On Thu, 2022-12-01 at 10:53 -0800, Eric Biggers wrote:
> On Thu, Dec 01, 2022 at 11:06:24AM +0100, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> >
> > Commit ac4e97abce9b8 ("scatterlist: sg_set_buf() argument must be in linear
> > mapping") checks that both the signature and the digest reside in the
> > linear mapping area.
> >
> > However, more recently commit ba14a194a434c ("fork: Add generic vmalloced
> > stack support"), made it possible to move the stack in the vmalloc area,
> > which is not contiguous, and thus not suitable for sg_set_buf() which needs
> > adjacent pages.
> >
> > Fix this by checking if CONFIG_VMAP_STACK is enabled. If yes, allocate an
> > evm_digest structure, and use that instead of the in-stack cbounterpart.
> >
> > Cc: stable@xxxxxxxxxxxxxxx # 4.9.x
> > Fixes: ba14a194a434 ("fork: Add generic vmalloced stack support")
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> > security/integrity/evm/evm_main.c | 26 +++++++++++++++++++++-----
> > 1 file changed, 21 insertions(+), 5 deletions(-)
> >
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index 23d484e05e6f..7f76d6103f2e 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -174,6 +174,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
> > struct signature_v2_hdr *hdr;
> > enum integrity_status evm_status = INTEGRITY_PASS;
> > struct evm_digest digest;
> > + struct evm_digest *digest_ptr = &digest;
> > struct inode *inode;
> > int rc, xattr_len, evm_immutable = 0;
> >
> > @@ -231,14 +232,26 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
> > }
> >
> > hdr = (struct signature_v2_hdr *)xattr_data;
> > - digest.hdr.algo = hdr->hash_algo;
> > +
> > + if (IS_ENABLED(CONFIG_VMAP_STACK)) {
> > + digest_ptr = kmalloc(sizeof(*digest_ptr), GFP_NOFS);
> > + if (!digest_ptr) {
> > + rc = -ENOMEM;
> > + break;
> > + }
> > + }
> > +
> > + digest_ptr->hdr.algo = hdr->hash_algo;
> > +
> > rc = evm_calc_hash(dentry, xattr_name, xattr_value,
> > - xattr_value_len, xattr_data->type, &digest);
> > + xattr_value_len, xattr_data->type,
> > + digest_ptr);
> > if (rc)
> > break;
> > rc = integrity_digsig_verify(INTEGRITY_KEYRING_EVM,
> > (const char *)xattr_data, xattr_len,
> > - digest.digest, digest.hdr.length);
> > + digest_ptr->digest,
> > + digest_ptr->hdr.length);
> > if (!rc) {
> > inode = d_backing_inode(dentry);
> >
> > @@ -268,8 +281,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
> > else
> > evm_status = INTEGRITY_FAIL;
> > }
> > - pr_debug("digest: (%d) [%*phN]\n", digest.hdr.length, digest.hdr.length,
> > - digest.digest);
> > + pr_debug("digest: (%d) [%*phN]\n", digest_ptr->hdr.length,
> > + digest_ptr->hdr.length, digest_ptr->digest);
> > +
> > + if (digest_ptr && digest_ptr != &digest)
> > + kfree(digest_ptr);
>
> What is the actual problem here? Where is a scatterlist being created from this
> buffer? AFAICS it never happens.
Enabling CONFIG_VMAP_STACK is the culprit, which triggers the BUG_ON
only when CONFIG_DEBUG_SG is enabled as well.
Refer to commit ba14a194a434 ("fork: Add generic vmalloced stack
support").
--
thanks,
Mimi
