Re: [PATCH v3 03/10] crash_dump: Disallow writing to dm-crypt configfs during kexec_file_load syscall

[Sourabh Jain]

On 29/07/26 09:06, Coiby Xu wrote:
> If writing to the configfs group happens concurrently during
> kexec_file_load syscall, it may lead to the following issues,
>    - buffer overflow if dm-crypt keys are added after allocation
>    - stale total_keys if dm-crypt keys are removed during iteration
>    - keys_header will not be freed if config/crash_dm_crypt_key/reuse is
>      set true
>
> So hold config_keys_subsys.su_mutex for the entire sequence during the
> kexec_file_load syscall to ensure a consistent snapshot.
>
> Fixes: 479e58549b0f ("crash_dump: store dm crypt keys in kdump reserved memory")
> Suggested-by: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
> Signed-off-by: Coiby Xu <coiby.xu@xxxxxxxxx>
> ---
>   kernel/crash_dump_dm_crypt.c | 23 +++++++++++++++++++++--
>   1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
> index 4335b6cb1fc4..d2e66c6fe6f3 100644
> --- a/kernel/crash_dump_dm_crypt.c
> +++ b/kernel/crash_dump_dm_crypt.c
> @@ -293,6 +293,7 @@ static ssize_t config_keys_reuse_show(struct config_item *item, char *page)
>   static ssize_t config_keys_reuse_store(struct config_item *item,
>   					   const char *page, size_t count)
>   {
> +	struct mutex *lock;
>   	bool val;
>   	int r;
>   
> @@ -302,8 +303,12 @@ static ssize_t config_keys_reuse_store(struct config_item *item,
>   		return -EINVAL;
>   	}
>   
> +	lock = &to_config_group(item)->cg_subsys->su_mutex;
> +	mutex_lock(lock);

Is this lock only protecting against races between key reuse and 
kexec_file_load(),
or does it also handle the case where a new key is added during key reuse or
kexec_file_load() is running?

If it is only intended to protect key reuse versus kexec_file_load(), 
why can't we use
the kexec lock instead?

The reason I'm asking is that, in upcoming patches, the key reuse path 
accesses
kexec_crash_image properties and the crash reserved region directly. 
Doing so
without taking the kexec lock (using kexec_trylock()) could lead to race 
conditions.

- Sourabh Jain
> +
> +	r = -EINVAL;
>   	if (kstrtobool(page, &val) || !val)
> -		return -EINVAL;
> +		goto unlock;

The jump above skips setting count, causing the function to return count 
instead of -EINVAL.
Is this really intended?

>   
>   	if (is_dm_key_reused) {
>   		pr_info("Already got dm-crypt keys, please continue with kexec_file_load syscall\n");
> @@ -311,11 +316,15 @@ static ssize_t config_keys_reuse_store(struct config_item *item,
>   		r = get_keys_from_kdump_reserved_memory();
>   		if (r) {
>   			pr_warn("Failed to get dm-crypt keys from reserved memory\n");
> -			return r;
> +			goto unlock;
>   		}
>   		is_dm_key_reused = true;
>   	}
>   
> +	r = count;
> +
> +unlock:
> +	mutex_unlock(lock);
>   	return count;
>   }
>   
> @@ -421,6 +430,8 @@ static int build_keys_header(void)
>   	return 0;
>   }
>   
> +static bool mutex_acquired;
> +
>   int crash_load_dm_crypt_keys(struct kimage *image)
>   {
>   	struct kexec_buf kbuf = {
> @@ -432,6 +443,9 @@ int crash_load_dm_crypt_keys(struct kimage *image)
>   	};
>   	int r = 0;
>   
> +	mutex_lock(&config_keys_subsys.su_mutex);
> +	mutex_acquired = true;
> +
>   	if (key_count <= 0) {
>   		kexec_dprintk("No dm-crypt keys\n");
>   		return 0;
> @@ -481,6 +495,11 @@ void kexec_file_post_load_cleanup_dm_crypt(struct kimage *image)
>   		kfree_sensitive(keys_header);
>   		keys_header = NULL;
>   	}
> +
> +	if (mutex_acquired) {
> +		mutex_unlock(&config_keys_subsys.su_mutex);
> +		mutex_acquired = false;
> +	}
>   }
>   
>   static int __init configfs_dmcrypt_keys_init(void)