[syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    6b95cc562de2 ftrace: Fix direct_functions leak in update_f..
git tree:       bpf-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11a5fbfa580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b5a1ee0109784fc2
dashboard link: https://syzkaller.appspot.com/bug?extid=5a0f1995634f7c1dadbf
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1035fc5a580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12a1625a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e902619c9a45/disk-6b95cc56.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c8a15c0caf9a/vmlinux-6b95cc56.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3e7c64a9dbaf/bzImage-6b95cc56.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: syz.1.44/6186
Modules linked in:
CPU: 0 UID: 0 PID: 6186 Comm: syz.1.44 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 2b 5b 23 fd 48 8d 3d 04 7d 58 0b 67 48 0f b9 3a eb 4a e8 18 5b 23 fd 48 8d 3d 01 7d 58 0b <67> 48 0f b9 3a eb 37 e8 05 5b 23 fd 48 8d 3d fe 7c 58 0b 67 48 0f
RSP: 0018:ffffc90003337380 EFLAGS: 00010293
RAX: ffffffff84a11b58 RBX: 0000000000000002 RCX: ffff88802f648000
RDX: 0000000000000000 RSI: ffffffff8ece7f00 RDI: ffffffff8ff99860
RBP: 0000000000000000 R08: ffff88802f648000 R09: 0000000000000005
R10: 0000000000000004 R11: 0000000000000000 R12: ffff8880762d8854
R13: 1ffff9200078f60c R14: ffff888079bc6258 R15: ffff888079bc6200
FS:  00007fb9d62266c0(0000) GS:ffff8881256f8000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb9d53e8600 CR3: 00000000329a6000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __add_used_btf+0x152/0x2e0 kernel/bpf/verifier.c:21107
 check_pseudo_btf_id+0x764/0xbb0 kernel/bpf/verifier.c:21238
 resolve_pseudo_ldimm64+0x3f4/0xc90 kernel/bpf/verifier.c:21489
 bpf_check+0x1d82/0x1ce00 kernel/bpf/verifier.c:25715
 bpf_prog_load+0x1484/0x1ae0 kernel/bpf/syscall.c:3081
 __sys_bpf+0x618/0x950 kernel/bpf/syscall.c:6218
 __do_sys_bpf kernel/bpf/syscall.c:6331 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:6329 [inline]
 __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6329
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb9d539aeb9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fb9d6226028 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007fb9d5615fa0 RCX: 00007fb9d539aeb9
RDX: 0000000000000094 RSI: 0000200000000180 RDI: 0000000000000005
RBP: 00007fb9d5408c1f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fb9d5616038 R14: 00007fb9d5615fa0 R15: 00007ffda6487988
 </TASK>
----------------
Code disassembly (best guess):
   0:	eb 66                	jmp    0x68
   2:	85 db                	test   %ebx,%ebx
   4:	74 3e                	je     0x44
   6:	83 fb 01             	cmp    $0x1,%ebx
   9:	75 4c                	jne    0x57
   b:	e8 2b 5b 23 fd       	call   0xfd235b3b
  10:	48 8d 3d 04 7d 58 0b 	lea    0xb587d04(%rip),%rdi        # 0xb587d1b
  17:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  1c:	eb 4a                	jmp    0x68
  1e:	e8 18 5b 23 fd       	call   0xfd235b3b
  23:	48 8d 3d 01 7d 58 0b 	lea    0xb587d01(%rip),%rdi        # 0xb587d2b
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	eb 37                	jmp    0x68
  31:	e8 05 5b 23 fd       	call   0xfd235b3b
  36:	48 8d 3d fe 7c 58 0b 	lea    0xb587cfe(%rip),%rdi        # 0xb587d3b
  3d:	67                   	addr32
  3e:	48                   	rex.W
  3f:	0f                   	.byte 0xf


---
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Re: [syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[Alexei Starovoitov]

On Tue, Feb 3, 2026 at 4:52 PM syzbot
<syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com> wrote:
>
> refcount_t: addition on 0; use-after-free.
> WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: syz.1.44/6186
> Modules linked in:
> CPU: 0 UID: 0 PID: 6186 Comm: syz.1.44 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
> RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
> Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 2b 5b 23 fd 48 8d 3d 04 7d 58 0b 67 48 0f b9 3a eb 4a e8 18 5b 23 fd 48 8d 3d 01 7d 58 0b <67> 48 0f b9 3a eb 37 e8 05 5b 23 fd 48 8d 3d fe 7c 58 0b 67 48 0f
> RSP: 0018:ffffc90003337380 EFLAGS: 00010293
> RAX: ffffffff84a11b58 RBX: 0000000000000002 RCX: ffff88802f648000
> RDX: 0000000000000000 RSI: ffffffff8ece7f00 RDI: ffffffff8ff99860
> RBP: 0000000000000000 R08: ffff88802f648000 R09: 0000000000000005
> R10: 0000000000000004 R11: 0000000000000000 R12: ffff8880762d8854
> R13: 1ffff9200078f60c R14: ffff888079bc6258 R15: ffff888079bc6200
> FS:  00007fb9d62266c0(0000) GS:ffff8881256f8000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fb9d53e8600 CR3: 00000000329a6000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  __add_used_btf+0x152/0x2e0 kernel/bpf/verifier.c:21107
>  check_pseudo_btf_id+0x764/0xbb0 kernel/bpf/verifier.c:21238
>  resolve_pseudo_ldimm64+0x3f4/0xc90 kernel/bpf/verifier.c:21489
>  bpf_check+0x1d82/0x1ce00 kernel/bpf/verifier.c:25715
>  bpf_prog_load+0x1484/0x1ae0 kernel/bpf/syscall.c:3081
>  __sys_bpf+0x618/0x950 kernel/bpf/syscall.c:6218
>  __do_sys_bpf kernel/bpf/syscall.c:6331 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:6329 [inline]
>  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6329
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f

Anton,

commit 76145f725532 ("bpf: Refactor check_pseudo_btf_id")
looks buggy and I think syzbot spotted it correctly.

This chunk of code:
        if (btf_fd) {
                CLASS(fd, f)(btf_fd);

                btf = __btf_get_by_fd(f);
                if (IS_ERR(btf)) {
                        verbose(env, "invalid module BTF object FD
specified.\n");
                        return -EINVAL;
                }
        } else {


doesn't hold btf.
As soon as FD gets out of scope btf->refcnt can be zero.
Either btf_get_by_fd() is needed or CLASS(fd, f) needs to span
the whole function which is harder.

Note add_fd_from_fd_array() is using __btf_get_by_fd() correctly.

Re: [syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[Anton Protopopov]

On 26/02/03 05:06PM, Alexei Starovoitov wrote:
> On Tue, Feb 3, 2026 at 4:52 PM syzbot
> <syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com> wrote:
> >
> > refcount_t: addition on 0; use-after-free.
> > WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: syz.1.44/6186
> > Modules linked in:
> > CPU: 0 UID: 0 PID: 6186 Comm: syz.1.44 Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
> > RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
> > Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 2b 5b 23 fd 48 8d 3d 04 7d 58 0b 67 48 0f b9 3a eb 4a e8 18 5b 23 fd 48 8d 3d 01 7d 58 0b <67> 48 0f b9 3a eb 37 e8 05 5b 23 fd 48 8d 3d fe 7c 58 0b 67 48 0f
> > RSP: 0018:ffffc90003337380 EFLAGS: 00010293
> > RAX: ffffffff84a11b58 RBX: 0000000000000002 RCX: ffff88802f648000
> > RDX: 0000000000000000 RSI: ffffffff8ece7f00 RDI: ffffffff8ff99860
> > RBP: 0000000000000000 R08: ffff88802f648000 R09: 0000000000000005
> > R10: 0000000000000004 R11: 0000000000000000 R12: ffff8880762d8854
> > R13: 1ffff9200078f60c R14: ffff888079bc6258 R15: ffff888079bc6200
> > FS:  00007fb9d62266c0(0000) GS:ffff8881256f8000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fb9d53e8600 CR3: 00000000329a6000 CR4: 00000000003526f0
> > Call Trace:
> >  <TASK>
> >  __add_used_btf+0x152/0x2e0 kernel/bpf/verifier.c:21107
> >  check_pseudo_btf_id+0x764/0xbb0 kernel/bpf/verifier.c:21238
> >  resolve_pseudo_ldimm64+0x3f4/0xc90 kernel/bpf/verifier.c:21489
> >  bpf_check+0x1d82/0x1ce00 kernel/bpf/verifier.c:25715
> >  bpf_prog_load+0x1484/0x1ae0 kernel/bpf/syscall.c:3081
> >  __sys_bpf+0x618/0x950 kernel/bpf/syscall.c:6218
> >  __do_sys_bpf kernel/bpf/syscall.c:6331 [inline]
> >  __se_sys_bpf kernel/bpf/syscall.c:6329 [inline]
> >  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6329
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Anton,
> 
> commit 76145f725532 ("bpf: Refactor check_pseudo_btf_id")
> looks buggy and I think syzbot spotted it correctly.
> 
> This chunk of code:
>         if (btf_fd) {
>                 CLASS(fd, f)(btf_fd);
> 
>                 btf = __btf_get_by_fd(f);
>                 if (IS_ERR(btf)) {
>                         verbose(env, "invalid module BTF object FD
> specified.\n");
>                         return -EINVAL;
>                 }
>         } else {
> 
> 
> doesn't hold btf.
> As soon as FD gets out of scope btf->refcnt can be zero.
> Either btf_get_by_fd() is needed or CLASS(fd, f) needs to span
> the whole function which is harder.
> 
> Note add_fd_from_fd_array() is using __btf_get_by_fd() correctly.

Thanks Alexei! I will send a fix.

Re: [syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[Anton Protopopov]

On Wed, Feb 4, 2026 at 9:20 AM Anton Protopopov
<a.s.protopopov@gmail.com> wrote:
>
> On 26/02/03 05:06PM, Alexei Starovoitov wrote:
> > On Tue, Feb 3, 2026 at 4:52 PM syzbot
> > <syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com> wrote:
> > >
> > > refcount_t: addition on 0; use-after-free.
> > > WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: syz.1.44/6186
> > > Modules linked in:
> > > CPU: 0 UID: 0 PID: 6186 Comm: syz.1.44 Not tainted syzkaller #0 PREEMPT(full)
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
> > > RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
> > > Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 2b 5b 23 fd 48 8d 3d 04 7d 58 0b 67 48 0f b9 3a eb 4a e8 18 5b 23 fd 48 8d 3d 01 7d 58 0b <67> 48 0f b9 3a eb 37 e8 05 5b 23 fd 48 8d 3d fe 7c 58 0b 67 48 0f
> > > RSP: 0018:ffffc90003337380 EFLAGS: 00010293
> > > RAX: ffffffff84a11b58 RBX: 0000000000000002 RCX: ffff88802f648000
> > > RDX: 0000000000000000 RSI: ffffffff8ece7f00 RDI: ffffffff8ff99860
> > > RBP: 0000000000000000 R08: ffff88802f648000 R09: 0000000000000005
> > > R10: 0000000000000004 R11: 0000000000000000 R12: ffff8880762d8854
> > > R13: 1ffff9200078f60c R14: ffff888079bc6258 R15: ffff888079bc6200
> > > FS:  00007fb9d62266c0(0000) GS:ffff8881256f8000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 00007fb9d53e8600 CR3: 00000000329a6000 CR4: 00000000003526f0
> > > Call Trace:
> > >  <TASK>
> > >  __add_used_btf+0x152/0x2e0 kernel/bpf/verifier.c:21107
> > >  check_pseudo_btf_id+0x764/0xbb0 kernel/bpf/verifier.c:21238
> > >  resolve_pseudo_ldimm64+0x3f4/0xc90 kernel/bpf/verifier.c:21489
> > >  bpf_check+0x1d82/0x1ce00 kernel/bpf/verifier.c:25715
> > >  bpf_prog_load+0x1484/0x1ae0 kernel/bpf/syscall.c:3081
> > >  __sys_bpf+0x618/0x950 kernel/bpf/syscall.c:6218
> > >  __do_sys_bpf kernel/bpf/syscall.c:6331 [inline]
> > >  __se_sys_bpf kernel/bpf/syscall.c:6329 [inline]
> > >  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6329
> > >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > >  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
> > >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > Anton,
> >
> > commit 76145f725532 ("bpf: Refactor check_pseudo_btf_id")
> > looks buggy and I think syzbot spotted it correctly.
> >
> > This chunk of code:
> >         if (btf_fd) {
> >                 CLASS(fd, f)(btf_fd);
> >
> >                 btf = __btf_get_by_fd(f);
> >                 if (IS_ERR(btf)) {
> >                         verbose(env, "invalid module BTF object FD
> > specified.\n");
> >                         return -EINVAL;
> >                 }
> >         } else {
> >
> >
> > doesn't hold btf.
> > As soon as FD gets out of scope btf->refcnt can be zero.
> > Either btf_get_by_fd() is needed or CLASS(fd, f) needs to span
> > the whole function which is harder.
> >
> > Note add_fd_from_fd_array() is using __btf_get_by_fd() correctly.
>
> Thanks Alexei! I will send a fix.

#syz test: https://github.com/aspsk/bpf-next.git fix-btf-refcount

Re: [syzbot] [bpf?] WARNING: refcount bug in __add_used_btf

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com
Tested-by: syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com

Tested on:

commit:         ea7bf7f2 bpf: Fix a potential use-after-free
git tree:       https://github.com/aspsk/bpf-next.git fix-btf-refcount
console output: https://syzkaller.appspot.com/x/log.txt?x=150eda5a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b5a1ee0109784fc2
dashboard link: https://syzkaller.appspot.com/bug?extid=5a0f1995634f7c1dadbf
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.