[syzbot] [bpf?] WARNING in bpf_check (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 172a9d94339c Merge tag '6.15-rc6-smb3-client-fixes' of git..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11d15ef4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=4f080d149583fe67
dashboard link: https://syzkaller.appspot.com/bug?extid=0ef84a7bdf5301d4cbec
compiler: arm-linux-gnueabi-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
userspace arch: arm
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=130462d4580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14efaef4580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/98a89b9f34e4/non_bootable_disk-172a9d94.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/88f3b6a8815a/vmlinux-172a9d94.xz
kernel image: https://storage.googleapis.com/syzbot-assets/8835063aa13d/zImage-172a9d94.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0ef84a...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 3102 at kernel/bpf/verifier.c:20723 opt_subreg_zext_lo32_rnd_hi32 kernel/bpf/verifier.c:20723 [inline]
WARNING: CPU: 1 PID: 3102 at kernel/bpf/verifier.c:20723 bpf_check+0x2d58/0x2ed4 kernel/bpf/verifier.c:24078
Modules linked in:
Kernel panic - not syncing: kernel: panic_on_warn set ...
CPU: 1 UID: 0 PID: 3102 Comm: syz-executor107 Not tainted 6.15.0-rc6-syzkaller #0 PREEMPT
Hardware name: ARM-Versatile Express
Call trace:
[<802019e4>] (dump_backtrace) from [<80201ae0>] (show_stack+0x18/0x1c arch/arm/kernel/traps.c:257)
r7:00000000 r6:828227fc r5:00000000 r4:82257e84
[<80201ac8>] (show_stack) from [<8021ff7c>] (__dump_stack lib/dump_stack.c:94 [inline])
[<80201ac8>] (show_stack) from [<8021ff7c>] (dump_stack_lvl+0x54/0x7c lib/dump_stack.c:120)
[<8021ff28>] (dump_stack_lvl) from [<8021ffbc>] (dump_stack+0x18/0x1c lib/dump_stack.c:129)
r5:00000000 r4:82a70d4c
[<8021ffa4>] (dump_stack) from [<802025f8>] (panic+0x120/0x374 kernel/panic.c:354)
[<802024d8>] (panic) from [<802619e8>] (check_panic_on_warn kernel/panic.c:243 [inline])
[<802024d8>] (panic) from [<802619e8>] (get_taint+0x0/0x1c kernel/panic.c:238)
r3:8280c604 r2:00000001 r1:8223ea4c r0:8224654c
r7:804020d0
[<80261974>] (check_panic_on_warn) from [<80261b4c>] (__warn+0x80/0x188 kernel/panic.c:749)
[<80261acc>] (__warn) from [<80261dcc>] (warn_slowpath_fmt+0x178/0x1f4 kernel/panic.c:776)
r8:00000009 r7:8225e3a4 r6:df989c44 r5:844f0000 r4:00000000
[<80261c58>] (warn_slowpath_fmt) from [<804020d0>] (opt_subreg_zext_lo32_rnd_hi32 kernel/bpf/verifier.c:20723 [inline])
[<80261c58>] (warn_slowpath_fmt) from [<804020d0>] (bpf_check+0x2d58/0x2ed4 kernel/bpf/verifier.c:24078)
r10:00000002 r9:84850000 r8:00000004 r7:00000002 r6:00000003 r5:000000c3
r4:ffffffff
[<803ff378>] (bpf_check) from [<803d66d0>] (bpf_prog_load+0x68c/0xc20 kernel/bpf/syscall.c:2971)
r10:844f0000 r9:842a6f30 r8:00000048 r7:df989d90 r6:00000000 r5:00000000
r4:df989ec0
[<803d6044>] (bpf_prog_load) from [<803d7e24>] (__sys_bpf+0x578/0x1fd0 kernel/bpf/syscall.c:5834)
r10:b5403587 r9:2000e000 r8:00000000 r7:00000000 r6:00000005 r5:df989e90
r4:00000048
[<803d78ac>] (__sys_bpf) from [<803d9e1c>] (__do_sys_bpf kernel/bpf/syscall.c:5941 [inline])
[<803d78ac>] (__sys_bpf) from [<803d9e1c>] (sys_bpf+0x2c/0x48 kernel/bpf/syscall.c:5939)
r10:00000182 r9:844f0000 r8:8020029c r7:00000182 r6:0008e048 r5:00000000
r4:ffffffff
[<803d9df0>] (sys_bpf) from [<80200060>] (ret_fast_syscall+0x0/0x1c arch/arm/mm/proc-v7.S:67)
Exception stack(0xdf989fa8 to 0xdf989ff0)
9fa0: ffffffff 00000000 00000005 2000e000 00000048 00000000
9fc0: ffffffff 00000000 0008e048 00000182 00000002 0000fd90 000f4240 00000000
9fe0: 7ebc8c70 7ebc8c60 000106bc 0002e810
Rebooting in 86400 seconds..


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[Puranjay Mohan]

I think this issue is triggered because insn_def_regno() doesn't
understand BPF_LOAD_ACQ and returns -1 for it. On arm32 and other
architectures that need zext, BPF_LOAD_ACQ will be marked for zext if
the load is < 64 bit, but when opt_subreg_zext_lo32_rnd_hi32() tries to
find the destination register to zext for BPF_LOAD_ACQ, it gets -1 from
insn_def_regno() and triggers the WARN().

This should be fixed by the the patch at the end.

Thanks,
Puranjay

#syz test

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 54c6953a8b84..9aa67e46cb8b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3643,6 +3643,9 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
/* Return the regno defined by the insn, or -1. */
static int insn_def_regno(const struct bpf_insn *insn)
{
+ if (is_atomic_load_insn(insn))
+ return insn->dst_reg;
+
switch (BPF_CLASS(insn->code)) {
case BPF_JMP:
case BPF_JMP32:

[syzbot]

Hello,

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

Reported-by: syzbot+0ef84a...@syzkaller.appspotmail.com
Tested-by: syzbot+0ef84a...@syzkaller.appspotmail.com

Tested on:

commit: 4a95bc12 Merge tag 'rproc-v6.15-fixes' of git://git.ke..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=125d60e8580000
kernel config: https://syzkaller.appspot.com/x/.config?x=1f68e7ba43fbfd48

dashboard link: https://syzkaller.appspot.com/bug?extid=0ef84a7bdf5301d4cbec
compiler: arm-linux-gnueabi-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
userspace arch: arm

patch: https://syzkaller.appspot.com/x/patch.diff?x=152c4ad4580000

Note: testing is done by a robot and is best-effort only.

[syzbot]

syzbot has bisected this issue to:

commit 0df1a55afa832f463f9ad68ddc5de92230f1bc8a
Author: Paul Chaignon <paul.c...@gmail.com>
Date: Tue Jul 1 18:36:15 2025 +0000

bpf: Warn on internal verifier errors

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17014bd4580000
start commit: cce3fee729ee selftests/bpf: Enable dynptr/test_probe_read_..
git tree: bpf-next
final oops: https://syzkaller.appspot.com/x/report.txt?x=14814bd4580000
console output: https://syzkaller.appspot.com/x/log.txt?x=10814bd4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=79da270cec5ffd65
dashboard link: https://syzkaller.appspot.com/bug?extid=0ef84a7bdf5301d4cbec
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=109df88c580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11705770580000

Reported-by: syzbot+0ef84a...@syzkaller.appspotmail.com
Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection