Re: [musl] Re: [RFC] Possible new execveat(2) Linux syscall

[Rich Felker <dalias at aerifal dot cx>]

On Mon, Nov 17, 2014 at 03:42:15PM +0000, David Drysdale wrote:
> I'm not familiar with O_EXEC either, I'm afraid, so to be clear -- does
> O_EXEC mean the permission check is explicitly skipped later, at execute
> time?  In other words, if you open(O_EXEC) an executable then remove the
> execute bit from the file, does a subsequent fexecve() still work?

Yes. It's just like how read and write permissions work. If you open a
file for read then remove read permissions, or open it for write then
remove write permissions, the existing permissions to the open file
are not lost. Of course open with O_EXEC/O_SEARCH needs to fail if the
caller does not have +x access to the file/directory at the time of
open.

> If it does, then from an implementation perspective that presumably implies
> the need for a record of the permission check in the struct file (and that
> this property would be inherited by any dup()ed file descriptors).  From a
> security perspective, having a gap between time-of-check and time-of-use
> always sounds worrying...

This record already exists for read and write. All that's needed is
for an extra bit to be added to record exec/search permission.

Rich