Re: [PATCH v2] KEYS: encrypted: fix integer overflow of datablob_len

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Aug 26, 2026 at 11:44:56AM -0400, Cen Zhang (Microsoft Security FORGE Labs) wrote:
> From: "Cen Zhang (Microsoft Security FORGE Labs)" <cenzhang@microsoft.com>
> 
> The datablob_len field in struct encrypted_key_payload and the local
> variable in encrypted_key_alloc() are declared as unsigned short, which
> has a maximum value of 65535. The datablob_len is computed as:
> 
>   format_len + 1 + strlen(master_desc) + 1 + strlen(datalen) + 1
>   + ivsize + 1 + encrypted_datalen
> 
> An attacker can create an encrypted key with a very long datalen string
> (e.g., 32756 characters of leading zeros followed by "4096", which
> kstrtol() happily parses as 4096), and then update it with a very long
> master_desc string (~32760 characters). The combined lengths exceed
> 65535, causing the unsigned short to silently wrap around. This results
> in a grossly undersized kzalloc() allocation, and the subsequent
> memcpy() in __ekey_init() writes ~32KB past the end of the buffer,
> corrupting adjacent slab objects.
> 
> Fix this by:
> 1. Using check_add_overflow() to calculate datablob_len directly into
>    its existing unsigned short destination.
> 2. Checking the total payload length the same way before passing it to
>    key_payload_reserve(), since key->datalen is also unsigned short.
> 3. Using kzalloc_flex() to allocate encrypted_key_payload together with
>    its trailing payload_data[] array.
> 
> Fixes: 7e70cb497850 ("keys: add new key-type encrypted")
> Cc: stable@vger.kernel.org
> Assisted-by: GitHub-Copilot:claude-opus-4.6
> Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs) <cenzhang@microsoft.com>
> Signed-off-by: Francis Perron (Akrites SIRT) <francis@akrites.dev>
> ---
> Changes in v2:
> - Keep datablob_len unchanged and check both unsigned short bounds with
>   check_add_overflow().
> - Use kzalloc_flex() for the trailing payload_data[] array.
> - Correct the attribution and sign-off trailers.
> 
> The initial version was discussed off-list and is not publicly archived.

I have no recollection of being part of this discussion.

> 
>  security/keys/encrypted-keys/encrypted.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
> index 59cb77b237b3..e07092ea301a 100644
> --- a/security/keys/encrypted-keys/encrypted.c
> +++ b/security/keys/encrypted-keys/encrypted.c
> @@ -19,6 +19,7 @@
>  #include <linux/parser.h>
>  #include <linux/string.h>
>  #include <linux/err.h>
> +#include <linux/overflow.h>
>  #include <keys/user-type.h>
>  #include <keys/trusted-type.h>
>  #include <keys/encrypted-type.h>
> @@ -579,6 +580,7 @@ static struct encrypted_key_payload *encrypted_key_alloc(struct key *key,
>  {
>  	struct encrypted_key_payload *epayload = NULL;
>  	unsigned short datablob_len;
> +	unsigned short payload_totallen;
>  	unsigned short decrypted_datalen;
>  	unsigned short payload_datalen;
>  	unsigned int encrypted_datalen;
> @@ -632,16 +634,22 @@ static struct encrypted_key_payload *encrypted_key_alloc(struct key *key,
>  
>  	encrypted_datalen = roundup(decrypted_datalen, blksize);
>  
> -	datablob_len = format_len + 1 + strlen(master_desc) + 1
> -	    + strlen(datalen) + 1 + ivsize + 1 + encrypted_datalen;
> +	if (check_add_overflow(format_len + 1 + strlen(master_desc) + 1
> +			       + strlen(datalen) + 1 + ivsize + 1,
> +			       encrypted_datalen, &datablob_len))
> +		return ERR_PTR(-EINVAL);
> +
> +	if (check_add_overflow(datablob_len,
> +			       payload_datalen + HASH_SIZE + 1,
> +			       &payload_totallen))
> +		return ERR_PTR(-EINVAL);
>  
> -	ret = key_payload_reserve(key, payload_datalen + datablob_len
> -				  + HASH_SIZE + 1);
> +	ret = key_payload_reserve(key, payload_totallen);
>  	if (ret < 0)
>  		return ERR_PTR(ret);
>  
> -	epayload = kzalloc(sizeof(*epayload) + payload_datalen +
> -			   datablob_len + HASH_SIZE + 1, GFP_KERNEL);
> +	epayload = kzalloc_flex(*epayload, payload_data, payload_totallen,
> +				GFP_KERNEL);
>  	if (!epayload)
>  		return ERR_PTR(-ENOMEM);
>  
> -- 
> 2.55.0

BR, Jarkko