[syzbot] [bpf?] BUG: using smp_processor_id() in preemptible code in bpf_mem_cache_free_rcu

[syzbot] [bpf?] BUG: using smp_processor_id() in preemptible code in bpf_mem_cache_free_rcu

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    e771677c937d Merge tag 'for-linus-iommufd' of git://git.ke..
git tree:       bpf
console output: https://syzkaller.appspot.com/x/log.txt?x=144f8aae580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1ce9592ed82f8125
dashboard link: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f
compiler:       Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=124c12ae580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13b89986580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5c69b301ec25/disk-e771677c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8267811b5065/vmlinux-e771677c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/bcc54586e6c4/bzImage-e771677c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+fd7e415d891073b83e1f@syzkaller.appspotmail.com

BUG: using smp_processor_id() in preemptible [00000000] code: syz.0.17/5820
caller is bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
CPU: 1 UID: 0 PID: 5820 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 check_preemption_disabled+0xd3/0xe0 lib/smp_processor_id.c:47
 bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
 rhtab_delete_elem+0x185a/0x1b30 kernel/bpf/hashtab.c:2969
 __rhtab_map_lookup_and_delete_batch+0x935/0xcb0 kernel/bpf/hashtab.c:3349
 bpf_map_do_batch+0x445/0x630 kernel/bpf/syscall.c:-1
 __sys_bpf+0x906/0xd90 kernel/bpf/syscall.c:-1
 __do_sys_bpf kernel/bpf/syscall.c:6537 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:6534 [inline]
 __x64_sys_bpf+0xba/0xd0 kernel/bpf/syscall.c:6534
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fce2819ce59
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:00007ffcea2ad4b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007fce28415fa0 RCX: 00007fce2819ce59
RDX: 0000000000000038 RSI: 00002000000018c0 RDI: 0000000000000019
RBP: 00007fce28232d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fce28415fac R14: 00007fce28415fa0 R15: 00007fce28415fa0
 </TASK>


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[PATCH] bpf: Fix smp_processor_id() call trace for preemptible kernels

[Edward Adam Davis]

bpf_mem_cache_free_rcu() maybe called in preemptible context, this
will trigger the below warning message:

BUG: using smp_processor_id() in preemptible [00000000] code: syz.0.17/5820
caller is bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
Call Trace:
 check_preemption_disabled+0xd3/0xe0 lib/smp_processor_id.c:47
 bpf_mem_cache_free_rcu+0x48/0xc0 kernel/bpf/memalloc.c:954
 rhtab_delete_elem+0x185a/0x1b30 kernel/bpf/hashtab.c:2969
 __rhtab_map_lookup_and_delete_batch+0x935/0xcb0 kernel/bpf/hashtab.c:3349
 bpf_map_do_batch+0x445/0x630 kernel/bpf/syscall.c:-1
 __sys_bpf+0x906/0xd90 kernel/bpf/syscall.c:-1

this_cpu_ptr() calls smp_processor_id() in a preemptible context.
Fix by using per_cpu_ptr() with raw_smp_processor_id() instead.

Fixes: 5af6807bdb10 ("bpf: Introduce bpf_mem_free_rcu() similar to kfree_rcu().")
Reported-by: syzbot+fd7e415d891073b83e1f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 kernel/bpf/memalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index bd45dda9dc35..151d9c8183bc 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -957,7 +957,7 @@ void notrace bpf_mem_cache_free_rcu(struct bpf_mem_alloc *ma, void *ptr)
 	if (!ptr)
 		return;
 
-	unit_free_rcu(this_cpu_ptr(ma->cache), ptr);
+	unit_free_rcu(per_cpu_ptr(ma->cache, raw_smp_processor_id()), ptr);
 }
 
 /* Directly does a kfree() without putting 'ptr' back to the free_llist
-- 
2.43.0