[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+5a0f19...@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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[Alexei Starovoitov]

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.