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Date: Mon, 17 Nov 2014 13:30:10 -0500
From: Rich Felker <dalias@...ifal.cx>
To: David Drysdale <drysdale@...gle.com>
Cc: Andy Lutomirski <luto@...capital.net>,
libc-alpha <libc-alpha@...rceware.org>, musl@...ts.openwall.com,
Andrew Morton <akpm@...ux-foundation.org>,
Linux API <linux-api@...r.kernel.org>,
Christoph Hellwig <hch@...radead.org>
Subject: Re: Re: [RFC] Possible new execveat(2) Linux syscall
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
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