[PATCH bpf 0/1] Follow-up fix for potential error pointer dereference in propagate_to_outer_instance()

[PATCH bpf 0/1] Follow-up fix for potential error pointer dereference in propagate_to_outer_instance()

[Shardul Bankar]

Hi BPF maintainers,

This patch follows up on my previous submission:

  [PATCH v2 bpf] bpf: Fix memory leak in __lookup_instance error path
  Link: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012189&state=*

During the review and CI discussions for that patch, a potential issue was
identified in propagate_to_outer_instance(), where get_outer_instance() may
return an ERR_PTR (e.g. -ENOMEM) that is not currently checked before use.

This patch adds a simple IS_ERR() guard and returns the error code to prevent
dereferencing the error pointer.

Thanks,
Shardul

---

Shardul Bankar (1):
  bpf: liveness: Handle ERR_PTR from get_outer_instance() in
    propagate_to_outer_instance()

 kernel/bpf/liveness.c | 2 ++
 1 file changed, 2 insertions(+)

-- 
2.34.1

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

[Shardul Bankar]

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>
---
 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);
-- 
2.34.1

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

[Eduard Zingerman]

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);

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

[Shardul Bankar]

On Thu, 2025-10-16 at 10:43 -0700, Eduard Zingerman wrote:
> We can land this change to avoid confusion, but the fixes tag is
> unnecessary.
> 
> [1]
> https://lore.kernel.org/bpf/8430f47f73d8d55a698e85341ece81955355c1fd.camel@gmail.com/
> 
Thanks, Eduard, for the clarification and reference.

That makes sense — since update_instance() guarantees the outer
instance’s existence, this case wouldn’t be hit in normal execution.

I agree, the Fixes: tag can be dropped. The check mainly serves to make
the intent explicit and avoid future confusion.

I’ve resent the patch without the Fixes: tag.
Link:
https://lore.kernel.org/all/20251020060712.4155702-1-shardulsb08@gmail.com/

Thanks again,
Shardul