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

[Nicolas Pitre]

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

> On Sun, 17 Jul 2016 23:31:56 -0400
> Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> 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@xxxxxxxxxx>
Date:   Mon Jul 18 11:28:57 2016 -0400

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

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
 	 */




>