Re: [bug report] bpf: Fix a potential use-after-free of BTF object

[Anton Protopopov <a.s.protopopov@gmail.com>]

On 26/02/13 08:56AM, Dan Carpenter wrote:
> [ Smatch checking is paused while we raise funding. #SadFace
>   https://lore.kernel.org/all/aTaiGSbWZ9DJaGo7@stanley.mountain/ -dan ]
> 
> Hello Anton Protopopov,
> 
> Commit c81e4322acf0 ("bpf: Fix a potential use-after-free of BTF
> object") from Feb 9, 2026 (linux-next), leads to the following Smatch
> static checker warning:
> 
> 	kernel/bpf/verifier.c:25375 add_fd_from_fd_array()
> 	warn: double fget(): 'fd'
> 
> kernel/bpf/verifier.c
>     25360 static int add_fd_from_fd_array(struct bpf_verifier_env *env, int fd)
>     25361 {
>     25362         struct bpf_map *map;
>     25363         struct btf *btf;
>     25364         CLASS(fd, f)(fd);
> 
> This assigns f = fdget(fd);
> 
>     25365         int err;
>     25366 
>     25367         map = __bpf_map_get(f);
>     25368         if (!IS_ERR(map)) {
>     25369                 err = __add_used_map(env, map);
>     25370                 if (err < 0)
>     25371                         return err;
>     25372                 return 0;
>     25373         }
>     25374 
> --> 25375         btf = btf_get_by_fd(fd);
>                                       ^^
> This re-uses the fd.  The reason behind the warning is that the user
> could have changed the fd to point to a different file from the
> start of the function.

True, this could happen. Not sure this is a real problem (if a user
replaced this by a valid BTF, well...)

>     25376         if (!IS_ERR(btf))
>     25377                 return __add_used_btf(env, btf);

The problem with this piece of code is that originally I wanted to
keep naming/appearance in sync, but the corresponding map/btf
functions, historically, behave a bit different...

To keep things working and to address the bug report in this
thread, one fix is required:

-       btf = btf_get_by_fd(fd);
-       if (!IS_ERR(btf))
+       btf = __btf_get_by_fd(f);
+       if (!IS_ERR(btf)) {
+               btf_get(btf);
                return __add_used_btf(env, btf);
+       }

I will send this fix later.

>     25379         verbose(env, "fd %d is not pointing to valid bpf_map or btf\n", fd);
>     25380         return PTR_ERR(map);
>     25381 }
> 
> regards,
> dan carpenter