Re: [PATCH v2 6/6] module: Move duplicate mod_check_sig users code to mod_parse_sig

Michal Suchánek
2021-12-13T18:06:20Z

Hello,

On Tue, Dec 07, 2021 at 05:10:34PM +0100, Philipp Rudo wrote:
> Hi Michal,
>
> On Thu, 25 Nov 2021 19:02:44 +0100
> Michal Suchanek <msuchanek@suse.de> wrote:
>
> > Multiple users of mod_check_sig check for the marker, then call
> > mod_check_sig, extract signature length, and remove the signature.
> >
> > Put this code in one place together with mod_check_sig.
> >
> > Signed-off-by: Michal Suchanek <msuchanek@suse.de>
> > ---
> > include/linux/module_signature.h | 1 +
> > kernel/module_signature.c | 56 ++++++++++++++++++++++++++++-
> > kernel/module_signing.c | 26 +++-----------
> > security/integrity/ima/ima_modsig.c | 22 ++----------
> > 4 files changed, 63 insertions(+), 42 deletions(-)
> >
> > diff --git a/include/linux/module_signature.h b/include/linux/module_signature.h
> > index 7eb4b00381ac..1343879b72b3 100644
> > --- a/include/linux/module_signature.h
> > +++ b/include/linux/module_signature.h
> > @@ -42,5 +42,6 @@ struct module_signature {
> >
> > int mod_check_sig(const struct module_signature *ms, size_t file_len,
> > const char *name);
> > +int mod_parse_sig(const void *data, size_t *len, size_t *sig_len, const char *name);
> >
> > #endif /* _LINUX_MODULE_SIGNATURE_H */
> > diff --git a/kernel/module_signature.c b/kernel/module_signature.c
> > index 00132d12487c..784b40575ee4 100644
> > --- a/kernel/module_signature.c
> > +++ b/kernel/module_signature.c
> > @@ -8,14 +8,36 @@
> >
> > #include <linux/errno.h>
> > #include <linux/printk.h>
> > +#include <linux/string.h>
> > #include <linux/module_signature.h>
> > #include <asm/byteorder.h>
> >
> > +/**
> > + * mod_check_sig_marker - check that the given data has signature marker at the end
> > + *
> > + * @data: Data with appended signature
> > + * @len: Length of data. Signature marker length is subtracted on success.
> > + */
> > +static inline int mod_check_sig_marker(const void *data, size_t *len)
>
> I personally don't like it when a function has a "check" in it's name
> as it doesn't describe what the function is checking for. For me

It is consistent with mod_check_sig

> mod_has_sig_marker is much more precise. I would use that instead.

It actually would not because it does more than that.

Thanks

Michal

>
> Thanks
> Philipp
>
> > +{
> > + const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
> > +
> > + if (markerlen > *len)
> > + return -ENODATA;
> > +
> > + if (memcmp(data + *len - markerlen, MODULE_SIG_STRING,
> > + markerlen))
> > + return -ENODATA;
> > +
> > + *len -= markerlen;
> > + return 0;
> > +}
> > +
> > /**
> > * mod_check_sig - check that the given signature is sane
> > *
> > * @ms: Signature to check.
> > - * @file_len: Size of the file to which @ms is appended.
> > + * @file_len: Size of the file to which @ms is appended (without the marker).
> > * @name: What is being checked. Used for error messages.
> > */
> > int mod_check_sig(const struct module_signature *ms, size_t file_len,
> > @@ -44,3 +66,35 @@ int mod_check_sig(const struct module_signature *ms, size_t file_len,
> >
> > return 0;
> > }
> > +
> > +/**
> > + * mod_parse_sig - check that the given signature is sane and determine signature length
> > + *
> > + * @data: Data with appended signature.
> > + * @len: Length of data. Signature and marker length is subtracted on success.
> > + * @sig_len: Length of signature. Filled on success.
> > + * @name: What is being checked. Used for error messages.
> > + */
> > +int mod_parse_sig(const void *data, size_t *len, size_t *sig_len, const char *name)
> > +{
> > + const struct module_signature *sig;
> > + int rc;
> > +
> > + rc = mod_check_sig_marker(data, len);
> > + if (rc)
> > + return rc;
> > +
> > + if (*len < sizeof(*sig))
> > + return -ENODATA;
> > +
> > + sig = (const struct module_signature *)(data + (*len - sizeof(*sig)));
> > +
> > + rc = mod_check_sig(sig, *len, name);
> > + if (rc)
> > + return rc;
> > +
> > + *sig_len = be32_to_cpu(sig->sig_len);
> > + *len -= *sig_len + sizeof(*sig);
> > +
> > + return 0;
> > +}
> > diff --git a/kernel/module_signing.c b/kernel/module_signing.c
> > index cef72a6f6b5d..02bbca90f467 100644
> > --- a/kernel/module_signing.c
> > +++ b/kernel/module_signing.c
> > @@ -25,35 +25,17 @@ int verify_appended_signature(const void *data, size_t *len,
> > struct key *trusted_keys,
> > enum key_being_used_for purpose)
> > {
> > - const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
> > struct module_signature ms;
> > - size_t sig_len, modlen = *len;
> > + size_t sig_len;
> > int ret;
> >
> > - pr_devel("==>%s %s(,%zu)\n", __func__, key_being_used_for[purpose], modlen);
> > + pr_devel("==>%s %s(,%zu)\n", __func__, key_being_used_for[purpose], *len);
> >
> > - if (markerlen > modlen)
> > - return -ENODATA;
> > -
> > - if (memcmp(data + modlen - markerlen, MODULE_SIG_STRING,
> > - markerlen))
> > - return -ENODATA;
> > - modlen -= markerlen;
> > -
> > - if (modlen <= sizeof(ms))
> > - return -EBADMSG;
> > -
> > - memcpy(&ms, data + (modlen - sizeof(ms)), sizeof(ms));
> > -
> > - ret = mod_check_sig(&ms, modlen, key_being_used_for[purpose]);
> > + ret = mod_parse_sig(data, len, &sig_len, key_being_used_for[purpose]);
> > if (ret)
> > return ret;
> >
> > - sig_len = be32_to_cpu(ms.sig_len);
> > - modlen -= sig_len + sizeof(ms);
> > - *len = modlen;
> > -
> > - return verify_pkcs7_signature(data, modlen, data + modlen, sig_len,
> > + return verify_pkcs7_signature(data, *len, data + *len, sig_len,
> > trusted_keys,
> > purpose,
> > NULL, NULL);
> > diff --git a/security/integrity/ima/ima_modsig.c b/security/integrity/ima/ima_modsig.c
> > index fb25723c65bc..46917eb37fd8 100644
> > --- a/security/integrity/ima/ima_modsig.c
> > +++ b/security/integrity/ima/ima_modsig.c
> > @@ -37,33 +37,17 @@ struct modsig {
> > *
> > * Return: 0 on success, error code otherwise.
> > */
> > -int ima_read_modsig(enum ima_hooks func, const void *buf, loff_t buf_len,
> > +int ima_read_modsig(enum ima_hooks func, const void *buf, loff_t len,
> > struct modsig **modsig)
> > {
> > - const size_t marker_len = strlen(MODULE_SIG_STRING);
> > - const struct module_signature *sig;
> > struct modsig *hdr;
> > - size_t sig_len;
> > - const void *p;
> > + size_t sig_len, buf_len = len;
> > int rc;
> >
> > - if (buf_len <= marker_len + sizeof(*sig))
> > - return -ENOENT;
> > -
> > - p = buf + buf_len - marker_len;
> > - if (memcmp(p, MODULE_SIG_STRING, marker_len))
> > - return -ENOENT;
> > -
> > - buf_len -= marker_len;
> > - sig = (const struct module_signature *)(p - sizeof(*sig));
> > -
> > - rc = mod_check_sig(sig, buf_len, func_tokens[func]);
> > + rc = mod_parse_sig(buf, &buf_len, &sig_len, func_tokens[func]);
> > if (rc)
> > return rc;
> >
> > - sig_len = be32_to_cpu(sig->sig_len);
> > - buf_len -= sig_len + sizeof(*sig);
> > -
> > /* Allocate sig_len additional bytes to hold the raw PKCS#7 data. */
> > hdr = kzalloc(sizeof(*hdr) + sig_len, GFP_KERNEL);
> > if (!hdr)
>