Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()

[Song Liu <song@kernel.org>]

On Tue, Jul 6, 2021 at 8:53 PM He Fengqing <hefengqing@huawei.com> wrote:
>
> In bpf_patch_insn_data, env->prog was input parameter of
> bpf_patch_insn_single function. bpf_patch_insn_single call
> bpf_prog_realloc to realloc ebpf prog. When we need to malloc new prog,
> bpf_prog_realloc will free the old prog, in this scenery is the
> env->prog.
> Then bpf_patch_insn_data function call adjust_insn_aux_data function, if
> adjust_insn_aux_data function return error, bpf_patch_insn_data will
> return NULL.
> In bpf_check->convert_ctx_accesses->bpf_patch_insn_data call chain, if
> bpf_patch_insn_data return NULL, env->prog has been freed in
> bpf_prog_realloc, then bpf_check will use the freed env->prog.

Besides "what is the bug", please also describe "how to fix it". For example,
add "Fix it by adding a free_old argument to bpf_prog_realloc(), and ...".
Also, for the subject of 0/3, it is better to say "fix potential
memory leak and ...".

>
> Signed-off-by: He Fengqing <hefengqing@huawei.com>
> ---
>  include/linux/filter.h |  2 +-
>  kernel/bpf/core.c      |  9 ++++---
>  kernel/bpf/verifier.c  | 53 ++++++++++++++++++++++++++++++++----------
>  net/core/filter.c      |  2 +-
>  4 files changed, 49 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index f39e008a377d..ec11a5ae92c2 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -881,7 +881,7 @@ void bpf_prog_jit_attempt_done(struct bpf_prog *prog);
>  struct bpf_prog *bpf_prog_alloc(unsigned int size, gfp_t gfp_extra_flags);
>  struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flags);
>  struct bpf_prog *bpf_prog_realloc(struct bpf_prog *fp_old, unsigned int size,
> -                                 gfp_t gfp_extra_flags);
> +                                 gfp_t gfp_extra_flags, bool free_old);
>  void __bpf_prog_free(struct bpf_prog *fp);
>
>  static inline void bpf_prog_clone_free(struct bpf_prog *fp)
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 49b0311f48c1..e5616bb1665b 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -218,7 +218,7 @@ void bpf_prog_fill_jited_linfo(struct bpf_prog *prog,
>  }
>
>  struct bpf_prog *bpf_prog_realloc(struct bpf_prog *fp_old, unsigned int size,
> -                                 gfp_t gfp_extra_flags)
> +                                 gfp_t gfp_extra_flags, bool free_old)
>  {
>         gfp_t gfp_flags = GFP_KERNEL_ACCOUNT | __GFP_ZERO | gfp_extra_flags;
>         struct bpf_prog *fp;
> @@ -238,7 +238,8 @@ struct bpf_prog *bpf_prog_realloc(struct bpf_prog *fp_old, unsigned int size,
>                 /* We keep fp->aux from fp_old around in the new
>                  * reallocated structure.
>                  */
> -               bpf_prog_clone_free(fp_old);
> +               if (free_old)
> +                       bpf_prog_clone_free(fp_old);
>         }
>
>         return fp;
> @@ -456,7 +457,7 @@ struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
>          * last page could have large enough tailroom.
>          */
>         prog_adj = bpf_prog_realloc(prog, bpf_prog_size(insn_adj_cnt),
> -                                   GFP_USER);
> +                                   GFP_USER, false);
>         if (!prog_adj)
>                 return ERR_PTR(-ENOMEM);
>
> @@ -1150,6 +1151,8 @@ struct bpf_prog *bpf_jit_blind_constants(struct bpf_prog *prog)
>                         return tmp;
>                 }
>
> +               if (tmp != clone)
> +                       bpf_prog_clone_free(clone);
>                 clone = tmp;
>                 insn_delta = rewritten - 1;
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 41109f49b724..e75b933f69e4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11855,7 +11855,10 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
>                 new_prog = bpf_patch_insn_data(env, adj_idx, patch, patch_len);
>                 if (!new_prog)
>                         return -ENOMEM;
> -               env->prog = new_prog;
> +               if (new_prog != env->prog) {
> +                       bpf_prog_clone_free(env->prog);
> +                       env->prog = new_prog;
> +               }

Can we move this check into bpf_patch_insn_data()?

>                 insns = new_prog->insnsi;
>                 aux = env->insn_aux_data;
>                 delta += patch_len - 1;
[...]

> diff --git a/net/core/filter.c b/net/core/filter.c
> index d70187ce851b..8a8d1a3ba5c2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1268,7 +1268,7 @@ static struct bpf_prog *bpf_migrate_filter(struct bpf_prog *fp)
>
>         /* Expand fp for appending the new filter representation. */
>         old_fp = fp;
> -       fp = bpf_prog_realloc(old_fp, bpf_prog_size(new_len), 0);
> +       fp = bpf_prog_realloc(old_fp, bpf_prog_size(new_len), 0, true);

Can we add some logic here and not add free_old to bpf_prog_realloc()?

Thanks,
Song