Re: [PATCH bpf-next v1 8/9] bpf: Add MEM_RDONLY for helper args that are pointers to rdonly mem.

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

On Mon, Dec 6, 2021 at 3:22 PM Hao Luo <haoluo@google.com> wrote:
>
> Some helper functions may modify its arguments, for example,
> bpf_d_path, bpf_get_stack etc. Previously, their argument types
> were marked as ARG_PTR_TO_MEM, which is compatible with read-only
> mem types, such as PTR_TO_RDONLY_BUF. Therefore it's legitimate
> to modify a read-only memory by passing it into one of such helper
> functions.
>
> This patch tags the bpf_args compatible with immutable memory with
> MEM_RDONLY flag. The arguments that don't have this flag will be
> only compatible with mutable memory types, preventing the helper
> from modifying a read-only memory. The bpf_args that have
> MEM_RDONLY are compatible with both mutable memory and immutable
> memory.
>
> Signed-off-by: Hao Luo <haoluo@google.com>
> ---
>  include/linux/bpf.h      |  4 ++-
>  kernel/bpf/btf.c         |  2 +-
>  kernel/bpf/cgroup.c      |  2 +-
>  kernel/bpf/helpers.c     |  8 ++---
>  kernel/bpf/ringbuf.c     |  2 +-
>  kernel/bpf/syscall.c     |  2 +-
>  kernel/bpf/verifier.c    | 14 +++++++--
>  kernel/trace/bpf_trace.c | 26 ++++++++--------
>  net/core/filter.c        | 64 ++++++++++++++++++++--------------------
>  9 files changed, 67 insertions(+), 57 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 03418ab3f416..5151d6b1f392 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -311,7 +311,9 @@ enum bpf_type_flag {
>         /* PTR may be NULL. */
>         PTR_MAYBE_NULL          = BIT(0 + BPF_BASE_TYPE_BITS),
>
> -       /* MEM is read-only. */
> +       /* MEM is read-only. When applied on bpf_arg, it indicates the arg is
> +        * compatible with both mutable and immutable memory.
> +        */
>         MEM_RDONLY              = BIT(1 + BPF_BASE_TYPE_BITS),
>
>         __BPF_TYPE_LAST_FLAG    = MEM_RDONLY,
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 7adc099bb24b..e09b5a7bfdc3 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6350,7 +6350,7 @@ const struct bpf_func_proto bpf_btf_find_by_name_kind_proto = {
>         .func           = bpf_btf_find_by_name_kind,
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
> -       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg1_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg2_type      = ARG_CONST_SIZE,
>         .arg3_type      = ARG_ANYTHING,
>         .arg4_type      = ARG_ANYTHING,
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 2ca643af9a54..b8fe671af13c 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1789,7 +1789,7 @@ static const struct bpf_func_proto bpf_sysctl_set_new_value_proto = {
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_CTX,
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg3_type      = ARG_CONST_SIZE,
>  };
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index a5e349c9d3e3..66b466903a4e 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -530,7 +530,7 @@ const struct bpf_func_proto bpf_strtol_proto = {
>         .func           = bpf_strtol,
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
> -       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg1_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg2_type      = ARG_CONST_SIZE,
>         .arg3_type      = ARG_ANYTHING,
>         .arg4_type      = ARG_PTR_TO_LONG,
> @@ -558,7 +558,7 @@ const struct bpf_func_proto bpf_strtoul_proto = {
>         .func           = bpf_strtoul,
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
> -       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg1_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg2_type      = ARG_CONST_SIZE,
>         .arg3_type      = ARG_ANYTHING,
>         .arg4_type      = ARG_PTR_TO_LONG,
> @@ -630,7 +630,7 @@ const struct bpf_func_proto bpf_event_output_data_proto =  {
>         .arg1_type      = ARG_PTR_TO_CTX,
>         .arg2_type      = ARG_CONST_MAP_PTR,
>         .arg3_type      = ARG_ANYTHING,
> -       .arg4_type      = ARG_PTR_TO_MEM,
> +       .arg4_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg5_type      = ARG_CONST_SIZE_OR_ZERO,
>  };
>
> @@ -1011,7 +1011,7 @@ const struct bpf_func_proto bpf_snprintf_proto = {
>         .arg1_type      = ARG_PTR_TO_MEM_OR_NULL,
>         .arg2_type      = ARG_CONST_SIZE_OR_ZERO,
>         .arg3_type      = ARG_PTR_TO_CONST_STR,
> -       .arg4_type      = ARG_PTR_TO_MEM_OR_NULL,
> +       .arg4_type      = ARG_PTR_TO_MEM | PTR_MAYBE_NULL | MEM_RDONLY,
>         .arg5_type      = ARG_CONST_SIZE_OR_ZERO,
>  };
>
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index 9e0c10c6892a..638d7fd7b375 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -444,7 +444,7 @@ const struct bpf_func_proto bpf_ringbuf_output_proto = {
>         .func           = bpf_ringbuf_output,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_CONST_MAP_PTR,
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>         .arg4_type      = ARG_ANYTHING,
>  };
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 50f96ea4452a..301afb44e47f 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4757,7 +4757,7 @@ static const struct bpf_func_proto bpf_sys_bpf_proto = {
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_ANYTHING,
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg3_type      = ARG_CONST_SIZE,
>  };
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 44af65f07a82..a7c015a7b52d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4966,6 +4966,7 @@ static int resolve_map_arg_type(struct bpf_verifier_env *env,
>         return 0;
>  }
>
> +

nit: unnecessary extra empty line?

>  struct bpf_reg_types {
>         const enum bpf_reg_type types[10];
>         u32 *btf_id;
> @@ -5012,7 +5013,6 @@ static const struct bpf_reg_types mem_types = {
>                 PTR_TO_MAP_VALUE,
>                 PTR_TO_MEM,
>                 PTR_TO_BUF,
> -               PTR_TO_BUF | MEM_RDONLY,
>         },
>  };
>
> @@ -5074,6 +5074,7 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
>         enum bpf_reg_type expected, type = reg->type;
>         const struct bpf_reg_types *compatible;
> +       u32 compatible_flags;
>         int i, j;
>
>         compatible = compatible_reg_types[base_type(arg_type)];
> @@ -5082,6 +5083,13 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
>                 return -EFAULT;
>         }
>
> +       /* If arg_type is tagged with MEM_RDONLY, it's compatible with both
> +        * RDONLY and non-RDONLY reg values. Therefore fold this flag before
> +        * comparison. PTR_MAYBE_NULL is similar.
> +        */
> +       compatible_flags = arg_type & (MEM_RDONLY | PTR_MAYBE_NULL);
> +       type &= ~compatible_flags;
> +

wouldn't

type &= ~MEM_RDONLY; /* clear read-only flag, if any */
type &= ~PTR_MAYBE_NULL; /* clear nullable flag, if any */

be cleaner and more straightforward?


>         for (i = 0; i < ARRAY_SIZE(compatible->types); i++) {
>                 expected = compatible->types[i];
>                 if (expected == NOT_INIT)

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