[PATCH bpf-next 2/3] bpf: Disallow freplace on kprobe with mismatched kprobe_write_ctx values

[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 avoid the abuse of kprobe_write_ctx=true via kprobe+freplace, disallow
freplace on kprobe programs with mismatched 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/verifier.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 12330466d58b..f8257bae6081 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6404,6 +6404,14 @@ static int check_ctx_access(struct bpf_verifier_env 
*env, int insn_idx, int off,
                /* remember the offset of last byte accessed in ctx */
                if (env->prog->aux->max_ctx_offset < off + size)
                        env->prog->aux->max_ctx_offset = off + size;
+               if (env->prog->type == BPF_PROG_TYPE_EXT) {
+                       struct bpf_prog *dst_prog = env->prog->aux->dst_prog;
+
+                       if (env->prog->aux->kprobe_write_ctx != 
dst_prog->aux->kprobe_write_ctx) {
+                               verbose(env, "Extension program cannot have 
different kprobe_write_ctx value with target prog\n");
+                               return -EINVAL;
+                       }
+               }
                return 0;
        }
 
-- 
2.53.0