[PATCH bpf] bpf: Fix stale offload->prog pointer after constant blinding

[MingTao Huang]

From: MingTao Huang <mintaohuang@xxxxxxxxxxx>

When a dev-bound-only BPF program (BPF_F_XDP_DEV_BOUND_ONLY) undergoes
JIT compilation with constant blinding enabled (bpf_jit_harden >= 2),
bpf_jit_blind_constants() clones the program. The original prog is then
freed in bpf_jit_prog_release_other(), which updates aux->prog to point
to the surviving clone, but fails to update offload->prog.

This leaves offload->prog pointing to the freed original program. When
the network namespace is subsequently destroyed, cleanup_net() triggers
bpf_dev_bound_netdev_unregister(), which iterates ondev->progs and calls
__bpf_prog_offload_destroy(offload->prog). Accessing the freed prog
causes a page fault:

BUG: unable to handle page fault for address: ffffc900085f1038
Workqueue: netns cleanup_net
RIP: 0010:__bpf_prog_offload_destroy+0xc/0x80
Call Trace:
__bpf_offload_dev_netdev_unregister+0x257/0x350
bpf_dev_bound_netdev_unregister+0x4a/0x90
unregister_netdevice_many_notify+0x2a2/0x660
...
cleanup_net+0x21a/0x320

The test sequence that triggers this reliably is:

1. Set net.core.bpf_jit_harden=2 (echo 2 > /proc/sys/net/core/bpf_jit_harden)
2. Run xdp_metadata selftest, which creates a dev-bound-only XDP
   program on a veth inside a netns (./test_progs -t xdp_metadata)
3. cleanup_net -> page fault in __bpf_prog_offload_destroy

Dev-bound-only programs are unique in that they have an offload structure 
but go through the normal JIT path instead of bpf_prog_offload_compile(). 
This means they are subject to constant blinding's prog clone-and-replace, 
while also having offload->prog that must stay in sync.

Fix this by updating offload->prog in bpf_jit_prog_release_other(),
alongside the existing aux->prog update. Both are back-pointers to
the prog that must be kept in sync when the prog is replaced.

Fixes: 2b3486bc2d23 ("bpf: Introduce device-bound XDP programs")
Signed-off-by: MingTao Huang <mintaohuang@xxxxxxxxxxx>
---

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

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 7b675a451ec8..048d275accae 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1487,6 +1487,8 @@ void bpf_jit_prog_release_other(struct bpf_prog *fp, struct bpf_prog *fp_other)
 	 * know whether fp here is the clone or the original.
 	 */
 	fp->aux->prog = fp;
+	if (fp->aux->offload)
+		fp->aux->offload->prog = fp;
 	bpf_prog_clone_free(fp_other);
 }
 
-- 
2.43.7