Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data objects

[David Rientjes <rientjes@google.com>]

On Sun, 5 Apr 2020, Waiman Long wrote:

> For kvmalloc'ed data object that contains sensitive information like
> cryptographic key, we need to make sure that the buffer is always
> cleared before freeing it. Using memset() alone for buffer clearing may
> not provide certainty as the compiler may compile it away. To be sure,
> the special memzero_explicit() has to be used.
> 
> This patch introduces a new kvfree_sensitive() for freeing those
> sensitive data objects allocated by kvmalloc(). The relevnat places
> where kvfree_sensitive() can be used are modified to use it.
> 
> Fixes: 4f0882491a14 ("KEYS: Avoid false positive ENOMEM error on key read")
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/mm.h       | 17 +++++++++++++++++
>  security/keys/internal.h | 11 -----------
>  security/keys/keyctl.c   | 16 +++++-----------
>  3 files changed, 22 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7dd5c4ccbf85..c26f279f1956 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -758,6 +758,23 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
>  
>  extern void kvfree(const void *addr);
>  
> +/**
> + * kvfree_sensitive - free a data object containing sensitive information
> + * @addr - address of the data object to be freed
> + * @len  - length of the data object
> + *
> + * Use the special memzero_explicit() function to clear the content of a
> + * kvmalloc'ed object containing sensitive data to make sure that the
> + * compiler won't optimize out the data clearing.
> + */
> +static inline void kvfree_sensitive(const void *addr, size_t len)
> +{
> +	if (addr) {

Shouldn't this be if (unlikely(ZERO_OR_NULL_PTR(addr))?

> +		memzero_explicit((void *)addr, len);
> +		kvfree(addr);
> +	}
> +}
> +
>  static inline int compound_mapcount(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageCompound(page), page);
> diff --git a/security/keys/internal.h b/security/keys/internal.h
> index 6d0ca48ae9a5..153d35c20d3d 100644
> --- a/security/keys/internal.h
> +++ b/security/keys/internal.h
> @@ -350,15 +350,4 @@ static inline void key_check(const struct key *key)
>  #define key_check(key) do {} while(0)
>  
>  #endif
> -
> -/*
> - * Helper function to clear and free a kvmalloc'ed memory object.
> - */
> -static inline void __kvzfree(const void *addr, size_t len)
> -{
> -	if (addr) {
> -		memset((void *)addr, 0, len);
> -		kvfree(addr);
> -	}
> -}
>  #endif /* _INTERNAL_H */
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 5e01192e222a..edde63a63007 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -142,10 +142,7 @@ SYSCALL_DEFINE5(add_key, const char __user *, _type,
>  
>  	key_ref_put(keyring_ref);
>   error3:
> -	if (payload) {
> -		memzero_explicit(payload, plen);
> -		kvfree(payload);
> -	}
> +	kvfree_sensitive(payload, plen);
>   error2:
>  	kfree(description);
>   error:
> @@ -360,7 +357,7 @@ long keyctl_update_key(key_serial_t id,
>  
>  	key_ref_put(key_ref);
>  error2:
> -	__kvzfree(payload, plen);
> +	kvfree_sensitive(payload, plen);
>  error:
>  	return ret;
>  }
> @@ -914,7 +911,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>  		 */
>  		if (ret > key_data_len) {
>  			if (unlikely(key_data))
> -				__kvzfree(key_data, key_data_len);
> +				kvfree_sensitive(key_data, key_data_len);
>  			key_data_len = ret;
>  			continue;	/* Allocate buffer */
>  		}
> @@ -923,7 +920,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>  			ret = -EFAULT;
>  		break;
>  	}
> -	__kvzfree(key_data, key_data_len);
> +	kvfree_sensitive(key_data, key_data_len);
>  
>  key_put_out:
>  	key_put(key);
> @@ -1225,10 +1222,7 @@ long keyctl_instantiate_key_common(key_serial_t id,
>  		keyctl_change_reqkey_auth(NULL);
>  
>  error2:
> -	if (payload) {
> -		memzero_explicit(payload, plen);
> -		kvfree(payload);
> -	}
> +	kvfree_sensitive(payload, plen);
>  error:
>  	return ret;
>  }
> -- 
> 2.18.1
> 
> 
>