Re: [PATCH 3/5] ima: Reset EVM status upon detecting changes to overlay backing file

[Stefan Berger]

On 1/31/24 08:56, Amir Goldstein wrote:
> On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > To avoid caching effects to take effect reset the EVM status upon
> > detecting changes to the overlay backing files. This prevents a not-yet-
> > copied-up file on the overlay from executing if for example the
> > security.evm xattr on the file on the 'lower' layer has been removed.
>
> And what is expected to happen when file is executed after copy up?

The copy-up may be triggered by changing file content or file metadata.
For EVM file metadata (file attributes and xattrs) are important and if 
they change EVM would re-evaluate the file, meaning that it would 
determine the file mode bits, uid, gid and xattrs and calculate a hash 
over them and compare this hash against the signature in security.evm.

> Doesn't this change also protect the same threat after copy up?

From what I remember from my testing is that file attribute or extended 
attribute changes on an already copied-up file were already handled 
correctly, meaning they caused the re-evaluation of the file as 
described above.

> > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > ---
> >   include/linux/evm.h               | 8 ++++++++
> >   security/integrity/evm/evm_main.c | 7 +++++++
> >   security/integrity/ima/ima_main.c | 2 ++
> >   3 files changed, 17 insertions(+)
> >
> > diff --git a/include/linux/evm.h b/include/linux/evm.h
> > index d8c0343436b8..e7d6742eee9d 100644
> > --- a/include/linux/evm.h
> > +++ b/include/linux/evm.h
> > @@ -66,6 +66,8 @@ extern int evm_protected_xattr_if_enabled(const char *req_xattr_name);
> >   extern int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
> >                                       int buffer_size, char type,
> >                                       bool canonical_fmt);
> > +extern void evm_reset_cache_status(struct dentry *dentry,
> > +                                  struct integrity_iint_cache *iint);
> >   #ifdef CONFIG_FS_POSIX_ACL
> >   extern int posix_xattr_acl(const char *xattrname);
> >   #else
> > @@ -189,5 +191,11 @@ static inline int evm_read_protected_xattrs(struct dentry *dentry, u8 *buffer,
> >          return -EOPNOTSUPP;
> >   }
> >
> > +static inline void evm_reset_cache_status(struct dentry *dentry,
> > +                                         struct integrity_iint_cache *iint)
> > +{
> > +       return;
> > +}
> > +
> >   #endif /* CONFIG_EVM */
> >   #endif /* LINUX_EVM_H */
> > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > index 22a5e26860ea..e96d127b48a2 100644
> > --- a/security/integrity/evm/evm_main.c
> > +++ b/security/integrity/evm/evm_main.c
> > @@ -721,6 +721,13 @@ static void evm_reset_status(struct inode *inode)
> >                  iint->evm_status = INTEGRITY_UNKNOWN;
> >   }
> >
> > +void evm_reset_cache_status(struct dentry *dentry,
> > +                           struct integrity_iint_cache *iint)
> > +{
> > +       if (d_real_inode(dentry) != d_backing_inode(dentry))
> > +               iint->evm_status = INTEGRITY_UNKNOWN;
> > +}
> > +
> >   /**
> >    * evm_revalidate_status - report whether EVM status re-validation is necessary
> >    * @xattr_name: pointer to the affected extended attribute name
> > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > index cc1217ac2c6f..84bdc6e58329 100644
> > --- a/security/integrity/ima/ima_main.c
> > +++ b/security/integrity/ima/ima_main.c
> > @@ -26,6 +26,7 @@
> >   #include <linux/ima.h>
> >   #include <linux/fs.h>
> >   #include <linux/iversion.h>
> > +#include <linux/evm.h>
> >
> >   #include "ima.h"
> >
> > @@ -295,6 +296,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
> >                      !inode_eq_iversion(backing_inode, iint->version)) {
> >                          iint->flags &= ~IMA_DONE_MASK;
> >                          iint->measured_pcrs = 0;
> > +                       evm_reset_cache_status(file_dentry(file), iint);
> >                  }
> >          }
>
> Make sense.
> Unrelated to your change, I now noticed something odd about Mimi's change:
>
>          backing_inode = d_real_inode(file_dentry(file));
>
> I find the choice of variable name to be quite confusing, because ima/evm code
> uses  d_backing_inode() all over the place and d_real_inode() !=
> d_backing_inode().
>
> First of all, there is never any reason to use d_backing_inode() and its name is
> quite confusing in the first place, but it will be a big cleanup to
> remove them all.
>
> Suggest to rename the variable to real_inode, same as in
> ima_collect_measurement()
> to be consistent and reduce confusion factor, which is already high enough ;)
>
> Thanks,
> Amir.