[PATCH bpf-next v3 1/2] bpf: Fix abuse of kprobe_write_ctx via freplace

Leon Hwang
2026-03-31T14:54:37Z

uprobe programs are allowed to modify struct pt_regs.

Since the actual program type of uprobe is KPROBE, it can be abused to
modify struct pt_regs via kprobe+freplace when the kprobe attaches to
kernel functions.

For example,

SEC("?kprobe")
int kprobe(struct pt_regs *regs)
{
return 0;
}

SEC("?freplace")
int freplace_kprobe(struct pt_regs *regs)
{
regs->di = 0;
return 0;
}

freplace_kprobe prog will attach to kprobe prog.
kprobe prog will attach to a kernel function.

Without this patch, when the kernel function runs, its first arg will
always be set as 0 via the freplace_kprobe prog.

To fix the abuse of kprobe_write_ctx=true via kprobe+freplace, disallow
attaching freplace programs on kprobe programs with different
kprobe_write_ctx values.

Fixes: 7384893d970e ("bpf: Allow uprobe program to change context registers")
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
kernel/bpf/syscall.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 51ade3cde8bb..e1505c9cd09e 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3733,6 +3733,23 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
tr = prog->aux->dst_trampoline;
tgt_prog = prog->aux->dst_prog;
}
+ /*
+ * It is to prevent modifying struct pt_regs via kprobe_write_ctx=true
+ * freplace prog. Without this check, kprobe_write_ctx=true freplace
+ * prog is allowed to attach to kprobe_write_ctx=false kprobe prog, and
+ * then modify the registers of the kprobe prog's target kernel
+ * function.
+ *
+ * This also blocks the combination of uprobe+freplace, because it is
+ * unable to recognize the use of the tgt_prog as an uprobe or a kprobe
+ * by tgt_prog itself. At attach time, uprobe/kprobe is recognized by
+ * the target perf event flags in __perf_event_set_bpf_prog().
+ */
+ if (prog->type == BPF_PROG_TYPE_EXT &&
+ prog->aux->kprobe_write_ctx != tgt_prog->aux->kprobe_write_ctx) {
+ err = -EINVAL;
+ goto out_unlock;
+ }

err = bpf_link_prime(&link->link.link, &link_primer);
if (err)
--
2.53.0