bpf: Fix resetting logic for unreferenced kptrs

Sparse reported a warning at bpf_map_free_kptrs()
"warning: Using plain integer as NULL pointer"
During the process of fixing this warning, it was discovered that the current
code erroneously writes to the pointer variable instead of deferencing and
writing to the actual kptr. Hence, Sparse tool accidentally helped to uncover
this problem. Fix this by doing WRITE_ONCE(*p, 0) instead of WRITE_ONCE(p, 0).

Note that the effect of this bug is that unreferenced kptrs will not be cleared
during check_and_free_fields. It is not a problem if the clearing is not done
during map_free stage, as there is nothing to free for them.

Fixes: 14a324f6a67e ("bpf: Wire up freeing of referenced kptr")
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/Yxi3pJaK6UDjVJSy@playground
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4fb08c43420d19fd9e5c59bcd8800494e3d5fc01..d35a6aa3aa963fd8259b8506223b03532c64751d 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -598,7 +598,7 @@ void bpf_map_free_kptrs(struct bpf_map *map, void *map_value)
                if (off_desc->type == BPF_KPTR_UNREF) {
                        u64 *p = (u64 *)btf_id_ptr;
 
-                       WRITE_ONCE(p, 0);
+                       WRITE_ONCE(*p, 0);
                        continue;
                }
                old_ptr = xchg(btf_id_ptr, 0);