Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)

["Mickaël Salaün" <mic@digikod.net>]

On Tue, Aug 26, 2025 at 08:30:41AM -0400, Theodore Ts'o wrote:
> Is there a single, unified design and requirements document that
> describes the threat model, and what you are trying to achieve with
> AT_EXECVE_CHECK and O_DENY_WRITE?  I've been looking at the cover
> letters for AT_EXECVE_CHECK and O_DENY_WRITE, and the documentation
> that has landed for AT_EXECVE_CHECK and it really doesn't describe
> what *are* the checks that AT_EXECVE_CHECK is trying to achieve:
> 
>    "The AT_EXECVE_CHECK execveat(2) flag, and the
>    SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE
>    securebits are intended for script interpreters and dynamic linkers
>    to enforce a consistent execution security policy handled by the
>    kernel."

From the documentation:

  Passing the AT_EXECVE_CHECK flag to execveat(2) only performs a check
  on a regular file and returns 0 if execution of this file would be
  allowed, ignoring the file format and then the related interpreter
  dependencies (e.g. ELF libraries, script’s shebang).

> 
> Um, what security policy?

Whether the file is allowed to be executed.  This includes file
permission, mount point option, ACL, LSM policies...

> What checks?

Executability checks?

> What is a sample exploit
> which is blocked by AT_EXECVE_CHECK?

Executing/interpreting any data: sh script.txt

> 
> And then on top of it, why can't you do these checks by modifying the
> script interpreters?

The script interpreter requires modification to use AT_EXECVE_CHECK.

There is no other way for user space to reliably check executability of
files (taking into account all enforced security
policies/configurations).