[PATCH v4] evm: Correct inode_init_security hooks behaviors

[Roberto Sassu]

On Sat, 2022-11-05 at 07:06 -0400, Paul Moore wrote:
> On Wed, Oct 26, 2022 at 10:30 AM Nicolas Bouchinet
> <nicolas.bouchinet at clip-os.org> wrote:
> > From: Nicolas Bouchinet <nicolas.bouchinet at ssi.gouv.fr>
> > 
> > Fixes a NULL pointer dereference occurring in the
> > `evm_protected_xattr_common` function of the EVM LSM. The bug is
> > triggered if a `inode_init_security` hook returns 0 without initializing
> > the given `struct xattr` fields (which is the case of BPF) and if no
> > other LSM overrides thoses fields after. This also leads to memory
> > leaks.
> > 
> > The `call_int_hook_xattr` macro has been inlined into the
> > `security_inode_init_security` hook in order to check hooks return
> > values and skip ones who doesn't init `xattrs`.
> > 
> > Modify `evm_init_hmac` function to init the EVM hmac using every
> > entry of the given xattr array.
> > 
> > The `MAX_LSM_EVM_XATTR` value is now based on the security modules
> > compiled in, which gives room for SMACK, SELinux, Apparmor, BPF and
> > IMA/EVM security attributes.
> > 
> > Changes the default return value of the `inode_init_security` hook
> > definition to `-EOPNOTSUPP`.
> > 
> > Changes the hook documentation to match the behavior of the LSMs using
> > it (only xattr->value is initialised with kmalloc and thus is the only
> > one that should be kfreed by the caller).
> > 
> > Cc: roberto.sassu at huaweicloud.com
> > Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet at ssi.gouv.fr>
> > ---
> > Changes since v3:
> > https://lore.kernel.org/linux-integrity/Y1fu4jofqLHVDprT@archlinux/
> > 
> > * Fixes compilation error reported by the kernel test robot.
> > ---
> >  include/linux/lsm_hook_defs.h       |  2 +-
> >  include/linux/lsm_hooks.h           |  4 ++--
> >  security/integrity/evm/evm.h        |  1 +
> >  security/integrity/evm/evm_crypto.c |  9 +++++++--
> >  security/integrity/evm/evm_main.c   |  7 ++++---
> >  security/security.c                 | 31 ++++++++++++++++++++++-------
> >  6 files changed, 39 insertions(+), 15 deletions(-)
> 
> ...
> 
> > diff --git a/security/security.c b/security/security.c
> > index 14d30fec8a00..79524f8734f1 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -30,7 +30,11 @@
> >  #include <linux/msg.h>
> >  #include <net/flow.h>
> > 
> > -#define MAX_LSM_EVM_XATTR      2
> > +#define MAX_LSM_EVM_XATTR                                \
> > +       ((IS_ENABLED(CONFIG_EVM) ? 1 : 0) +              \
> > +        (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \
> > +        (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) +   \
> > +        (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0))
> 
> ...
> 
> > @@ -1091,9 +1095,11 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >                                  const struct qstr *qstr,
> >                                  const initxattrs initxattrs, void *fs_data)
> >  {
> > +       int i = 0;
> > +       int ret = -EOPNOTSUPP;
> >         struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
> >         struct xattr *lsm_xattr, *evm_xattr, *xattr;
> > -       int ret;
> > +       struct security_hook_list *hook_ptr;
> > 
> >         if (unlikely(IS_PRIVATE(inode)))
> >                 return 0;
> > @@ -1103,15 +1109,26 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> >                                      dir, qstr, NULL, NULL, NULL);
> >         memset(new_xattrs, 0, sizeof(new_xattrs));
> >         lsm_xattr = new_xattrs;
> > -       ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
> > -                                               &lsm_xattr->name,
> > -                                               &lsm_xattr->value,
> > -                                               &lsm_xattr->value_len);
> > +       hlist_for_each_entry(hook_ptr, &security_hook_heads.inode_init_security,
> > +                            list) {
> > +               ret = hook_ptr->hook.inode_init_security(inode, dir, qstr,
> > +                               &lsm_xattr->name,
> > +                               &lsm_xattr->value,
> > +                               &lsm_xattr->value_len);
> > +               if (ret == -EOPNOTSUPP)
> > +                       continue;
> > +               if (WARN_ON_ONCE(i >= MAX_LSM_EVM_XATTR))
> > +                       ret = -ENOMEM;
> 
> It would really like to see us get rid of the MAX_LSM_EVM_XATTR macro
> and determine the array size similar to what we do with the security
> blob sizes.  The macro definition is a kludgy hack that is bound to
> get out of sync at some point and this extra checking inside the hook
> is something we should work to remove.

In this case, I already implemented this, as it was originally
suggested by Casey. I will resend this:

https://lore.kernel.org/linux-integrity/20210427113732.471066-1-roberto.sassu@huawei.com/

with few minor tweaks.

Roberto

> > +               if (ret != 0)
> > +                       break;
> > +               lsm_xattr++;
> > +               i++;
> > +       }
> >         if (ret)
> >                 goto out;
> > 
> >         evm_xattr = lsm_xattr + 1;
> > -       ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
> > +       ret = evm_inode_init_security(inode, new_xattrs, evm_xattr);
> >         if (ret)
> >                 goto out;
> >         ret = initxattrs(inode, new_xattrs, fs_data);
> > --
> > 2.38.1