On Wed, Aug 26, 2026 at 11:44:56AM -0400, Cen Zhang (Microsoft Security FORGE
Labs) wrote:
> From: "Cen Zhang (Microsoft Security FORGE Labs)" <[email protected]>
>
> 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
Please use concrete sizes as possible. E.g., 16-bit word or u16 would be
much less involved than "unsigned short".
> 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.
One more: please describe the issue as ergnomic as possible (but still
complete). I care neither Alice nor Bob.
>
> 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: [email protected]
> Assisted-by: GitHub-Copilot:claude-opus-4.6
> Signed-off-by: Cen Zhang (Microsoft Security FORGE Labs)
> <[email protected]>
> Signed-off-by: Francis Perron (Akrites SIRT) <[email protected]>
> ---
> 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.
>
> 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
