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Date: Sun, 14 Jan 2018 22:38:59 -0800
From: Y Song <ys114321@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Alexei Starovoitov <ast@...com>,
Edward Cree <ecree@...arflare.com>,
netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: do not modify min/max bounds on scalars with
constant values
On Fri, Jan 12, 2018 at 11:23 AM, Daniel Borkmann <daniel@...earbox.net> wrote:
> 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.
>
> Fix is to not update any bounds of the register holding the
> constant, thus in reg_set_min_max() and reg_set_min_max_inv()
> we return right away, similarly as in the case when the dst
> register holds a pointer value. Reason for doing so is rather
> straight forward: when we have a register holding a constant
> as dst, then {s,u}min_val == {s,u}max_val, thus it cannot get
> any more specific in terms of upper/lower bounds than this.
>
> In reg_set_min_max() and reg_set_min_max_inv() it's fine to
> only check false_reg similarly as with __is_pointer_value()
> check since at this point in time, false_reg and true_reg
> both hold the same state, so we only need to pick one. This
> fixes the bug and properly rejects the program with an error
> of 'R0 tried to subtract pointer from scalar'.
>
> I've been thinking to additionally reject arithmetic on ctx
> pointer in adjust_ptr_min_max_vals() right upfront as well
> since we reject actual access in such case later on anyway,
> but there's a use case in tracing (in bcc) in combination
> with passing such ctx to bpf_probe_read(), so we cannot do
> that part.
There is a reason why bcc does this. For example, suppose that we want to
trace kernel tracepoint, sched_process_exec,
TRACE_EVENT(sched_process_exec,
TP_PROTO(struct task_struct *p, pid_t old_pid,
struct linux_binprm *bprm),
TP_ARGS(p, old_pid, bprm),
TP_STRUCT__entry(
__string( filename, bprm->filename )
__field( pid_t, pid )
__field( pid_t, old_pid )
),
TP_fast_assign(
__assign_str(filename, bprm->filename);
__entry->pid = p->pid;
__entry->old_pid = old_pid;
),
TP_printk("filename=%s pid=%d old_pid=%d", __get_str(filename),
__entry->pid, __entry->old_pid)
);
Eventually structure at
/sys/kernel/debug/tracing/event/sched/sched_process_exec/format:
......
field:__data_loc char[] filename; offset:8;
size:4; signed:1;
field:pid_t pid; offset:12; size:4; signed:1;
field:pid_t old_pid; offset:16; size:4; signed:1;
and "data_loc filename" is the offset in the structure where
"filename" is stored.
Therefore, in bcc, the access sequence is:
offset = args->filename; /* field __data_loc filename */
bpf_probe_read(&dst, len, (char *)args + offset);
For this kind of dynamic array in the tracepoint, the offset to access
certain field in ctx will be unknown at verification time.
So I suggest to remove the above paragraph regarding to potential ctx+offset
rejection.
>
> Reported-by: syzbot+6d362cadd45dc0a12ba4@...kaller.appspotmail.com
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
> kernel/bpf/verifier.c | 11 ++++
> tools/testing/selftests/bpf/test_verifier.c | 95 +++++++++++++++++++++++++++++
> 2 files changed, 106 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index b414d6b..6bf1275 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2617,6 +2617,14 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg,
> */
> if (__is_pointer_value(false, false_reg))
> return;
> + /* Same goes for constant values. They have {s,u}min == {s,u}max,
> + * it cannot get narrower than this. Same here, at the branch
> + * point false_reg and true_reg have the same type, so we only
> + * check false_reg here as well.
> + */
> + if (false_reg->type == SCALAR_VALUE &&
> + tnum_is_const(false_reg->var_off))
> + return;
>
> switch (opcode) {
> case BPF_JEQ:
> @@ -2689,6 +2697,9 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
> {
> if (__is_pointer_value(false, false_reg))
> return;
> + if (false_reg->type == SCALAR_VALUE &&
> + tnum_is_const(false_reg->var_off))
> + return;
>
> switch (opcode) {
> case BPF_JEQ:
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index b510174..162d497 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -8569,6 +8569,101 @@ static struct bpf_test tests[] = {
> .flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
> },
> {
> + "check deducing bounds from const, 1",
> + .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, 2",
> + .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, 3",
> + .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, 4",
> + .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, 5",
> + .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, 6",
> + .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, 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_0, BPF_REG_1),
> + BPF_EXIT_INSN(),
> + },
> + .result = REJECT,
> + .errstr = "R0 tried to subtract pointer from scalar",
> + },
> + {
> + "check deducing bounds from const, 8",
> + .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 = {
> BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),
> --
> 2.9.5
>
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