Merge branch 'bpf-fix-abuse-of-kprobe_write_ctx-via-freplace'

Leon Hwang says:

====================
bpf: Fix abuse of kprobe_write_ctx via freplace

The potential issue of kprobe_write_ctx+freplace was mentioned in
"bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs" [1].

It is true issue, that the test in patch #2 verifies that kprobe_write_ctx=false
kprobe progs can be abused to modify struct pt_regs via kprobe_write_ctx=true
freplace progs.

When struct pt_regs is modified, bpf_prog_test_run_opts() gets -EFAULT instead
of 0.

test_freplace_kprobe_write_ctx:FAIL:bpf_prog_test_run_opts unexpected error: -14 (errno 14)

We will disallow attaching freplace programs on kprobe programs with different
kprobe_write_ctx values.

Links:
[1] https://lore.kernel.org/bpf/CAP01T74w4KVMn9bEwpQXrk+bqcUxzb6VW1SQ_QvNy0A4EY-9Jg@mail.gmail.com/

Changes:
v2 -> v3:
* Add comment to the rejection of kprobe_write_ctx (per Jiri).
* Use libbpf_get_error() instead of errno in test (per Jiri).
* Collect Acked-by tags from Jiri and Song, thanks.
v2: https://lore.kernel.org/bpf/20260326141718.17731-1-leon.hwang@linux.dev/

v1 -> v2:
* Drop patch #1 in v1, as it wasn't an issue (per Toke).
* Check kprobe_write_ctx value at attach time instead of at load time, to
  prevent attaching kprobe_write_ctx=true freplace progs on
  kprobe_write_ctx=false kprobe progs (per Gemini/sashiko).
* Move kprobe_write_ctx test code to attach_probe.c and kprobe_write_ctx.c.
v1: https://lore.kernel.org/bpf/20260324150444.68166-1-leon.hwang@linux.dev/
====================

Link: https://patch.msgid.link/20260331145353.87606-1-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 17 insertions, 0 deletions

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)