[v2,10/10] binfmt_flat: allow compressed flat binary format to work on MMU systems

Message ID alpine.DEB.2.20.1607181006190.9597@Yrabib
State New
Headers show

Commit Message

Nicolas Pitre July 18, 2016, 3:45 p.m.
On Mon, 18 Jul 2016, One Thousand Gnomes wrote:

> On Sun, 17 Jul 2016 23:31:56 -0400

> Nicolas Pitre <nicolas.pitre@linaro.org> wrote:

> 

> > Let's take the simple and obvious approach by decompressing the binary

> > into a kernel buffer and then copying it to user space.  Those who are

> > looking for more performance on a MMU system are unlikely to choose this

> > executable format anyway.

> 

> The flat loader takes a very casual attitude to overruns and corrupted

> binaries. It's after all MMUless so has no real security model. If you

> enable flat for an MMU system then IMHO those all need to be fixed

> including all the missing overflow checks on the maths on textlen and the

> like.


What about the following patch?  This with existing user accessors and 
allocation error checks should cover it all.

----- >8
commit cc1051c9c57202772568600e96b75229a2a7cf19
Author: Nicolas Pitre <nicolas.pitre@linaro.org>
Date:   Mon Jul 18 11:28:57 2016 -0400

    binfmt_flat: prevent kernel dammage from corrupted executable headers
    
    Signed-off-by: Nicolas Pitre <nico@linaro.org>






>

Comments

Nicolas Pitre July 18, 2016, 4:58 p.m. | #1
On Mon, 18 Jul 2016, One Thousand Gnomes wrote:

> On Mon, 18 Jul 2016 11:45:53 -0400 (EDT)

> Nicolas Pitre <nicolas.pitre@linaro.org> wrote:

> 

> > On Mon, 18 Jul 2016, One Thousand Gnomes wrote:

> > 

> > > On Sun, 17 Jul 2016 23:31:56 -0400

> > > Nicolas Pitre <nicolas.pitre@linaro.org> wrote:

> > >   

> > > > Let's take the simple and obvious approach by decompressing the binary

> > > > into a kernel buffer and then copying it to user space.  Those who are

> > > > looking for more performance on a MMU system are unlikely to choose this

> > > > executable format anyway.  

> > > 

> > > The flat loader takes a very casual attitude to overruns and corrupted

> > > binaries. It's after all MMUless so has no real security model. If you

> > > enable flat for an MMU system then IMHO those all need to be fixed

> > > including all the missing overflow checks on the maths on textlen and the

> > > like.  

> > 

> > What about the following patch?  This with existing user accessors and 

> > allocation error checks should cover it all.

> > 

> > ----- >8  

> > commit cc1051c9c57202772568600e96b75229a2a7cf19

> > Author: Nicolas Pitre <nicolas.pitre@linaro.org>

> > Date:   Mon Jul 18 11:28:57 2016 -0400

> > 

> >     binfmt_flat: prevent kernel dammage from corrupted executable headers

> >     

> >     Signed-off-by: Nicolas Pitre <nico@linaro.org>

> > 

> > diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c

> > index 24deae4dcb..fa0054c1c3 100644

> > --- a/fs/binfmt_flat.c

> > +++ b/fs/binfmt_flat.c

> > @@ -498,6 +498,17 @@ static int load_flat_file(struct linux_binprm * bprm,

> >  	}

> >  

> >  	/*

> > +	 * Make sure the header params are sane.

> > +	 * 28 bits (256 MB) is way more than reasonable in this case.

> > +	 * If some top bits are set we have probable binary corruption.

> > +	*/

> > +	if ((text_len | data_len | bss_len | stack_len | full_data) >> 28) {

> > +		printk("BINFMT_FLAT: bad header\n");

> 

> Apart from the printk that looks good for the header but I think the rest

> could do with a fair bit more review (eg relocations in range checks).


Given that they all go through put_user() now, the worst that could 
happen is an executable that craps onto itself.  I don't think there is 
much we can do here besides letting the user task crash.


Nicolas

Patch

diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index 24deae4dcb..fa0054c1c3 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -498,6 +498,17 @@  static int load_flat_file(struct linux_binprm * bprm,
 	}
 
 	/*
+	 * Make sure the header params are sane.
+	 * 28 bits (256 MB) is way more than reasonable in this case.
+	 * If some top bits are set we have probable binary corruption.
+	*/
+	if ((text_len | data_len | bss_len | stack_len | full_data) >> 28) {
+		printk("BINFMT_FLAT: bad header\n");
+		ret = -ENOEXEC;
+		goto err;
+	}
+
+	/*
 	 * fix up the flags for the older format,  there were all kinds
 	 * of endian hacks,  this only works for the simple cases
 	 */