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Message-ID: <CAADnVQJrjPynzVZTDvDh7qosBVFO8+iKEKDbC4=yK+4HVZ6Tng@mail.gmail.com>
Date: Fri, 4 Sep 2020 17:10:18 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Yonghong Song <yhs@...com>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/2] bpf: permit map_ptr arithmetic with opcode
add and offset 0
On Fri, Sep 4, 2020 at 5:08 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Fri, Sep 4, 2020 at 4:20 PM Yonghong Song <yhs@...com> wrote:
> >
> >
> >
> > On 9/4/20 1:30 PM, Andrii Nakryiko wrote:
> > > On Fri, Sep 4, 2020 at 12:49 PM Yonghong Song <yhs@...com> wrote:
> > >>
> > >> Commit 41c48f3a98231 ("bpf: Support access
> > >> to bpf map fields") added support to access map fields
> > >> with CORE support. For example,
> > >>
> > >> struct bpf_map {
> > >> __u32 max_entries;
> > >> } __attribute__((preserve_access_index));
> > >>
> > >> struct bpf_array {
> > >> struct bpf_map map;
> > >> __u32 elem_size;
> > >> } __attribute__((preserve_access_index));
> > >>
> > >> struct {
> > >> __uint(type, BPF_MAP_TYPE_ARRAY);
> > >> __uint(max_entries, 4);
> > >> __type(key, __u32);
> > >> __type(value, __u32);
> > >> } m_array SEC(".maps");
> > >>
> > >> SEC("cgroup_skb/egress")
> > >> int cg_skb(void *ctx)
> > >> {
> > >> struct bpf_array *array = (struct bpf_array *)&m_array;
> > >>
> > >> /* .. array->map.max_entries .. */
> > >> }
> > >>
> > >> In kernel, bpf_htab has similar structure,
> > >>
> > >> struct bpf_htab {
> > >> struct bpf_map map;
> > >> ...
> > >> }
> > >>
> > >> In the above cg_skb(), to access array->map.max_entries, with CORE, the clang will
> > >> generate two builtin's.
> > >> base = &m_array;
> > >> /* access array.map */
> > >> map_addr = __builtin_preserve_struct_access_info(base, 0, 0);
> > >> /* access array.map.max_entries */
> > >> max_entries_addr = __builtin_preserve_struct_access_info(map_addr, 0, 0);
> > >> max_entries = *max_entries_addr;
> > >>
> > >> In the current llvm, if two builtin's are in the same function or
> > >> in the same function after inlining, the compiler is smart enough to chain
> > >> them together and generates like below:
> > >> base = &m_array;
> > >> max_entries = *(base + reloc_offset); /* reloc_offset = 0 in this case */
> > >> and we are fine.
> > >>
> > >> But if we force no inlining for one of functions in test_map_ptr() selftest, e.g.,
> > >> check_default(), the above two __builtin_preserve_* will be in two different
> > >> functions. In this case, we will have code like:
> > >> func check_hash():
> > >> reloc_offset_map = 0;
> > >> base = &m_array;
> > >> map_base = base + reloc_offset_map;
> > >> check_default(map_base, ...)
> > >> func check_default(map_base, ...):
> > >> max_entries = *(map_base + reloc_offset_max_entries);
> > >>
> > >> In kernel, map_ptr (CONST_PTR_TO_MAP) does not allow any arithmetic.
> > >> The above "map_base = base + reloc_offset_map" will trigger a verifier failure.
> > >> ; VERIFY(check_default(&hash->map, map));
> > >> 0: (18) r7 = 0xffffb4fe8018a004
> > >> 2: (b4) w1 = 110
> > >> 3: (63) *(u32 *)(r7 +0) = r1
> > >> R1_w=invP110 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0
> > >> ; VERIFY_TYPE(BPF_MAP_TYPE_HASH, check_hash);
> > >> 4: (18) r1 = 0xffffb4fe8018a000
> > >> 6: (b4) w2 = 1
> > >> 7: (63) *(u32 *)(r1 +0) = r2
> > >> R1_w=map_value(id=0,off=0,ks=4,vs=8,imm=0) R2_w=invP1 R7_w=map_value(id=0,off=4,ks=4,vs=8,imm=0) R10=fp0
> > >> 8: (b7) r2 = 0
> > >> 9: (18) r8 = 0xffff90bcb500c000
> > >> 11: (18) r1 = 0xffff90bcb500c000
> > >> 13: (0f) r1 += r2
> > >> R1 pointer arithmetic on map_ptr prohibited
> > >>
> > >> To fix the issue, let us permit map_ptr + 0 arithmetic which will
> > >> result in exactly the same map_ptr.
> > >>
> > >> Signed-off-by: Yonghong Song <yhs@...com>
> > >> ---
> > >> kernel/bpf/verifier.c | 3 +++
> > >> 1 file changed, 3 insertions(+)
> > >>
> > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > >> index b4e9c56b8b32..92aa985e99df 100644
> > >> --- a/kernel/bpf/verifier.c
> > >> +++ b/kernel/bpf/verifier.c
> > >> @@ -5317,6 +5317,9 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
> > >> dst, reg_type_str[ptr_reg->type]);
> > >> return -EACCES;
> > >> case CONST_PTR_TO_MAP:
> > >> + if (known && smin_val == 0 && opcode == BPF_ADD)
> > >
> > > does smin_val imply that var_off is strictly zero? if that's the case,
> > > can you please leave a comment stating this clearly, it's hard to tell
> > > if that's enough of a check.
> >
> > It should be, if register state is maintained properly, the following
> > function (or its functionality) should have been called.
> >
> > static void __update_reg64_bounds(struct bpf_reg_state *reg)
> > {
> > /* min signed is max(sign bit) | min(other bits) */
> > reg->smin_value = max_t(s64, reg->smin_value,
> > reg->var_off.value | (reg->var_off.mask
> > & S64_MIN));
> > /* max signed is min(sign bit) | max(other bits) */
> > reg->smax_value = min_t(s64, reg->smax_value,
> > reg->var_off.value | (reg->var_off.mask
> > & S64_MAX));
> > reg->umin_value = max(reg->umin_value, reg->var_off.value);
> > reg->umax_value = min(reg->umax_value,
> > reg->var_off.value | reg->var_off.mask);
> > }
> >
> > for scalar constant, reg->var_off.mask should be 0. so we will have
> > reg->smin_value = reg->smax_value = (s64)reg->var_off.value.
> >
> > The smin_val is also used below, e.g., BPF_ADD, for a known value.
> > That is why I am using smin_val here.
> >
> > Will add a comment and submit v2.
>
> it would be way-way more obvious (and reliable in the long run,
> probably) if you just used (known && reg->var_off.value == 0). or just
> tnum_equals_const(reg->var_off, 0)?
Pls dont. smin_val == 0 is a standard way to do this.
Just check all other places in this function and everywhere else.
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