[bpf-next,1/3] bpf: verifier: Do not extract constant map keys for irrelevant maps

Commit Message

Daniel Xu Feb. 1, 2025, 7:58 p.m. UTC

Previously, we were trying to extract constant map keys for all
bpf_map_lookup_elem(), regardless of map type. This is an issue if the
map has a u64 key and the value is very high, as it can be interpreted
as a negative signed value. This in turn is treated as an error value by
check_func_arg() which causes a valid program to be incorrectly
rejected.

Fix by only extracting constant map keys for relevant maps. See next
commit for an example via selftest.

Reported-by: Marc Hartmayer <[e-mail]>
Reported-by: Ilya Leoshkevich <[e-mail]>
Tested-by: Marc Hartmayer <[e-mail]>
Signed-off-by: Daniel Xu <[e-mail]>
---
 kernel/bpf/verifier.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Comments

Eduard Zingerman Feb. 3, 2025, 6:45 p.m. UTC | #1

On Sat, 2025-02-01 at 12:58 -0700, Daniel Xu wrote:
> Previously, we were trying to extract constant map keys for all
> bpf_map_lookup_elem(), regardless of map type. This is an issue if the
> map has a u64 key and the value is very high, as it can be interpreted
> as a negative signed value. This in turn is treated as an error value by
> check_func_arg() which causes a valid program to be incorrectly
> rejected.
> 
> Fix by only extracting constant map keys for relevant maps. See next
> commit for an example via selftest.
> 
> Reported-by: Marc Hartmayer <[e-mail]>
> Reported-by: Ilya Leoshkevich <[e-mail]>
> Tested-by: Marc Hartmayer <[e-mail]>
> Signed-off-by: Daniel Xu <[e-mail]>

Acked-by: Eduard Zingerman <[e-mail]>

Nit:
  would be good if commit message said something along the lines:
  ... the fix works because nullness elision is only allowed for
      {PERCPU_}ARRAY maps, and keys for these are within u32 range ...

> ---
>  kernel/bpf/verifier.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9971c03adfd5..e9176a5ce215 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9206,6 +9206,8 @@ static s64 get_constant_map_key(struct bpf_verifier_env *env,
>  	return reg->var_off.value;
>  }
>  
> +static bool can_elide_value_nullness(enum bpf_map_type type);
> +
>  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>  			  struct bpf_call_arg_meta *meta,
>  			  const struct bpf_func_proto *fn,
> @@ -9354,9 +9356,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>  		err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL);
>  		if (err)
>  			return err;
> -		meta->const_map_key = get_constant_map_key(env, reg, key_size);
> -		if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
> -			return meta->const_map_key;
> +		if (can_elide_value_nullness(meta->map_ptr->map_type)) {
> +			meta->const_map_key = get_constant_map_key(env, reg, key_size);
> +			if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
> +				return meta->const_map_key;
> +		}
>  		break;
>  	case ARG_PTR_TO_MAP_VALUE:
>  		if (type_may_be_null(arg_type) && register_is_null(reg))

Daniel Xu Feb. 4, 2025, 9:09 a.m. UTC | #2

On Mon, Feb 03, 2025 at 10:45:35AM -0800, Eduard Zingerman wrote:
> On Sat, 2025-02-01 at 12:58 -0700, Daniel Xu wrote:
> > Previously, we were trying to extract constant map keys for all
> > bpf_map_lookup_elem(), regardless of map type. This is an issue if the
> > map has a u64 key and the value is very high, as it can be interpreted
> > as a negative signed value. This in turn is treated as an error value by
> > check_func_arg() which causes a valid program to be incorrectly
> > rejected.
> > 
> > Fix by only extracting constant map keys for relevant maps. See next
> > commit for an example via selftest.
> > 
> > Reported-by: Marc Hartmayer <[e-mail]>
> > Reported-by: Ilya Leoshkevich <[e-mail]>
> > Tested-by: Marc Hartmayer <[e-mail]>
> > Signed-off-by: Daniel Xu <[e-mail]>
> 
> Acked-by: Eduard Zingerman <[e-mail]>
> 
> Nit:
>   would be good if commit message said something along the lines:
>   ... the fix works because nullness elision is only allowed for
>       {PERCPU_}ARRAY maps, and keys for these are within u32 range ...

Ack. I can respin if necessary. Otherwise, here's the edited commit msg:

    bpf: verifier: Do not extract constant map keys for irrelevant maps

    Previously, we were trying to extract constant map keys for all
    bpf_map_lookup_elem(), regardless of map type. This is an issue if the
    map has a u64 key and the value is very high, as it can be interpreted
    as a negative signed value. This in turn is treated as an error value by
    check_func_arg() which causes a valid program to be incorrectly
    rejected.

    Fix by only extracting constant map keys for relevant maps. This fix
    works because nullness elision is only allowed for {PERCPU_}ARRAY maps,
    and keys for these are within u32 range. See next commit for an example
    via selftest.

    Acked-by: Eduard Zingerman <[e-mail]>
    Reported-by: Marc Hartmayer <[e-mail]>
    Reported-by: Ilya Leoshkevich <[e-mail]>
    Tested-by: Marc Hartmayer <[e-mail]>

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9971c03adfd5..e9176a5ce215 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9206,6 +9206,8 @@  static s64 get_constant_map_key(struct bpf_verifier_env *env,
 	return reg->var_off.value;
 }
 
+static bool can_elide_value_nullness(enum bpf_map_type type);
+
 static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			  struct bpf_call_arg_meta *meta,
 			  const struct bpf_func_proto *fn,
@@ -9354,9 +9356,11 @@  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL);
 		if (err)
 			return err;
-		meta->const_map_key = get_constant_map_key(env, reg, key_size);
-		if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
-			return meta->const_map_key;
+		if (can_elide_value_nullness(meta->map_ptr->map_type)) {
+			meta->const_map_key = get_constant_map_key(env, reg, key_size);
+			if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
+				return meta->const_map_key;
+		}
 		break;
 	case ARG_PTR_TO_MAP_VALUE:
 		if (type_may_be_null(arg_type) && register_is_null(reg))