On Mon, 10 Dec 2018 11:35:12 -0800, Jakub Kicinski wrote:
> Currently for liveness and state pruning the register parentage
> chains don't include states of the callee. This makes some sense
> as the callee can't access those registers. However, this means
> that READs done after the callee returns will not propagate into
> the states of the callee. Callee will then perform pruning
> disregarding differences in caller state.
>
> Example:
>
> 0: (85) call bpf_user_rnd_u32
> 1: (b7) r8 = 0
> 2: (55) if r0 != 0x0 goto pc+1
> 3: (b7) r8 = 1
> 4: (bf) r1 = r8
> 5: (85) call pc+4
> 6: (15) if r8 == 0x1 goto pc+1
> 7: (05) *(u64 *)(r9 - 8) = r3
> 8: (b7) r0 = 0
> 9: (95) exit
>
> 10: (15) if r1 == 0x0 goto pc+0
> 11: (95) exit
>
> Here we acquire unknown state with call to get_random() [1]. Then
> we store this random state in r8 (either 0 or 1) [1 - 3], and make
> a call on line 5. Callee does nothing but a trivial conditional
> jump (to create a pruning point). Upon return caller checks the
> state of r8 and either performs an unsafe read or not.
>
> Verifier will first explore the path with r8 == 1, creating a pruning
> point at [11]. The parentage chain for r8 will include only callers
> states so once verifier reaches [6] it will mark liveness only on states
> in the caller, and not [11]. Now when verifier walks the paths with
> r8 == 0 it will reach [11] and since REG_LIVE_READ on r8 was not
> propagated there it will prune the walk entirely (stop walking
> the entire program, not just the callee). Since [6] was never walked
> with r8 == 0, [7] will be considered dead and replaced with "goto -1"
> causing hang at runtime.
>
> This patch weaves the callee's explored states onto the callers
> parentage chain.
>
> v1: don't unnecessarily link caller saved regs (Jiong)
>
> Fixes: f4d7e40a5b71 ("bpf: introduce function calls (verification)")
> Reported-by: David Beckett <david.beck...@netronome.com>
> Signed-off-by: Jakub Kicinski <jakub.kicin...@netronome.com>
> Reviewed-by: Jiong Wang <jiong.w...@netronome.com>
> ---
> resend with netdev included
And CC Ed, sorry..
> kernel/bpf/verifier.c | 10 +++++++-
> tools/testing/selftests/bpf/test_verifier.c | 28 +++++++++++++++++++++
> 2 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index fc760d00a38c..60d57d9d3663 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5114,11 +5114,19 @@ static int is_state_visited(struct bpf_verifier_env
> *env, int insn_idx)
> for (i = 0; i < BPF_REG_FP; i++)
> cur->frame[cur->curframe]->regs[i].live = REG_LIVE_NONE;
>
> - /* all stack frames are accessible from callee, clear them all */
> + /* connect new state to parentage chain:
> + * - all stack frames are accessible from callee
> + * - even though other stack frames' registers are not accessible
> + * liveness must propagate through the callees' states otherwise
> + * not knowing the liveness callee may prune caller
> + */
> for (j = 0; j <= cur->curframe; j++) {
> struct bpf_func_state *frame = cur->frame[j];
> struct bpf_func_state *newframe = new->frame[j];
>
> + for (i = BPF_REG_6; i < BPF_REG_FP; i++)
> + frame->regs[i].parent = &newframe->regs[i];
> +
> for (i = 0; i < frame->allocated_stack / BPF_REG_SIZE; i++) {
> frame->stack[i].spilled_ptr.live = REG_LIVE_NONE;
> frame->stack[i].spilled_ptr.parent =
> diff --git a/tools/testing/selftests/bpf/test_verifier.c
> b/tools/testing/selftests/bpf/test_verifier.c
> index df6f751cc1e8..373611ae9f52 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -13915,6 +13915,34 @@ static struct bpf_test tests[] = {
> .result_unpriv = REJECT,
> .result = ACCEPT,
> },
> + {
> + "calls: cross frame pruning",
> + .insns = {
> + /* r8 = !!random();
> + * call pruner()
> + * if (r8)
> + * do something bad;
> + */
> + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
> + BPF_FUNC_get_prandom_u32),
> + BPF_MOV64_IMM(BPF_REG_8, 0),
> + BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
> + BPF_MOV64_IMM(BPF_REG_8, 1),
> + BPF_MOV64_REG(BPF_REG_1, BPF_REG_8),
> + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 1, 0, 4),
> + BPF_JMP_IMM(BPF_JEQ, BPF_REG_8, 1, 1),
> + BPF_LDX_MEM(BPF_B, BPF_REG_9, BPF_REG_1, 0),
> + BPF_MOV64_IMM(BPF_REG_0, 0),
> + BPF_EXIT_INSN(),
> + BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 0, 0),
> + BPF_EXIT_INSN(),
> + },
> + .prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
> + .errstr_unpriv = "function calls to other bpf functions are
> allowed for root only",
> + .result_unpriv = REJECT,
> + .errstr = "!read_ok",
> + .result = REJECT,
> + },
> };
>
> static int probe_filter_length(const struct bpf_insn *fp)
