[PATCH bpf v2] bpf: guard uprobes against private-stack corruption

[PATCH bpf v2] bpf: guard uprobes against private-stack corruption

[Christian Simon]

Eligible BPF programs use one private stack per program and CPU. Both
bpf_prog_run_array_uprobe() and uprobe_prog_run() use migrate_disable()
to keep an invocation on one CPU, but another task can still preempt it
and run the same program on that CPU. The second invocation then reuses
and can overwrite the first invocation's private stack.

Protect each real program invocation with the existing per-program
recursion context. When the program is already active on this CPU,
account for the missed invocation and skip it. Return zero when skipping
an uprobe-multi invocation so session handling does not suppress its
return probe.

Skip dummy_bpf_prog in the classic array path before acquiring the
recursion context because its active pointer is NULL.

Add a regression test that pins two threads to one CPU and overlaps
classic and multi uprobe invocations while preserving a sentinel in a
private stack frame. Without the guards, the second invocation executes
and corrupts the first invocation's sentinel.

Fixes: 7d1cd70d4b16 ("bpf, x86: Support private stack in jit")
Fixes: 6c17a882d380 ("bpf, arm64: JIT support for private stack")
Closes: https://github.com/open-telemetry/opentelemetry-ebpf-instrumentation/issues/3056
Cc: stable@vger.kernel.org
Signed-off-by: Christian Simon <simon@swine.de>
---
Changes in v2:
- Address review comments from sashiko-bot
  - Guard the uprobe-multi path as well.
  - Remove const from correct line
- Add a regression selftest for uprobe-classic/multi paths.
- Add the arm64 Fixes tag.

 include/linux/bpf.h                           |  33 +++--
 kernel/trace/bpf_trace.c                      |  11 +-
 .../bpf/prog_tests/uprobe_private_stack.c     | 118 ++++++++++++++++++
 .../bpf/progs/uprobe_private_stack.c          |  54 ++++++++
 4 files changed, 206 insertions(+), 10 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/uprobe_private_stack.c
 create mode 100644 tools/testing/selftests/bpf/progs/uprobe_private_stack.c

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 7719f6528445..a94fc9898ece 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2572,6 +2572,14 @@ static inline void bpf_reset_run_ctx(struct bpf_run_ctx *old_ctx)
 
 typedef u32 (*bpf_prog_run_fn)(const struct bpf_prog *prog, const void *ctx);
 
+#ifdef CONFIG_BPF_SYSCALL
+void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog);
+#else
+static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
+{
+}
+#endif
+
 static __always_inline u32
 bpf_prog_run_array(const struct bpf_prog_array *array,
 		   const void *ctx, bpf_prog_run_fn run_prog)
@@ -2617,7 +2625,7 @@ bpf_prog_run_array_uprobe(const struct bpf_prog_array *array,
 			  const void *ctx, bpf_prog_run_fn run_prog)
 {
 	const struct bpf_prog_array_item *item;
-	const struct bpf_prog *prog;
+	struct bpf_prog *prog;
 	struct bpf_run_ctx *old_run_ctx;
 	struct bpf_trace_run_ctx run_ctx;
 	u32 ret = 1;
@@ -2635,15 +2643,30 @@ bpf_prog_run_array_uprobe(const struct bpf_prog_array *array,
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
 	item = &array->items[0];
 	while ((prog = READ_ONCE(item->prog))) {
+		/* dummy_bpf_prog has no recursion state. */
+		if (unlikely(!prog->len)) {
+			item++;
+			continue;
+		}
+
+		if (unlikely(!bpf_prog_get_recursion_context(prog))) {
+			bpf_prog_inc_misses_counter(prog);
+			bpf_prog_put_recursion_context(prog);
+			item++;
+			continue;
+		}
+
 		if (!prog->sleepable)
 			rcu_read_lock();
 
 		run_ctx.bpf_cookie = item->bpf_cookie;
 		ret &= run_prog(prog, ctx);
-		item++;
 
 		if (!prog->sleepable)
 			rcu_read_unlock();
+
+		bpf_prog_put_recursion_context(prog);
+		item++;
 	}
 	bpf_reset_run_ctx(old_run_ctx);
 	migrate_enable();
@@ -3208,8 +3231,6 @@ static inline bool has_current_bpf_ctx(void)
 	return !!current->bpf_ctx;
 }
 
-void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog);
-
 void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
 		     enum bpf_dynptr_type type, u32 offset, u32 size);
 void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr);
@@ -3538,10 +3559,6 @@ static inline bool has_current_bpf_ctx(void)
 	return false;
 }
 
-static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
-{
-}
-
 static inline void bpf_cgrp_storage_free(struct cgroup *cgroup)
 {
 }
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 75495a5c3507..379d472590ec 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3139,7 +3139,7 @@ static int uprobe_prog_run(struct bpf_uprobe *uprobe,
 	struct bpf_prog *prog = link->link.prog;
 	bool sleepable = prog->sleepable;
 	struct bpf_run_ctx *old_run_ctx;
-	int err;
+	int err = 0;
 
 	if (link->task && !same_thread_group(current, link->task))
 		return 0;
@@ -3151,10 +3151,17 @@ static int uprobe_prog_run(struct bpf_uprobe *uprobe,
 
 	migrate_disable();
 
+	if (unlikely(!bpf_prog_get_recursion_context(prog))) {
+		bpf_prog_inc_misses_counter(prog);
+		goto out;
+	}
+
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
-	err = bpf_prog_run(link->link.prog, regs);
+	err = bpf_prog_run(prog, regs);
 	bpf_reset_run_ctx(old_run_ctx);
 
+out:
+	bpf_prog_put_recursion_context(prog);
 	migrate_enable();
 
 	if (sleepable)
diff --git a/tools/testing/selftests/bpf/prog_tests/uprobe_private_stack.c b/tools/testing/selftests/bpf/prog_tests/uprobe_private_stack.c
new file mode 100644
index 000000000000..20a5b87b1de6
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/uprobe_private_stack.c
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <pthread.h>
+#include <sched.h>
+#include <unistd.h>
+
+#include <test_progs.h>
+#include "uprobe_private_stack.skel.h"
+
+static noinline void uprobe_private_stack_trigger(void)
+{
+	asm volatile("");
+}
+
+static void *trigger_uprobe(void *arg)
+{
+	uprobe_private_stack_trigger();
+	return NULL;
+}
+
+static void run_private_stack_test(struct uprobe_private_stack *skel, bool multi)
+{
+	LIBBPF_OPTS(bpf_uprobe_opts, opts);
+	LIBBPF_OPTS(bpf_uprobe_multi_opts, multi_opts);
+	const char *binary = "/proc/self/exe";
+	struct bpf_program *prog;
+	struct bpf_link *link;
+	pthread_t thread;
+	int err, i;
+
+	skel->bss->ready = 0;
+	skel->bss->release = 0;
+	skel->bss->executions = 0;
+	skel->bss->corruptions = 0;
+	skel->bss->loop_exhausted = 0;
+
+	if (multi) {
+		prog = skel->progs.uprobe_multi_private_stack;
+		link = bpf_program__attach_uprobe_multi(prog, 0, binary,
+							"uprobe_private_stack_trigger",
+							&multi_opts);
+	} else {
+		opts.func_name = "uprobe_private_stack_trigger";
+		prog = skel->progs.uprobe_private_stack;
+		link = bpf_program__attach_uprobe_opts(prog, 0, binary, 0, &opts);
+	}
+	if (!ASSERT_OK_PTR(link, "attach_uprobe"))
+		return;
+
+	err = pthread_create(&thread, NULL, trigger_uprobe, NULL);
+	if (!ASSERT_OK(err, "pthread_create"))
+		goto cleanup;
+
+	for (i = 0; i < 10000; i++) {
+		if (__atomic_load_n(&skel->bss->ready, __ATOMIC_ACQUIRE))
+			break;
+		usleep(1000);
+	}
+
+	if (ASSERT_LT(i, 10000, "first_uprobe_ready"))
+		uprobe_private_stack_trigger();
+	__atomic_store_n(&skel->bss->release, 1, __ATOMIC_RELEASE);
+
+	err = pthread_join(thread, NULL);
+	if (!ASSERT_OK(err, "pthread_join"))
+		goto cleanup;
+
+	ASSERT_EQ(skel->bss->loop_exhausted, 0, "loop_exhausted");
+	ASSERT_EQ(skel->bss->executions, 1, "executions");
+	ASSERT_EQ(skel->bss->corruptions, 0, "corruptions");
+
+cleanup:
+	bpf_link__destroy(link);
+}
+
+void test_uprobe_private_stack(void)
+{
+#if defined(__x86_64__) || defined(__aarch64__)
+	struct uprobe_private_stack *skel = NULL;
+	cpu_set_t old_mask, mask;
+	bool affinity_set = false;
+	int cpu;
+
+	if (!ASSERT_OK(sched_getaffinity(0, sizeof(old_mask), &old_mask),
+		       "get_affinity"))
+		return;
+
+	CPU_ZERO(&mask);
+	for (cpu = 0; cpu < CPU_SETSIZE; cpu++) {
+		if (CPU_ISSET(cpu, &old_mask)) {
+			CPU_SET(cpu, &mask);
+			break;
+		}
+	}
+	if (!ASSERT_LT(cpu, CPU_SETSIZE, "available_cpu"))
+		return;
+	/* Both triggers must contend for the same per-CPU private stack. */
+	if (!ASSERT_OK(sched_setaffinity(0, sizeof(mask), &mask), "set_affinity"))
+		return;
+	affinity_set = true;
+
+	skel = uprobe_private_stack__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		goto cleanup;
+
+	if (test__start_subtest("classic"))
+		run_private_stack_test(skel, false);
+	if (test__start_subtest("multi"))
+		run_private_stack_test(skel, true);
+
+cleanup:
+	uprobe_private_stack__destroy(skel);
+	if (affinity_set)
+		ASSERT_OK(sched_setaffinity(0, sizeof(old_mask), &old_mask),
+			  "restore_affinity");
+#else
+	test__skip();
+#endif
+}
diff --git a/tools/testing/selftests/bpf/progs/uprobe_private_stack.c b/tools/testing/selftests/bpf/progs/uprobe_private_stack.c
new file mode 100644
index 000000000000..a8d623a3d2fd
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/uprobe_private_stack.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+volatile int ready;
+volatile int release;
+volatile int executions;
+volatile int corruptions;
+int loop_exhausted;
+
+static long wait_for_release(__u32 index, void *ctx)
+{
+	return release ? 1 : 0;
+}
+
+static __always_inline int run_private_stack_test(void)
+{
+	/* A 64-byte frame makes the JIT select a private stack. */
+	volatile __u64 stack[8] = {};
+	__u64 id = bpf_get_current_pid_tgid();
+	int seq;
+
+	stack[0] = id;
+	seq = executions;
+	executions = seq + 1;
+	/* An unguarded second invocation overwrites the first invocation's frame. */
+	if (seq) {
+		stack[0] = ~id;
+		return 0;
+	}
+
+	ready = 1;
+	bpf_loop(1 << 23, wait_for_release, NULL, 0);
+	if (!release)
+		loop_exhausted = 1;
+	if (stack[0] != id)
+		corruptions++;
+
+	return 0;
+}
+
+SEC("uprobe")
+int uprobe_private_stack(struct pt_regs *ctx)
+{
+	return run_private_stack_test();
+}
+
+SEC("uprobe.multi")
+int uprobe_multi_private_stack(struct pt_regs *ctx)
+{
+	return run_private_stack_test();
+}
+
+char LICENSE[] SEC("license") = "GPL";
-- 
2.54.0

Re: [PATCH bpf v2] bpf: guard uprobes against private-stack corruption

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Incomplete fix for private stack corruption: BPF programs attached to both uprobes and kprobes/tracepoints bypass recursion protection.
--

--- Patch [1]: [PATCH bpf v2] bpf: guard uprobes against private-stack corruption ---
commit 91e05311e6d3a25ea1742394d64f625138f2c862
Author: Christian Simon <simon@swine.de>

bpf: guard uprobes against private-stack corruption

Eligible BPF programs use one private stack per program and CPU. This
commit adds per-program recursion protection to the uprobe execution
paths to prevent a second invocation from overwriting the first invocation's
private stack.

Closes: https://github.com/open-telemetry/opentelemetry-ebpf-instrumentation/issues/3056

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7719f65284456..a94fc9898ecef 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h

[ ... ]

> @@ -2635,15 +2643,30 @@ bpf_prog_run_array_uprobe(const struct bpf_prog_array *array,
>  	old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
>  	item = &array->items[0];
>  	while ((prog = READ_ONCE(item->prog))) {
> +		/* dummy_bpf_prog has no recursion state. */
> +		if (unlikely(!prog->len)) {
> +			item++;
> +			continue;
> +		}
> +
> +		if (unlikely(!bpf_prog_get_recursion_context(prog))) {

[Severity: High]
This is a pre-existing issue, but does this leave the private stack vulnerable
to corruption if a program is attached to both an uprobe and a kprobe?

The uprobe path increments the per-program recursion counter here via
bpf_prog_get_recursion_context(), but it does not increment the global
bpf_prog_active counter.

If an interrupt triggers a kprobe while the uprobe is executing on the same
CPU, the kprobe dispatcher only checks the global counter:

kernel/trace/bpf_trace.c:trace_call_bpf() {
    if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
        ...
        ret = bpf_prog_run_array(..., bpf_prog_run);
    ...
}

Because the uprobe didn't increment the global bpf_prog_active counter,
trace_call_bpf() will proceed and execute the same BPF program again,
bypassing the per-program recursion protection and potentially overwriting
the first invocation's stack frames.

Does kprobe_multi_link_prog_run() also have this same vulnerability since
it relies solely on bpf_prog_active before executing bpf_prog_run()?

> +			bpf_prog_inc_misses_counter(prog);
> +			bpf_prog_put_recursion_context(prog);
> +			item++;
> +			continue;
> +		}
> +
>  		if (!prog->sleepable)
>  			rcu_read_lock();
>  
>  		run_ctx.bpf_cookie = item->bpf_cookie;
>  		ret &= run_prog(prog, ctx);
> -		item++;
>  
>  		if (!prog->sleepable)
>  			rcu_read_unlock();
> +
> +		bpf_prog_put_recursion_context(prog);
> +		item++;
>  	}
>  	bpf_reset_run_ctx(old_run_ctx);
>  	migrate_enable();

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260818203234.1142913-1-simon@swine.de?part=1