[PATCH] fs: Return raw xattr for security.* if there is size disagreement with LSMs

[Roberto Sassu]

> From: Stefan Berger [mailto:stefanb at linux.ibm.com]
> Sent: Wednesday, June 16, 2021 4:40 PM
> On 6/16/21 9:22 AM, Roberto Sassu wrote:
> > vfs_getxattr() differs from vfs_setxattr() in the way it obtains the xattr
> > value. The former gives precedence to the LSMs, and if the LSMs don't
> > provide a value, obtains it from the filesystem handler. The latter does
> > the opposite, first invokes the filesystem handler, and if the filesystem
> > does not support xattrs, passes the xattr value to the LSMs.
> >
> > The problem is that not necessarily the user gets the same xattr value that
> > he set. For example, if he sets security.selinux with a value not
> > terminated with '\0', he gets a value terminated with '\0' because SELinux
> > adds it during the translation from xattr to internal representation
> > (vfs_setxattr()) and from internal representation to xattr
> > (vfs_getxattr()).
> >
> > Normally, this does not have an impact unless the integrity of xattrs is
> > verified with EVM. The kernel and the user see different values due to the
> > different functions used to obtain them:
> >
> > kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from
> >                the filesystem handler (raw value);
> >
> > user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value
> >                        from the LSMs (normalized value).
> 
> Maybe there should be another implementation similar to
> vfs_getxattr_alloc() (or modify it) to behave like vfs_getxattr() but do
> the memory allocation part so that the kernel sees what user space see
> rather than modifying it with your patch so that user space now sees
> something different than what it has been for years (previous
> NUL-terminated SELinux xattr may not be NUL-terminated anymore)?

I'm concerned that this would break HMACs/digital signatures
calculated with raw values.

An alternative would be to do the EVM verification twice if the
first time didn't succeed (with vfs_getxattr_alloc() and with the
new function that behaves like vfs_getxattr()).

Roberto

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

>      Stefan
> 
> 
> 
> 
> >
> > Given that the difference between the raw value and the normalized value
> > should be just the additional '\0' not the rest of the content, this patch
> > modifies vfs_getxattr() to compare the size of the xattr value obtained
> > from the LSMs to the size of the raw xattr value. If there is a mismatch
> > and the filesystem handler does not return an error, vfs_getxattr() returns
> > the raw value.
> >
> > This patch should have a minimal impact on existing systems, because if the
> > SELinux label is written with the appropriate tools such as setfiles or
> > restorecon, there will not be a mismatch (because the raw value also has
> > '\0').
> >
> > In the case where the SELinux label is written directly with setfattr and
> > without '\0', this patch helps to align EVM and ima-evm-utils in terms of
> > result provided (due to the fact that they both verify the integrity of
> > xattrs from raw values).
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > Tested-by: Mimi Zohar <zohar at linux.ibm.com>
> > ---
> >   fs/xattr.c | 15 +++++++++++++++
> >   1 file changed, 15 insertions(+)
> >
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index 5c8c5175b385..412ec875aa07 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -420,12 +420,27 @@ vfs_getxattr(struct user_namespace *mnt_userns,
> struct dentry *dentry,
> >   		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
> >   		int ret = xattr_getsecurity(mnt_userns, inode, suffix, value,
> >   					    size);
> > +		int ret_raw;
> > +
> >   		/*
> >   		 * Only overwrite the return value if a security module
> >   		 * is actually active.
> >   		 */
> >   		if (ret == -EOPNOTSUPP)
> >   			goto nolsm;
> > +
> > +		if (ret < 0)
> > +			return ret;
> > +
> > +		/*
> > +		 * Read raw xattr if the size from the filesystem handler
> > +		 * differs from that returned by xattr_getsecurity() and is
> > +		 * equal or greater than zero.
> > +		 */
> > +		ret_raw = __vfs_getxattr(dentry, inode, name, NULL, 0);
> > +		if (ret_raw >= 0 && ret_raw != ret)
> > +			goto nolsm;
> > +
> >   		return ret;
> >   	}
> >   nolsm: