RE: [PATCH v5 04/12] ima: Move ima_reset_appraise_flags() call to post hooks

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

> From: Casey Schaufler [mailto:casey@schaufler-ca.com]
> Sent: Wednesday, April 7, 2021 6:18 PM
> On 4/7/2021 3:52 AM, Roberto Sassu wrote:
> > 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),
> > +			xvalue->type == EVM_IMA_XATTR_DIGSIG);
> > +}
> > +
> 
> Now you're calling ima_protect_xattr() twice for each setxattr.
> Is that safe? Is it performant? Does it matter?

Hi Casey

I would expect that this does not have a significant impact on
the performance (it is just a strcmp on the xattr name).

Thanks

Roberto

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

> >  int ima_inode_removexattr(struct dentry *dentry, const char
> *xattr_name)
> >  {
> >  	int result;
> >
> >  	result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
> >  	if (result == 1) {
> > -		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
> >  		result = 0;
> >  	}
> >  	return result;
> >  }
> > +
> > +void ima_inode_post_removexattr(struct dentry *dentry, const char
> *xattr_name)
> > +{
> > +	int result;
> > +
> > +	result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
> > +	if (result == 1)
> > +		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
> > +}
> 
> Now you're calling ima_protect_xattr() twice for each removexattr.
> Is that safe? Is it performant? Does it matter?
> 
> > diff --git a/security/security.c b/security/security.c
> > index 5ac96b16f8fa..efb1f874dc41 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1319,6 +1319,7 @@ void security_inode_post_setxattr(struct dentry
> *dentry, const char *name,
> >  	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> >  		return;
> >  	call_void_hook(inode_post_setxattr, dentry, name, value, size,
> flags);
> > +	ima_inode_post_setxattr(dentry, name, value, size);
> >  	evm_inode_post_setxattr(dentry, name, value, size);
> >  }
> >