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

On Tue, Aug 18, 2026 at 09:32:27PM +0100, Christian Simon wrote:
> 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: [email protected]
> Signed-off-by: Christian Simon <[email protected]>
> ---
> 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

please split the change into 3 patches - uprobe, uprobe_multi, selftest

> 
> 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;

so uprobe program has this problem only when using private stack, right?
but we exclude all the programs, I think we should check program has private
stack as well

jirka

> +		}
> +
>  		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++;

SNIP