Re: [RFC PATCH v9 0/3] Add introspect_access(2) (was O_MAYEXEC)

[Igor Zhbanov]

On 10.09.2020 23:05, Matthew Wilcox wrote:
> On Thu, Sep 10, 2020 at 09:00:10PM +0100, Al Viro wrote:
> > On Thu, Sep 10, 2020 at 07:40:33PM +0100, Matthew Wilcox wrote:
> > > On Thu, Sep 10, 2020 at 08:38:21PM +0200, Mickaël Salaün wrote:
> > > > There is also the use case of noexec mounts and file permissions. From
> > > > user space point of view, it doesn't matter which kernel component is in
> > > > charge of defining the policy. The syscall should then not be tied with
> > > > a verification/integrity/signature/appraisal vocabulary, but simply an
> > > > access control one.
> > >
> > > permission()?
> >
> > int lsm(int fd, const char *how, char *error, int size);
> >
> > Seriously, this is "ask LSM to apply special policy to file"; let's
> > _not_ mess with flags, etc. for that; give it decent bandwidth
> > and since it's completely opaque for the rest of the kernel,
> > just a pass a string to be parsed by LSM as it sees fit.
>
> Hang on, it does have some things which aren't BD^W^WLSM.  It lets
> the interpreter honour the mount -o noexec option.  I presume it's
> not easily defeated by
> 	cat /home/salaun/bin/bad.pl | perl -

Hi!

It could be bypassed this way. There are several ways of executing some
script:

1) /unsigned.sh (Already handled by IMA)
2) bash /unsigned.sh (Not handled. Works even with "-o noexec" mount)
3) bash < /unsigned.sh (Not handled. Works even with "-o noexec" mount)
4) cat /unsigned.sh | bash (Not handled. Works even with "-o noexec" mount)

AFAIK, the proposed syscall solves #2 and may be #3. As for #4 in security
critical environments there should be system-wide options to disable
interpreting scripts from the standard input. I suppose, executing commands
from the stdin is a rare case, and could be avoided entirely in security
critical environments. And yes, some help from the interpreters is needed
for that.

As for the usage of the system call, I have a proposal to extend its usage
to validate systemd unit files. Because a unit file could specify what UID
to use for a service, also it contains ExecStartPre which is actually a script
and is running as root (for the system session services).

For the syscall name it could be:
- trusted_file()
- trusted_file_content()
- valid_file()
- file_integrity()
because what we are checking here is the file content integrity (IMA) and
may be file permissions/attrs integrity (EVM).