On Sat, Nov 5, 2022 at 7:34 AM Christian Brauner [off-list ref] wrote:

On Sat, Nov 05, 2022 at 12:38:57AM -0400, Paul Moore wrote:

quoted

On Thu, Nov 3, 2022 at 11:13 AM Ondrej Mosnacek [off-list ref] wrote:

quoted

The check being unconditional may lead to unwanted denials reported by
LSMs when a process has the capability granted by DAC, but denied by an
LSM. In the case of SELinux such denials are a problem, since they can't
be effectively filtered out via the policy and when not silenced, they
produce noise that may hide a true problem or an attack.

Checking for the capability only if any trusted xattr is actually
present wouldn't really address the issue, since calling listxattr(2) on
such node on its own doesn't indicate an explicit attempt to see the
trusted xattrs. Additionally, it could potentially leak the presence of
trusted xattrs to an unprivileged user if they can check for the denials
(e.g. through dmesg).

Therefore, it's best (and simplest) to keep the check unconditional and
instead use ns_capable_noaudit() that will silence any associated LSM
denials.

Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs")
Reported-by: Martin Pitt <redacted>
Suggested-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---

v1 -> v2: switch to simpler and better solution as suggested by Christian

v1: https://lore.kernel.org/selinux/CAFqZXNuC7c0Ukx_okYZ7rsKycQY5P1zpMPmmq_T5Qyzbg-x7yQ@mail.gmail.com/T/ (local)

 fs/xattr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

VFS folks, this should really go through a vfs tree, but if nobody
wants to pick it up *and* there are no objections to the change, I can
take this via the LSM tree.

I can pick this up as I'm currently massaging the simple xattr
infrastructure.

Thanks Christian.

I think the fix is pretty straightforward otherwise.

Agreed.

-- 
paul-moore.com