Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

On Mon, Jul 8, 2024 at 2:25 PM Steve Dower [off-list ref] wrote:

On 08/07/2024 22:15, Jeff Xu wrote:

quoted

IIUC:
CHECK=0, RESTRICT=0: do nothing, current behavior
CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results.
CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception.
CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except
those in the "checked-and-allowed" list.

I had much the same question for Mickaël while working on this.

Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking.
In the context of a script or macro interpreter, this just means it will
never interpret any scripts. Non-binary code execution is fully disabled
in any part of the process that respects these bits.

I see, so Mickaël does mean this will block all scripts.
I guess, in the context of dynamic linker, this means: no more .so
loading, even "dlopen" is called by an app ?  But this will make the
execve()  fail.

"CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This
case is the allow list (or whatever mechanism is being used to determine
the result of an AT_CHECK check). The actual mechanism isn't the
business of the script interpreter at all, it just has to refuse to
execute anything that doesn't pass the check. So a generic interpreter
can implement a generic mechanism and leave the specifics to whoever
configures the machine.

In the context of dynamic linker. this means:
if .so passed the AT_CHECK, ldopen() can still load it.
If .so fails the AT_CHECK, ldopen() will fail too.

Thanks
-Jeff

The other two case are more obvious. "CHECK=0, RESTRICT=0" is the
zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or
otherwise audit the result of the check, but it won't restrict execution.

Cheers,
Steve