[PATCH bpf-next v2 1/2] bpf: Fix abuse of kprobe_write_ctx via freplace

[Leon Hwang]

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")
Signed-off-by: Leon Hwang <leon.hw...@linux.dev>
---
 kernel/bpf/syscall.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 51ade3cde8bb..1dd2ea076d8b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3733,6 +3733,11 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
                tr = prog->aux->dst_trampoline;
                tgt_prog = prog->aux->dst_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