Re: [PATCHv2 bpf-next 2/2] bpf: Disable private stacks for untagged kprobe programs

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Nov 06, 2026 at 05:40:43PM +0100, Jiri Olsa wrote:
> Uprobe programs can be preempted and re-entered on the same CPU. Since
> the private stack is shared per program and CPU, another invocation can
> overwrite the preempted invocation's stack.
> 
> Programs without a multi or session attach type may be standard kprobes
> or uprobes. Disable private stacks for them, and retain private stacks
> only for kprobe multi and session programs, which have attach types that
> identify them as kprobes.
> 
> Update verifier tests that rely on private stacks to use kprobe.multi,
> and add coverage that untagged kprobe and uprobe programs do not use
> private stacks.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

hi,
I did not realize before, but I think this should be added to stable

Fixes: a76ab5731e32 ("bpf: Find eligible subprogs for private stack support")
Cc: stable@vger.kernel.org

jirka


> ---
>  kernel/bpf/verifier.c                         |  9 +++++
>  .../bpf/progs/verifier_large_stack.c          |  2 +-
>  .../bpf/progs/verifier_private_stack.c        | 38 ++++++++++++++++++-
>  3 files changed, 46 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index b7329d164771..1b58a1796a2e 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5749,6 +5749,15 @@ static enum priv_stack_mode bpf_enable_priv_stack(struct bpf_prog *prog)
>  	 */
>  	switch (prog->type) {
>  	case BPF_PROG_TYPE_KPROBE:
> +		/*
> +		 * Uprobes are preemptible and lack recursion protection. Classic
> +		 * kprobe and uprobe progs have no expected attach type, so enable
> +		 * private stacks only for kprobe.multi and kprobe.session progs.
> +		 */
> +		if (prog->expected_attach_type != BPF_TRACE_KPROBE_MULTI &&
> +		    prog->expected_attach_type != BPF_TRACE_KPROBE_SESSION)
> +			return NO_PRIV_STACK;
> +		fallthrough;
>  	case BPF_PROG_TYPE_TRACEPOINT:
>  	case BPF_PROG_TYPE_PERF_EVENT:
>  	case BPF_PROG_TYPE_RAW_TRACEPOINT:
> diff --git a/tools/testing/selftests/bpf/progs/verifier_large_stack.c b/tools/testing/selftests/bpf/progs/verifier_large_stack.c
> index 7e4f5e3463a2..cbabb41d973c 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_large_stack.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_large_stack.c
> @@ -243,7 +243,7 @@ static void priv_stack_frame_2048(void)
>  "	::: __clobber_all);
>  }
>  
> -SEC("kprobe")
> +SEC("kprobe.multi")
>  __description("private stack: two frames of 2048 bytes")
>  __load_if_large_stack()
>  __arch_x86_64
> diff --git a/tools/testing/selftests/bpf/progs/verifier_private_stack.c b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> index aa15361bcca5..0ce07bf6b566 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_private_stack.c
> @@ -22,7 +22,7 @@ struct {
>  	__type(value, struct elem);
>  } array SEC(".maps");
>  
> -SEC("kprobe")
> +SEC("kprobe.multi")
>  __description("Private stack, single prog")
>  __success
>  __arch_x86_64
> @@ -110,6 +110,40 @@ int no_private_stack_stack_args(void)
>  }
>  #endif
>  
> +SEC("kprobe")
> +__description("No private stack, untagged kprobe")
> +__success
> +__arch_x86_64
> +__jited("	subq	$0x100, %rsp")
> +__arch_arm64
> +__jited("	sub	sp, sp, #0x100")
> +__naked void no_private_stack_kprobe(void)
> +{
> +	asm volatile ("			\
> +	r1 = 42;			\
> +	*(u64 *)(r10 - 256) = r1;	\
> +	r0 = 0;				\
> +	exit;				\
> +"	::: __clobber_all);
> +}
> +
> +SEC("uprobe")
> +__description("No private stack, uprobe")
> +__success
> +__arch_x86_64
> +__jited("	subq	$0x100, %rsp")
> +__arch_arm64
> +__jited("	sub	sp, sp, #0x100")
> +__naked void no_private_stack_uprobe(void)
> +{
> +	asm volatile ("			\
> +	r1 = 42;			\
> +	*(u64 *)(r10 - 256) = r1;	\
> +	r0 = 0;				\
> +	exit;				\
> +"	::: __clobber_all);
> +}
> +
>  __used
>  __naked static void cumulative_stack_depth_subprog(void)
>  {
> @@ -123,7 +157,7 @@ __naked static void cumulative_stack_depth_subprog(void)
>  	: __clobber_all);
>  }
>  
> -SEC("kprobe")
> +SEC("kprobe.multi")
>  __description("Private stack, subtree > MAX_BPF_STACK")
>  __success
>  __log_level(4) __msg("stack depth max 512")
> -- 
> 2.43.0
>