
kernel/bpf/verifier.c | 61 +++++++++++++++++++++++++++++++------------
2 files changed, 46 insertions(+), 17 deletions(-)

--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -24,13 +24,13 @@ struct bpf_reg_state {
*/
s64 min_value;
u64 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;
};
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -212,9 +212,10 @@ static void print_verifier_state(struct
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=%lld",
(long long)reg->min_value);
@@ -447,6 +448,7 @@ static void mark_reg_unknown_value(struc
{
BUG_ON(regno >= MAX_BPF_REG);
regs[regno].type = UNKNOWN_VALUE;
+ regs[regno].id = 0;
regs[regno].imm = 0;
}

@@ -1252,6 +1254,7 @@ static int check_call(struct bpf_verifie
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);
@@ -1668,8 +1671,7 @@ static int check_alu_op(struct bpf_verif
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
@@ -1931,6 +1933,38 @@ static void reg_set_min_max_inv(struct b
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)
{
@@ -2018,18 +2052,13 @@ static int check_cond_jmp_op(struct bpf_
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) {

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