Re: [PATCH bpf 1/1] bpf: liveness: Handle ERR_PTR from get_outer_instance() in propagate_to_outer_instance()

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-10-16 at 15:43 +0530, Shardul Bankar wrote:
> propagate_to_outer_instance() calls get_outer_instance() and then uses the
> returned pointer to reset/commit stack write marks. When get_outer_instance()
> fails (e.g., __lookup_instance() returns -ENOMEM), it may return an ERR_PTR.
> Without a check, the code dereferences this error pointer.
> 
> Protect the call with IS_ERR() and propagate the error.
> 
> Fixes: b3698c356ad9 ("bpf: callchain sensitive stack liveness tracking
> using CFG")
> Reported-by: kernel-patches-review-bot (https://github.com/kernel-patches/bpf/pull/10006#issuecomment-3409419240)
> Signed-off-by: Shardul Bankar <shardulsb08@gmail.com>
> ---

This was brought up already in [1].  This is not a bug as check before
propagate_to_outer_instance() call in update_instance() guarantees
that outer instance exists.

We can land this change to avoid confusion, but the fixes tag is
unnecessary.

[1] https://lore.kernel.org/bpf/8430f47f73d8d55a698e85341ece81955355c1fd.camel@gmail.com/


>  kernel/bpf/liveness.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/bpf/liveness.c b/kernel/bpf/liveness.c
> index 3c611aba7f52..ae31f9ee4994 100644
> --- a/kernel/bpf/liveness.c
> +++ b/kernel/bpf/liveness.c
> @@ -522,6 +522,8 @@ static int propagate_to_outer_instance(struct bpf_verifier_env *env,
>  
>  	this_subprog_start = callchain_subprog_start(callchain);
>  	outer_instance = get_outer_instance(env, instance);
> +	if (IS_ERR(outer_instance))
> +		return PTR_ERR(outer_instance);
>  	callsite = callchain->callsites[callchain->curframe - 1];
>  
>  	reset_stack_write_marks(env, outer_instance, callsite);