Re: executable but not readable

[Frank van Maarseveen <frankvm@xs4all.nl>]

On Sun, Mar 28, 2004 at 06:46:51PM -0500, Trond Myklebust wrote:
> P=E5 lau , 27/03/2004 klokka 07:17, skreiv Frank van Maarseveen:
> > On Fri, Mar 26, 2004 at 02:17:51PM -0500, Trond Myklebust wrote:
> > >=20
> > > In NFSv3, the ACCESS call should then be used to decide whether or not
> > > the client is allowed to open the file for execution (and for reading=
 if
> > > that is required). Unfortunately ACCESS is not implemented in the sto=
ck
> > > Linux 2.4.x kernel.
> >=20
> > So the kernel does its own permission checking on the client side
> > for executables _knowing_ that it is going to execute the file but
> > unfortunately the interpreter has to open the file by itself and that
> > fails.
>=20
> No... The kernel should do permission checking for execute, then opens
> the file for reading (overriding the read permissions since this is the
> kernel).
>=20
> Unfortunately, the server is broken, and failing to follow RFC1813 when
> it comes to allowing reads on executables.

I don't understand this. The server doesn't see the difference between
a read for "read" purposes and a read for "execute" purposes IIRC. How
can the server then be broken?

>=20
> > But from a different perspective:
> >=20
> > Being able to create a (non-setuid) executable which cannot be read
> > for security reasons looks very weak to me unless of course it is not
> > possible to let it dump core, strace (ptrace) it, open /proc/... files
> > etc. But is that all actually the case in 2.6?
>=20
> Sounds like a good test of the 2.6 VFS. Have you tried it?

um, no, that's why I asked it. If security fails here and if it's
insolvable then it is a reason to drop the client side ACCESS check in 2.6
for read versus execute permission because it wouldn't buy you anything.

--=20
Frank


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