Re: [PATCH bpf-next v3 07/11] bpf: Fix a false rejection caused by AND operation

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sun, Apr 28, 2024 at 8:15 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> On 4/27/2024 4:36 AM, Andrii Nakryiko wrote:
> > On Tue, Apr 23, 2024 at 7:26 PM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
> >>
> >> On 4/24/2024 5:55 AM, Yonghong Song wrote:
> >>>
> >>> On 4/20/24 1:33 AM, Xu Kuohai wrote:
> >>>> On 4/20/2024 7:00 AM, Eduard Zingerman wrote:
> >>>>> On Thu, 2024-04-11 at 20:27 +0800, Xu Kuohai wrote:
> >>>>>> From: Xu Kuohai <xukuohai@huawei.com>
> >>>>>>
> >>>>>> With lsm return value check, the no-alu32 version test_libbpf_get_fd_by_id_opts
> >>>>>> is rejected by the verifier, and the log says:
> >>>>>>
> >>>>>>     0: R1=ctx() R10=fp0
> >>>>>>     ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
> >>>>>>     0: (b7) r0 = 0                        ; R0_w=0
> >>>>>>     1: (79) r2 = *(u64 *)(r1 +0)
> >>>>>>     func 'bpf_lsm_bpf_map' arg0 has btf_id 916 type STRUCT 'bpf_map'
> >>>>>>     2: R1=ctx() R2_w=trusted_ptr_bpf_map()
> >>>>>>     ; if (map != (struct bpf_map *)&data_input) @ test_libbpf_get_fd_by_id_opts.c:29
> >>>>>>     2: (18) r3 = 0xffff9742c0951a00       ; R3_w=map_ptr(map=data_input,ks=4,vs=4)
> >>>>>>     4: (5d) if r2 != r3 goto pc+4         ; R2_w=trusted_ptr_bpf_map() R3_w=map_ptr(map=data_input,ks=4,vs=4)
> >>>>>>     ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
> >>>>>>     5: (79) r0 = *(u64 *)(r1 +8)          ; R0_w=scalar() R1=ctx()
> >>>>>>     ; if (fmode & FMODE_WRITE) @ test_libbpf_get_fd_by_id_opts.c:32
> >>>>>>     6: (67) r0 <<= 62                     ; R0_w=scalar(smax=0x4000000000000000,umax=0xc000000000000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xc000000000000000))
> >>>>>>     7: (c7) r0 s>>= 63                    ; R0_w=scalar(smin=smin32=-1,smax=smax32=0)
> >>>>>>     ;  @ test_libbpf_get_fd_by_id_opts.c:0
> >>>>>>     8: (57) r0 &= -13                     ; R0_w=scalar(smax=0x7ffffffffffffff3,umax=0xfffffffffffffff3,smax32=0x7ffffff3,umax32=0xfffffff3,var_off=(0x0; 0xfffffffffffffff3))
> >>>>>>     ; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
> >>>>>>     9: (95) exit
> >
> > [...]
> >
> >>
> >>       As suggested by Eduard, this patch makes a special case for source
> >>       or destination register of '&=' operation being in range [-1, 0].
> >>
> >>       Meaning that one of the '&=' operands is either:
> >>       - all ones, in which case the counterpart is the result of the operation;
> >>       - all zeros, in which case zero is the result of the operation.
> >>
> >>       And MIN and MAX values could be derived based on above two observations.
> >>
> >>       [0] https://lore.kernel.org/bpf/e62e2971301ca7f2e9eb74fc500c520285cad8f5.camel@gmail.com/
> >>       [1] https://github.com/llvm/llvm-project/blob/4523a267829c807f3fc8fab8e5e9613985a51565/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> >>
> >>       Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
> >>       Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> >>
> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >> index 640747b53745..30c551d39329 100644
> >> --- a/kernel/bpf/verifier.c
> >> +++ b/kernel/bpf/verifier.c
> >> @@ -13374,6 +13374,24 @@ static void scalar32_min_max_and(struct bpf_reg_state *dst_reg,
> >>           dst_reg->u32_min_value = var32_off.value;
> >>           dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
> >>
> >> +       /* Special case: src_reg is known and dst_reg is in range [-1, 0] */
> >> +       if (src_known &&
> >> +               dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0 &&
> >> +               dst_reg->smin_value == -1 && dst_reg->smax_value == 0) {
> >
> > please keep if () condition aligned across multiple lines, it's super
> > confusing this way
> >
>
> OK, will update the align style
>
> >> +               dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
> >> +               dst_reg->s32_max_value = max_t(s32, src_reg->s32_min_value, 0);
> >
> > do we need to update tnum parts as well (or reset and re-derive, probably)?
> >
> > btw, can't we support src being a range here? the idea is that dst_reg
> > either all ones or all zeros. For and it means that it either stays
> > all zero, or will be *exactly equal* to src, right? So I think the
> > logic would be:
> >
> > a) if [s32_min, s32_max] is on the same side of zero, then resulting
> > range would be [min(s32_min, 0), max(s32_max, 0)], just like you have
> > here
> >
> > b) if [s32_min, s32_max] contains zero, then resulting range will be
> > exactly [s32_min, s32_max]
> >
> > Or did I make a mistake above?
> >
>
> Totally agree, the AND of any set with the range [-1,0] is equivalent
> to adding number 0 to the set!
>
> Based on this observation, I've rewritten the patch as follows.
>
> diff --git a/include/linux/tnum.h b/include/linux/tnum.h
> index 3c13240077b8..5e795d728b9f 100644
> --- a/include/linux/tnum.h
> +++ b/include/linux/tnum.h
> @@ -52,6 +52,9 @@ struct tnum tnum_mul(struct tnum a, struct tnum b);
>   /* Return a tnum representing numbers satisfying both @a and @b */
>   struct tnum tnum_intersect(struct tnum a, struct tnum b);
>
> +/* Return a tnum representing numbers satisfying either @a or @b */
> +struct tnum tnum_union(struct tnum a, struct tnum b);
> +
>   /* Return @a with all but the lowest @size bytes cleared */
>   struct tnum tnum_cast(struct tnum a, u8 size);
>
> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> index 9dbc31b25e3d..9d4480a683ca 100644
> --- a/kernel/bpf/tnum.c
> +++ b/kernel/bpf/tnum.c
> @@ -150,6 +150,29 @@ struct tnum tnum_intersect(struct tnum a, struct tnum b)
>          return TNUM(v & ~mu, mu);
>   }
>
> +/*
> + * Each bit has 3 states: unkown, known 0, known 1. If using x to represent
> + * unknown state, the result of the union of two bits is as follows:
> + *
> + *         | x    0    1
> + *    -----+------------
> + *     x   | x    x    x
> + *     0   | x    0    x
> + *     1   | x    x    1
> + *
> + * For tnum a and b, only the bits that are both known 0 or known 1 in a
> + * and b are known in the result of union a and b.
> + */
> +struct tnum tnum_union(struct tnum a, struct tnum b)
> +{
> +       u64 v0, v1, mu;
> +
> +       mu = a.mask | b.mask; // unkown bits either in a or b
> +       v1 = (a.value & b.value) & ~mu; // "known 1" bits in both a and b
> +       v0 = (~a.value & ~b.value) & ~mu; // "known 0" bits in both a and b

no C++-style comments, please

> +       return TNUM(v1, mu | ~(v0 | v1));
> +}
> +

I've CC'ed Edward, hopefully he can take a look as well. Please CC him
on future patches touching tnum as well.

>   struct tnum tnum_cast(struct tnum a, u8 size)
>   {
>          a.value &= (1ULL << (size * 8)) - 1;
>   {
>          a.value &= (1ULL << (size * 8)) - 1;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8f0f2e21699e..b69c89bc5cfc 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13478,6 +13478,28 @@ static void scalar32_min_max_and(struct bpf_reg_state *dst_reg,
>                  return;
>          }
>
> +       /* Special case: dst_reg is in range [-1, 0] */
> +       if (dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0) {
> +               var32_off = tnum_union(src_reg->var_off, tnum_const(0));
> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
> +               dst_reg->u32_min_value = var32_off.value;
> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);

can you explain the logic behing u32 min/max updates, especially that
we use completely different values for min/max and it's not clear why
u32_min <= u32_max invariant will always hold. Same below

> +               dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
> +               dst_reg->s32_max_value = max_t(s32, src_reg->s32_max_value, 0);
> +               return;
> +       }
> +
> +       /* Special case: src_reg is in range [-1, 0] */
> +       if (src_reg->s32_min_value == -1 && src_reg->s32_max_value == 0) {
> +               var32_off = tnum_union(dst_reg->var_off, tnum_const(0));
> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
> +               dst_reg->u32_min_value = var32_off.value;
> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
> +               dst_reg->s32_min_value = min_t(s32, dst_reg->s32_min_value, 0);
> +               dst_reg->s32_max_value = max_t(s32, dst_reg->s32_max_value, 0);
> +               return;
> +       }
> +
>          /* We get our minimum from the var_off, since that's inherently
>           * bitwise.  Our maximum is the minimum of the operands' maxima.
>           */
> @@ -13508,6 +13530,26 @@ static void scalar_min_max_and(struct bpf_reg_state *dst_reg,
>                  return;
>          }
>
> +       /* Special case: dst_reg is in range [-1, 0] */
> +       if (dst_reg->smin_value == -1 && dst_reg->smax_value == 0) {
> +               dst_reg->var_off = tnum_union(src_reg->var_off, tnum_const(0));
> +               dst_reg->umin_value = dst_reg->var_off.value;
> +               dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
> +               dst_reg->smin_value = min_t(s64, src_reg->smin_value, 0);
> +               dst_reg->smax_value = max_t(s64, src_reg->smax_value, 0);
> +               return;
> +       }
> +
> +       /* Special case: src_reg is in range [-1, 0] */
> +       if (src_reg->smin_value == -1 && src_reg->smax_value == 0) {
> +               dst_reg->var_off = tnum_union(dst_reg->var_off, tnum_const(0));
> +               dst_reg->umin_value = dst_reg->var_off.value;
> +               dst_reg->umax_value = min(dst_reg->umax_value, umax_val);
> +               dst_reg->smin_value = min_t(s64, dst_reg->smin_value, 0);
> +               dst_reg->smax_value = max_t(s64, dst_reg->smax_value, 0);
> +               return;
> +       }
> +
>
> >> +               return;
> >> +       }
> >> +
> >> +       /* Special case: dst_reg is known and src_reg is in range [-1, 0] */
> >> +       if (dst_known &&
> >> +               src_reg->s32_min_value == -1 && src_reg->s32_max_value == 0 &&
> >> +               src_reg->smin_value == -1 && src_reg->smax_value == 0) {
> >> +               dst_reg->s32_min_value = min_t(s32, dst_reg->s32_min_value, 0);
> >> +               dst_reg->s32_max_value = max_t(s32, dst_reg->s32_min_value, 0);
> >> +               return;
> >> +       }
> >> +
> >>           /* Safe to set s32 bounds by casting u32 result into s32 when u32
> >>            * doesn't cross sign boundary. Otherwise set s32 bounds to unbounded.
> >>            */
> >
> > [...]
> >
>