[PATCH v2] 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() access needs to be guarded against preemption.

Fixes: 5af6807bdb10 ("bpf: Introduce bpf_mem_free_rcu() similar to kfree_rcu().")
Reported-by: syzbot+fd7e415d891073b83e1f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=fd7e415d891073b83e1f
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
v1 -> v2: using guard against preemption

 kernel/bpf/memalloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index bd45dda9dc35..1d7ddfc1c9d1 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -957,7 +957,9 @@ void notrace bpf_mem_cache_free_rcu(struct bpf_mem_alloc *ma, void *ptr)
 	if (!ptr)
 		return;
 
+	get_cpu();
 	unit_free_rcu(this_cpu_ptr(ma->cache), ptr);
+	put_cpu();
 }
 
 /* Directly does a kfree() without putting 'ptr' back to the free_llist
-- 
2.43.0