Re: [PATCH v2] KEYS: trusted: dcp: fix key_len validation and calc_blob_len() return type

On Mon, Jul 20, 2026 at 12:34:54PM +0200, David Gstir wrote:
> Hi Fabrice,
> 
> > On 19.07.2026, at 18:41, Fabrice Derepas <[email protected]> wrote:
> > Two correctness and type-hygiene issues exist in the DCP trusted keys
> > implementation.
> > 
> > First, trusted_dcp_unseal() reads p->key_len from a user-supplied blob
> > without checking if it exceeds MAX_KEY_SIZE.  If a crafted blob provides
> > a payload_len larger than 128, the subsequent do_aead_crypto() call
> > writes past the end of the p->key array into the adjacent p->blob
> > buffer within the same struct trusted_key_payload -- the caller's own
> > input, not unrelated kernel memory.  While not exploitable, this
> > violates strict array bounds and triggers static analyzers.  Fix this by adding a validation check against
> > MIN_KEY_SIZE and MAX_KEY_SIZE immediately after reading the length,
> > matching the checks already done in trusted_core.c.
> > 
> > Second, calc_blob_len() calculates a sum in size_t that truncates to
> > unsigned int on 64-bit platforms.  Because the DCP hardware is only
> > present on 32-bit i.MX SoC platforms, size_t and unsigned int are
> > functionally equivalent in production, making this truncation harmless in
> > practice.  Nevertheless, updating the return type to size_t (and
> > subsequently updating 'blen' in the seal/unseal paths) resolves
> > type-narrowing warnings and improves overall code hygiene.
> > 
> > Fixes: 2e8a0f40a39c ("KEYS: trusted: Introduce NXP DCP-backed trusted keys")
> > Signed-off-by: Fabrice Derepas <[email protected]>
> > ---
> > Changes in v2:
> >  - Reframed commit message as correctness/hardening fix (Richard, Jarkko)
> >  - Changed bare return -EINVAL to goto out for consistency
> >  - Added Fixes tag
> > 
> > Compile-tested with arm-linux-gnueabihf-gcc on imx_v6_v7_defconfig
> > with CONFIG_TRUSTED_KEYS_DCP=y (W=1, zero warnings). No DCP hardware
> > available for runtime testing.
> > 
> > security/keys/trusted-keys/trusted_dcp.c | 15 +++++++++++----
> > 1 file changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/security/keys/trusted-keys/trusted_dcp.c b/security/keys/trusted-keys/trusted_dcp.c
> > index 7b6eb655d..c078adebe 100644
> > --- a/security/keys/trusted-keys/trusted_dcp.c
> > +++ b/security/keys/trusted-keys/trusted_dcp.c
> > @@ -69,7 +69,7 @@ static bool skip_zk_test;
> > module_param_named(dcp_skip_zk_test, skip_zk_test, bool, 0);
> > MODULE_PARM_DESC(dcp_skip_zk_test, "Don't test whether device keys are zero'ed");
> > 
> > -static unsigned int calc_blob_len(unsigned int payload_len)
> > +static size_t calc_blob_len(unsigned int payload_len)
> > {
> >    return sizeof(struct dcp_blob_fmt) + payload_len + DCP_BLOB_AUTHLEN;
> > }
> > @@ -200,7 +200,8 @@ static int encrypt_blob_key(u8 *plain_key, u8 *encrypted_key)
> > static int trusted_dcp_seal(struct trusted_key_payload *p, char *datablob)
> > {
> >    struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob;
> > -    int blen, ret;
> > +    size_t blen;
> > +    int ret;
> >    u8 *plain_blob_key;
> > 
> >    blen = calc_blob_len(p->key_len);
> > @@ -242,7 +243,8 @@ static int trusted_dcp_seal(struct trusted_key_payload *p, char *datablob)
> > static int trusted_dcp_unseal(struct trusted_key_payload *p, char *datablob)
> > {
> >    struct dcp_blob_fmt *b = (struct dcp_blob_fmt *)p->blob;
> > -    int blen, ret;
> > +    size_t blen;
> > +    int ret;
> >    u8 *plain_blob_key = NULL;
> > 
> >    if (b->fmt_version != DCP_BLOB_VERSION) {
> > @@ -253,9 +255,14 @@ static int trusted_dcp_unseal(struct trusted_key_payload *p, char *datablob)
> >    }
> > 
> >    p->key_len = le32_to_cpu(b->payload_len);
> > +    if (p->key_len < MIN_KEY_SIZE || p->key_len > MAX_KEY_SIZE) {
> > +        ret = -EINVAL;
> > +        goto out;
> > +    }
> > +
> >    blen = calc_blob_len(p->key_len);
> >    if (blen != p->blob_len) {
> > -        pr_err("DCP blob has bad length: %i != %i\n", blen,
> > +        pr_err("DCP blob has bad length: %zu != %u\n", blen,
> >               p->blob_len);
> >        ret = -EINVAL;
> >        goto out;
> > 
> > base-commit: 6d41c11ef3044788902a05e292cd7f0a9c7a8157
> > --
> > 2.53.0
> > 
> 
> LGTM and thanks for the fix!
> 
> Reviewed-by: David Gstir <[email protected]>
> 
> - David


Reviewed-by: Jarkko Sakkinen <[email protected]>

And applied (not yet pushed, will after end of my inbox)

BR, Jarkko