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bpf: mark dst unknown on inconsistent {s, u}bounds adjustments
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syzkaller generated a BPF proglet and triggered a warning with
the following:

  0: (b7) r0 = 0
  1: (d5) if r0 s<= 0x0 goto pc+0
   R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  2: (1f) r0 -= r1
   R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
  verifier internal error: known but bad sbounds

What happens is that in the first insn, r0's min/max value
are both 0 due to the immediate assignment, later in the jsle
test the bounds are updated for the min value in the false
path, meaning, they yield smin_val = 1, smax_val = 0, and when
ctx pointer is subtracted from r0, verifier bails out with the
internal error and throwing a WARN since smin_val != smax_val
for the known constant.

For min_val > max_val scenario it means that reg_set_min_max()
and reg_set_min_max_inv() (which both refine existing bounds)
demonstrated that such branch cannot be taken at runtime.

In above scenario for the case where it will be taken, the
existing [0, 0] bounds are kept intact. Meaning, the rejection
is not due to a verifier internal error, and therefore the
WARN() is not necessary either.

We could just reject such cases in adjust_{ptr,scalar}_min_max_vals()
when either known scalars have smin_val != smax_val or
umin_val != umax_val or any scalar reg with bounds
smin_val > smax_val or umin_val > umax_val. However, there
may be a small risk of breakage of buggy programs, so handle
this more gracefully and in adjust_{ptr,scalar}_min_max_vals()
just taint the dst reg as unknown scalar when we see ops with
such kind of src reg.

Reported-by: [email protected]
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
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@borkmann
borkmann authored and Alexei Starovoitov committed Jan 18, 2018
1 parent f37a8cb commit 6f16101
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Showing 2 changed files with 138 additions and 12 deletions.
27 changes: 16 additions & 11 deletions kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1895,17 +1895,13 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,

dst_reg = &regs[dst];

if (WARN_ON_ONCE(known && (smin_val != smax_val))) {
print_verifier_state(env, env->cur_state);
verbose(env,
"verifier internal error: known but bad sbounds\n");
return -EINVAL;
}
if (WARN_ON_ONCE(known && (umin_val != umax_val))) {
print_verifier_state(env, env->cur_state);
verbose(env,
"verifier internal error: known but bad ubounds\n");
return -EINVAL;
if ((known && (smin_val != smax_val || umin_val != umax_val)) ||
smin_val > smax_val || umin_val > umax_val) {
/* Taint dst register if offset had invalid bounds derived from
* e.g. dead branches.
*/
__mark_reg_unknown(dst_reg);
return 0;
}

if (BPF_CLASS(insn->code) != BPF_ALU64) {
Expand Down Expand Up @@ -2097,6 +2093,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
src_known = tnum_is_const(src_reg.var_off);
dst_known = tnum_is_const(dst_reg->var_off);

if ((src_known && (smin_val != smax_val || umin_val != umax_val)) ||
smin_val > smax_val || umin_val > umax_val) {
/* Taint dst register if offset had invalid bounds derived from
* e.g. dead branches.
*/
__mark_reg_unknown(dst_reg);
return 0;
}

if (!src_known &&
opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
__mark_reg_unknown(dst_reg);
Expand Down
123 changes: 122 additions & 1 deletion tools/testing/selftests/bpf/test_verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -6732,7 +6732,7 @@ static struct bpf_test tests[] = {
BPF_JMP_IMM(BPF_JA, 0, 0, -7),
},
.fixup_map1 = { 4 },
.errstr = "unbounded min value",
.errstr = "R0 invalid mem access 'inv'",
.result = REJECT,
},
{
Expand Down Expand Up @@ -8633,6 +8633,127 @@ static struct bpf_test tests[] = {
.prog_type = BPF_PROG_TYPE_XDP,
.flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
},
{
"check deducing bounds from const, 1",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 1),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 1, 0),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "R0 tried to subtract pointer from scalar",
},
{
"check deducing bounds from const, 2",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 1),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 1, 1),
BPF_EXIT_INSN(),
BPF_JMP_IMM(BPF_JSLE, BPF_REG_0, 1, 1),
BPF_EXIT_INSN(),
BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_0),
BPF_EXIT_INSN(),
},
.result = ACCEPT,
},
{
"check deducing bounds from const, 3",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSLE, BPF_REG_0, 0, 0),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "R0 tried to subtract pointer from scalar",
},
{
"check deducing bounds from const, 4",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSLE, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),
BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_0),
BPF_EXIT_INSN(),
},
.result = ACCEPT,
},
{
"check deducing bounds from const, 5",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "R0 tried to subtract pointer from scalar",
},
{
"check deducing bounds from const, 6",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "R0 tried to subtract pointer from scalar",
},
{
"check deducing bounds from const, 7",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, ~0),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 0),
BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_0),
BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
offsetof(struct __sk_buff, mark)),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "dereference of modified ctx ptr",
},
{
"check deducing bounds from const, 8",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, ~0),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),
BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_0),
BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
offsetof(struct __sk_buff, mark)),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "dereference of modified ctx ptr",
},
{
"check deducing bounds from const, 9",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 0),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "R0 tried to subtract pointer from scalar",
},
{
"check deducing bounds from const, 10",
.insns = {
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_JMP_IMM(BPF_JSLE, BPF_REG_0, 0, 0),
/* Marks reg as unknown. */
BPF_ALU64_IMM(BPF_NEG, BPF_REG_0, 0),
BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
BPF_EXIT_INSN(),
},
.result = REJECT,
.errstr = "math between ctx pointer and register with unbounded min value is not allowed",
},
{
"bpf_exit with invalid return code. test1",
.insns = {
Expand Down

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