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Message-ID: <202005150732.17C5EE0@keescook>
Date: Fri, 15 May 2020 07:37:16 -0700
From: Kees Cook <keescook@...omium.org>
To: Florian Weimer <fweimer@...hat.com>
Cc: Mickaël Salaün <mic@...ikod.net>,
Al Viro <viro@...iv.linux.org.uk>, Aleksa Sarai <cyphar@...har.com>,
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Subject: Re: How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to
enforce noexec mounts or file exec through O_MAYEXEC)
On Fri, May 15, 2020 at 10:43:34AM +0200, Florian Weimer wrote:
> * Kees Cook:
>
> > Maybe I've missed some earlier discussion that ruled this out, but I
> > couldn't find it: let's just add O_EXEC and be done with it. It actually
> > makes the execve() path more like openat2() and is much cleaner after
> > a little refactoring. Here are the results, though I haven't emailed it
> > yet since I still want to do some more testing:
> > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/o_exec/v1
>
> I think POSIX specifies O_EXEC in such a way that it does not confer
> read permissions. This seems incompatible with what we are trying to
> achieve here.
I was trying to retain this behavior, since we already make this
distinction between execve() and uselib() with the MAY_* flags:
execve():
struct open_flags open_exec_flags = {
.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
.acc_mode = MAY_EXEC,
uselib():
static const struct open_flags uselib_flags = {
.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
.acc_mode = MAY_READ | MAY_EXEC,
I tried to retain this in my proposal, in the O_EXEC does not imply
MAY_READ:
+ /* Should execution permissions be checked on open? */
+ if (flags & O_EXEC) {
+ flags |= __FMODE_EXEC;
+ acc_mode |= MAY_EXEC;
+ }
--
Kees Cook
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