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Date: Thu, 18 Dec 2014 09:24:19 +0100
From: Lionel Debroux <>
Subject: Re: How GNU/Linux distros deal with offset2lib attack?

Hello Greg,

Before someone (whoever he/she is) spends time on something that might,
from the beginning, have no chance of getting integrated into mainline,
I'd like your input on three questions I asked over a week ago, at the
end of an all too lengthy e-mail :)
Thanks in advance.

Reposting here, so that you don't have to dig, with added "=" lines:

> > If the latter, I think it wouldn't be good to see another
> > half-measure integrated to mainline, until the next mainline ASLR
> > defeat against which PaX has protected for over a decade. Just
> > my 2 cents.
> The reason PaX isn't in the main kernel tree is that no one has
> spent the time and effort to actually submit it in a mergable form.
> So please, do so if you think this is something that is needed.
In 2010(-2011 ?), I pushed some bits of constification work from PaX to
mainline, nothing complicated.
Pushing from PaX to mainline some of the more useful bits that I listed
in a previous e-mail is a very different matter... well beyond the
amount of time I could devote to it - and beyond my technical abilities,

However, pointing and pushing patches of lower complexity, why not.
For instance:
1) do you consider that using
static inline bool check_heap_stack_gap(const struct vm_area_struct
*vma, unsigned long addr, unsigned long len) {
return (!vma || addr + len <= vma->vm_start);
defined in include/linux/sched.h (where PaX defines that prototype, and
where most functions using it are defined) to replace 20- open-coded
(!vma || addr + len <= vma->vm_start) checks is a worthwhile, though
obviously no-op, cleanup in its own right ?

2) do you consider that, even before the integration of PaX's refcount
protection to mainline (which is there, like most of PaX, for security
reasons, there have been refcount-related holes in the past...),
switching to atomic_t for fs_struct.users,
kmem_cache.refcount (for SLAB and SLUB), tty_port.count,
tty_ldisc_ops.refcount is worthwhile ?
Most such refcounts at other places are already atomic, after all.

3) I see a patch in USB core, which looks like it's written to avoid
some potential integer overflows.
--- linux-3.17.4/drivers/usb/core/devio.c
+++ linux-3.17.4-pax/drivers/usb/core/devio.c
@@ -187,7 +187,7 @@ static ssize_t usbdev_read(struct file *
    struct usb_dev_state *ps = file->private_data;
    struct usb_device *dev = ps->dev;
    ssize_t ret = 0;
-    unsigned len;
+    size_t len;
    loff_t pos;
    int i;

@@ -229,22 +229,22 @@ static ssize_t usbdev_read(struct file *
    for (i = 0; nbytes && i < dev->descriptor.bNumConfigurations; i++){
        struct usb_config_descriptor *config =
          (struct usb_config_descriptor *)dev->rawdescriptors[i];
-        unsigned int length = le16_to_cpu(config->wTotalLength);
+        size_t length = le16_to_cpu(config->wTotalLength);

        if (*ppos < pos + length) {

        /* The descriptor may claim to be longer than it
        * really is.  Here is the actual allocated length. */
-            unsigned alloclen =
+            size_t alloclen =

-            len = length - (*ppos - pos);
+            len = length + pos - *ppos;
            if (len > nbytes)
                len = nbytes;

            /* Simply don't write (skip over) unallocated parts */
            if (alloclen > (*ppos - pos)) {
-                alloclen -= (*ppos - pos);
+                alloclen = alloclen + pos - *ppos;
                if (copy_to_user(buf,
                    dev->rawdescriptors[i] + (*ppos - pos),
                    min(len, alloclen))) {
Does that make sense ?

In addition to what I wrote earlier: PaX contains several hundreds of
lines of hunks dealing with local variables needlessly made static:
--- linux-3.17.6/drivers/mfd/max8925-i2c.c
+++ linux-3.17.6-pax/drivers/mfd/max8925-i2c.c
@@ -152,7 +152,7 @@ static int max8925_probe(struct i2c_clie
const struct i2c_device_id *id)
     struct max8925_platform_data *pdata = dev_get_platdata(&client->dev);
-    static struct max8925_chip *chip;
+    struct max8925_chip *chip;
     struct device_node *node = client->dev.of_node;

if (node && !pdata) {

(the first reference to the "chip" variable in that function is an
unconditional devm_kzalloc)
or local structs which are not meant to be modified and should therefore
probably be made static / static const (mainline doesn't use the GCC
plugin for constification):
--- linux-3.17.6/arch/arm/mach-omap2/wd_timer.c
+++ linux-3.17.6-pax/arch/arm/mach-omap2/wd_timer.c
@@ -110,7 +110,9 @@ static int __init omap_init_wdt(void)
    struct omap_hwmod *oh;
    char *oh_name = "wd_timer2";
    char *dev_name = "omap_wdt";
-    struct omap_wd_timer_platform_data pdata;
+    static struct omap_wd_timer_platform_data pdata = {
+        .read_reset_sources = prm_read_reset_sources
+    };

    if (!cpu_class_is_omap2() || of_have_populated_dt())
        return 0;
@@ -121,8 +123,6 @@ static int __init omap_init_wdt(void)
    return -EINVAL;

-    pdata.read_reset_sources = prm_read_reset_sources;
    pdev = omap_device_build(dev_name, id, oh, &pdata,
sizeof(struct omap_wd_timer_platform_data));
    WARN(IS_ERR(pdev), "Can't build omap_device for %s:%s.\n",

A tiny subset of the bits which have been in PaX/grsec for a while
slowly migrate to mainline, though the mainline patches are certainly
independent reimplementations, given that many people don't know the
breadth of changes PaX/grsec contains...
For instance, PaX/grsec has been using ARMv7's PXN bit for two years.
There are also e.g. changes related to ACCESS_ONCE, though those did not
aim at solving problems with some compilers / platforms.


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