On Mon, 2026-08-24 at 12:00 -0400, Mimi Zohar wrote:
> On Sat, 2026-08-22 at 08:24 +0000, Jérémy Jean wrote:
> > ima_collect_modsig() binds the file being appraised to the parsed PKCS#7
> > message with pkcs7_supply_detached_data(). If the message already contains
> > embedded data, the helper returns -EINVAL. However, ima_collect_modsig()
> > silently returns because its return type is void.
> >
> > ima_modsig_verify() subsequently calls verify_pkcs7_message_sig() with a
> > NULL data pointer. That verifies the embedded PKCS#7 data instead of the
> > file being appraised. An attacker can therefore take a valid
> > detached signature, insert the original signed bytes as embedded content
> > without the private key, and append the resulting message to an unrelated
> > file. IMA accepts the unrelated file.
> >
> > This bypasses the module authenticity boundary when an IMA modsig appraisal
> > rule is enforced while CONFIG_MODULE_SIG is disabled.
> >
> > Return detached-data binding errors and stop measurement collection when
> > binding fails. Keep digest extraction optional because algorithms such as
> > ML-DSA can verify the message directly. Appraisal then rejects the file
> > rather than reaching verification with embedded content active.
> >
> > Fixes: 15588227e086 ("ima: Collect modsig")
> > Signed-off-by: Jérémy Jean <[email protected]>
> > Assisted-by: Codex:gpt-5
> > ---
[...]
> > static inline int ima_get_modsig_digest(const struct modsig *modsig,
> > diff --git a/security/integrity/ima/ima_api.c
> > b/security/integrity/ima/ima_api.c
> > index 122d127e108d..e2b167f24778 100644
> > --- a/security/integrity/ima/ima_api.c
> > +++ b/security/integrity/ima/ima_api.c
> > @@ -262,8 +262,11 @@ int ima_collect_measurement(struct ima_iint_cache
> > *iint, struct file *file,
> > * the file digest without collecting the modsig in a previous
> > * measurement rule.
> > */
> > - if (modsig)
> > - ima_collect_modsig(modsig, buf, size);
> > + if (modsig) {
> > + result = ima_collect_modsig(modsig, buf, size);
> > + if (result)
> > + goto out;
> > +
>
> The normal processing of getting the xattr needs to continue even if the
> appended signatures doesn't exist. Exiting here is too early. Consider saving
> the result as a different variable and returning an error later, if needed.
By going to "out", the file will not be properly measured, nor would the file be
appraised based on security.ima xattr. A better solution would be to free
modsig and continue as if there wasn't an appended signature.
thanks,
Mimi
