Re: [PATCH v2 0/5] Add support for O_MAYEXEC

On Sep 6, 2019, at 12:07 PM, Steve Grubb [off-list ref] wrote:

quoted

On Friday, September 6, 2019 2:57:00 PM EDT Florian Weimer wrote:
* Steve Grubb:

quoted

Now with LD_AUDIT
$ LD_AUDIT=/home/sgrubb/test/openflags/strip-flags.so.0 strace ./test
2>&1 | grep passwd openat(3, "passwd", O_RDONLY)           = 4

No O_CLOEXEC flag.

I think you need to explain in detail why you consider this a problem.

Because you can strip the O_MAYEXEC flag from being passed into the kernel. 
Once you do that, you defeat the security mechanism because it never gets 
invoked. The issue is that the only thing that knows _why_ something is being 
opened is user space. With this mechanism, you can attempt to pass this 
reason to the kernel so that it may see if policy permits this. But you can 
just remove the flag.

I’m with Florian here. Once you are executing code in a process, you could just emulate some other unapproved code. This series is not intended to provide the kind of absolute protection you’re imagining.

What the kernel *could* do is prevent mmapping a non-FMODE_EXEC file with PROT_EXEC, which would indeed have a real effect (in an iOS-like world, for example) but would break many, many things.