Re: [PATCH] bpf: Unregister fentry when bpf_trampoline_unlink_prog fails to update image

[Yonghong Song]

On 4/26/23 2:55 AM, Chen Zhongjin wrote:
> In bpf_link_free, bpf trampoline will update the image and remove the
> unlinked prog.
>
> bpf_trampoline_unlink_prog is committed as 'never fail', however it depends
> on the result of image update. It is possible to fail if memory allocation
> fail in bpf_trampoline_update.

Could you give more details which memory allocation fail here?
bpf_tramp_image_alloc()? Are you using some error injection or
this happens in your production workload?

> The error result of bpf_trampoline_update can't be passed to bpf_link_free
> because link release callback returns void. Then it will free the prog
> whether image updating is successful or not.
> If the old image tries to call a freed prog, it makes kernel panic.
>
>      BUG: unable to handle page fault for address: ffffffffc04a8d20
>      #PF: supervisor instruction fetch in kernel mode
>      #PF: error_code(0x0010) - not-present page
>      RIP: 0010:0xffffffffc04a8d20
>      Code: Unable to access opcode bytes at RIP 0xffffffffc04a8cf6.
>      ...
>      Call Trace:
>      ? bpf_trampoline_78223_0
>      bpf_traced_function
>      ...

Could you explain how 'the old image tries to call a freed prog'?
IIUC, the previous bpf_link_free() should not be available to
call the bpf prog, right?

> Fix this when bpf_trampoline_update failed in bpf_trampoline_unlink_prog,
> unregister fentry to disable the trampoline. Then other progs on the
> trampoline can be unlinked safely and finally the trampoline will be
> released.

Do we still leak tr->cur_image here?

> Fixes: 88fd9e5352fe ("bpf: Refactor trampoline update code")

If the above is a refactoring patch, you should not use that
as 'Fixes' patch, you should pick one truely introduced the issue.

> Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
> ---
>   kernel/bpf/trampoline.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index d0ed7d6f5eec..6daa93b30e81 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -604,7 +604,10 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
>   	}
>   	hlist_del_init(&link->tramp_hlist);
>   	tr->progs_cnt[kind]--;
> -	return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> +	err =  bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> +	if (err && tr->cur_image)
> +		unregister_fentry(tr, tr->cur_image->image);

If error happens for the all subsequent programs,
unregister_fentry() will be called multiple times. Any side effect?

Overall, I think this is an extreme corner case which happens
when kernel memory is extreme tight. If this is the case, not
sure whether it is worthwhile to fix it or not.

> +	return err;
>   }
>   
>   /* bpf_trampoline_unlink_prog() should never fail. */