Re: [PATCH bpf v3 1/5] bpf: Reject key-less BTF for hash maps

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 8/31/26 4:01 AM, Jiayuan Chen wrote:
> map_check_btf() allows a key-less BTF (btf_key_type_id == 0) only for
> maps that have a ->map_check_btf callback, and leaves the actual
> decision to that callback. Hash maps used to have no ->map_check_btf,
> so a key-less BTF was rejected outright.
> 
> That changed when htab and rhtab gained a ->map_check_btf to register a
> dtor - htab in commit 1df97a7453ee ("bpf: Register dtor for freeing
> special fields") and rhtab in commit 6905f8601298 ("bpf: Allow special
> fields in resizable hashtab"). Neither looks at the key, so a key-less
> hash map now passes map_check_btf() and gets created. Reading it back
> through bpffs feeds the key type_id 0 into btf_type_seq_show();
> btf_type_by_id() returns the void type, kind_ops[BTF_KIND_UNKN] is NULL,
> and btf_type_show() dereferences it:
> 
> KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
> RIP: 0010:btf_type_show+0x223/0x2e0 kernel/bpf/btf.c:8232
> Call Trace:
>  <TASK>
>  btf_type_seq_show_flags+0xca/0x120 kernel/bpf/btf.c:8250
>  htab_map_seq_show_elem+0x12e/0x350 kernel/bpf/hashtab.c:1669
>  map_seq_show+0x13d/0x1e0 kernel/bpf/inode.c:293
>  seq_read_iter+0x93f/0x1270 fs/seq_file.c:196
>  seq_read+0x344/0x4d0 fs/seq_file.c:163
>  vfs_read+0x1e4/0xb40 fs/read_write.c:572
>  __x64_sys_pread64+0x1eb/0x250 fs/read_write.c:769
>  do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>  </TASK>
> 
> Reject a key-less BTF in htab_map_check_btf() and rhtab_map_check_btf(),
> restoring the previous behavior.
> 
> Fixes: 1df97a7453ee ("bpf: Register dtor for freeing special fields")
> Fixes: 6905f8601298 ("bpf: Allow special fields in resizable hashtab")
> Reported-by: syzbot+37b56485bbbf90ad8489@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6a8f4e88.27659fcc.2ceef7.0008.GAE@google.com/T/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  kernel/bpf/hashtab.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index d40cb5dd446c..43123424183c 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -530,6 +530,10 @@ static int htab_map_check_btf(struct bpf_map *map, const struct btf *btf,
>  {
>  	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
>  
> +	/* Hash maps have no use for a key-less BTF. */

nit: I think this comment is unnecessary in both places.

Other than that the fix makes sense to me, thanks.

Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>

> +	if (btf_type_is_void(key_type))
> +		return -EINVAL;
> +
>  	if (htab_is_prealloc(htab))
>  		return 0;
>  	/*
> @@ -3110,6 +3114,10 @@ static int rhtab_map_check_btf(struct bpf_map *map, const struct btf *btf,
>  {
>  	struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map);
>  
> +	/* Hash maps have no use for a key-less BTF. */
> +	if (btf_type_is_void(key_type))
> +		return -EINVAL;
> +
>  	return bpf_ma_set_dtor(map, &rhtab->ma, rhtab_mem_dtor);
>  }
>