RE: [PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to post hooks

[Roberto Sassu <roberto.sassu@huawei.com>]

> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Friday, April 30, 2021 8:00 PM
> On Wed, 2021-04-28 at 15:35 +0000, Roberto Sassu wrote:
> > > From: Roberto Sassu
> > > Sent: Friday, March 5, 2021 4:19 PM
> > > ima_inode_setxattr() and ima_inode_removexattr() hooks are called
> before
> > > an
> > > operation is performed. Thus, ima_reset_appraise_flags() should not be
> > > called there, as flags might be unnecessarily reset if the operation is
> > > denied.
> > >
> > > This patch introduces the post hooks ima_inode_post_setxattr() and
> > > ima_inode_post_removexattr(), and adds the call to
> > > ima_reset_appraise_flags() in the new functions.
> > >
> > > Cc: Casey Schaufler <casey@schaufler-ca.com>
> > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> > > ---
> > >  fs/xattr.c                            |  2 ++
> > >  include/linux/ima.h                   | 18 ++++++++++++++++++
> > >  security/integrity/ima/ima_appraise.c | 25 ++++++++++++++++++++++---
> > >  security/security.c                   |  1 +
> > >  4 files changed, 43 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/xattr.c b/fs/xattr.c
> > > index b3444e06cded..81847f132d26 100644
> > > --- a/fs/xattr.c
> > > +++ b/fs/xattr.c
> > > @@ -16,6 +16,7 @@
> > >  #include <linux/namei.h>
> > >  #include <linux/security.h>
> > >  #include <linux/evm.h>
> > > +#include <linux/ima.h>
> > >  #include <linux/syscalls.h>
> > >  #include <linux/export.h>
> > >  #include <linux/fsnotify.h>
> > > @@ -502,6 +503,7 @@ __vfs_removexattr_locked(struct
> user_namespace
> > > *mnt_userns,
> > >
> > >  	if (!error) {
> > >  		fsnotify_xattr(dentry);
> > > +		ima_inode_post_removexattr(dentry, name);
> > >  		evm_inode_post_removexattr(dentry, name);
> > >  	}
> > >
> > > diff --git a/include/linux/ima.h b/include/linux/ima.h
> > > index 61d5723ec303..5e059da43857 100644
> > > --- a/include/linux/ima.h
> > > +++ b/include/linux/ima.h
> > > @@ -171,7 +171,13 @@ extern void ima_inode_post_setattr(struct
> > > user_namespace *mnt_userns,
> > >  				   struct dentry *dentry);
> > >  extern int ima_inode_setxattr(struct dentry *dentry, const char
> *xattr_name,
> > >  		       const void *xattr_value, size_t xattr_value_len);
> > > +extern void ima_inode_post_setxattr(struct dentry *dentry,
> > > +				    const char *xattr_name,
> > > +				    const void *xattr_value,
> > > +				    size_t xattr_value_len);
> > >  extern int ima_inode_removexattr(struct dentry *dentry, const char
> > > *xattr_name);
> > > +extern void ima_inode_post_removexattr(struct dentry *dentry,
> > > +				       const char *xattr_name);
> > >  #else
> > >  static inline bool is_ima_appraise_enabled(void)
> > >  {
> > > @@ -192,11 +198,23 @@ static inline int ima_inode_setxattr(struct
> dentry
> > > *dentry,
> > >  	return 0;
> > >  }
> > >
> > > +static inline void ima_inode_post_setxattr(struct dentry *dentry,
> > > +					   const char *xattr_name,
> > > +					   const void *xattr_value,
> > > +					   size_t xattr_value_len)
> > > +{
> > > +}
> > > +
> > >  static inline int ima_inode_removexattr(struct dentry *dentry,
> > >  					const char *xattr_name)
> > >  {
> > >  	return 0;
> > >  }
> > > +
> > > +static inline void ima_inode_post_removexattr(struct dentry *dentry,
> > > +					      const char *xattr_name)
> > > +{
> > > +}
> > >  #endif /* CONFIG_IMA_APPRAISE */
> > >
> > >  #if defined(CONFIG_IMA_APPRAISE) &&
> > > defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> > > diff --git a/security/integrity/ima/ima_appraise.c
> > > b/security/integrity/ima/ima_appraise.c
> > > index 565e33ff19d0..1f029e4c8d7f 100644
> > > --- a/security/integrity/ima/ima_appraise.c
> > > +++ b/security/integrity/ima/ima_appraise.c
> > > @@ -577,21 +577,40 @@ int ima_inode_setxattr(struct dentry *dentry,
> const
> > > char *xattr_name,
> > >  	if (result == 1) {
> > >  		if (!xattr_value_len || (xvalue->type >= IMA_XATTR_LAST))
> > >  			return -EINVAL;
> > > -		ima_reset_appraise_flags(d_backing_inode(dentry),
> > > -			xvalue->type == EVM_IMA_XATTR_DIGSIG);
> > >  		result = 0;
> > >  	}
> > >  	return result;
> > >  }
> > >
> > > +void ima_inode_post_setxattr(struct dentry *dentry, const char
> > > *xattr_name,
> > > +			     const void *xattr_value, size_t xattr_value_len)
> > > +{
> > > +	const struct evm_ima_xattr_data *xvalue = xattr_value;
> > > +	int result;
> > > +
> > > +	result = ima_protect_xattr(dentry, xattr_name, xattr_value,
> > > +				   xattr_value_len);
> > > +	if (result == 1)
> > > +		ima_reset_appraise_flags(d_backing_inode(dentry),
> >
> > I found an issue in this patch.
> >
> > Moving ima_reset_appraise_flags() to the post hook causes this
> > function to be executed also when __vfs_setxattr_noperm() is
> > called.
> >
> > The problem is that at the end of a write IMA calls
> > ima_collect_measurement() to recalculate the file digest and
> > update security.ima. ima_collect_measurement() sets
> > IMA_COLLECTED.
> >
> > However, after that __vfs_setxattr_noperm() causes
> > IMA_COLLECTED to be reset, and to unnecessarily recalculate
> > the file digest. This wouldn't happen if ima_reset_appraise_flags()
> > is in the pre hook.
> >
> > I solved by replacing:
> > 	iint->flags &= ~IMA_DONE_MASK;
> > with:
> > 	iint->flags &= ~(IMA_DONE_MASK & ~IMA_COLLECTED);
> >
> > just when the IMA_CHANGE_XATTR bit is set. It should
> > not be a problem since setting an xattr does not influence
> > the file content.
> >
> > Mimi, what do you think?
> 
> Thank yor for noticing this.
> 
> Without seeing the actual change it is hard to tell.   The only place
> that "iint->flags &= ~IMA_DONE_MASK;" occurs is in neither of the above
> functions, but in process_measurement().  There it is a part of a
> compound "if" statement.  Perhaps it would be ok to change it for just
> the IMA_CHANGE_XATTR test, but definitely not for the other conditions,
> like untrusted mounts.

Ok. Should I include this change in this patch or in a separate patch?
	
> Moving ima_reset_appraise_flags() to the post hooks is to minimize
> resetting the flags unnecessarily.  That is really a performance fix,
> not something necessary for making the EVM portable & immutable
> signatures more usable.  As much as possible, please minimize the
> changes to facilitate review and testing.

Ok.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> thanks,
> 
> Mimi