On 09/25/2018 09:33 PM, Dave Chinner wrote:
On Tue, Sep 25, 2018 at 08:51:50PM -0400, TongZhang wrote:
Hi,
I'm bringing up this issue again to let of LSM developers know the situation, and would like to know your thoughts.
Several weeks ago I sent an email to the security list to discuss the issue where
XFS's ioctl interface can do things like vfs_readlink without asking LSM's
permission, which we think is kind of weird and this kind of operation should be
audited by LSM.
These aren't user interfaces. They are filesystem maintenance and
extension interfaces. They are intended for low level filesystem
utilities that require complete, unrestricted access to the
underlying filesystem via holding CAP_SYSADMIN in the initns.
i.e. they are used to perform filesystem maintenance and extension
operations that need to be completely invisible to users from
userspace. e.g. online file defragmentation (xfs_fsr), data
migration (e.g. HSM products), efficient backup of data (xfsdump),
metadata and data scrubbing, online repair, etc.
IOWs, I really don't think these interfaces are something the LSMs
should be trying to intercept or audit, because they are essentially
internal filesystem interfaces used by trusted code and not general
user application facing APIs.
If they are interfaces exposed to userspace, then they should be
mediated via LSM. We only omit the LSM hook when the usage is purely
kernel-internal. Security modules are often used to limit even
"trusted" applications to least privilege and protect them from
untrustworthy inputs, moving from binary trust notions to only trusting
them for what they must be trusted to do. CAP_SYS_ADMIN doesn't
necessarily indicate that they are trusted to override any given MAC
policy restrictions.
Wondering why we don't perform the security_inode_readlink() call inside
of vfs_readlink() currently. The general pattern is that we do perform
security_inode_*() calls inside the other vfs_*() helpers, so
vfs_readlink() is an exception currently.
