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Message-ID: <202005150847.2B1ED8F81@keescook>
Date: Fri, 15 May 2020 08:50:16 -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 04:43:37PM +0200, Florian Weimer wrote:
> * Kees Cook:
>
> > 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:
>
> That doesn't quite parse for me, sorry.
>
> The point is that the script interpreter actually needs to *read* those
> files in order to execute them.
I think I misunderstood what you meant (Mickaël got me sorted out
now). If O_EXEC is already meant to be "EXEC and _not_ READ nor WRITE",
then yes, this new flag can't be O_EXEC. I was reading the glibc
documentation (which treats it as a permission bit flag, not POSIX,
which treats it as a complete mode description).
--
Kees Cook
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