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Date: Mon, 26 Jun 2017 06:40:36 -0400
From: Brad Spengler <>
Subject: Re: Can someone explain all the CONFIG_VMAP_STACK
 CVEs lately?

Hi Andy,

As I recently learned, you'll have to go through the CVE
dispute/reject process for each one of them.  Since I'm the reporter
I believe I would have to reject them (you could only dispute them),
however I'm not willing to reject any but the USB ones given the
information I found below, since the vulnerability (for the crypto
etc cases) is also the BUG() when CONFIG_DEBUG_VIRTUAL is enabled
(aka the thing you seem to depend on to find these bugs in the first
place since no static analysis is apparently being done), which will
panic the system when panic_on_oops is on.

unsigned long __phys_addr(unsigned long x)
        unsigned long y = x - __START_KERNEL_map;

        /* use the carry flag to determine if x was < __START_KERNEL_map */
        if (unlikely(x > y)) {
                x = y + phys_base;

                VIRTUAL_BUG_ON(y >= KERNEL_IMAGE_SIZE);
        } else {
                x = y + (__START_KERNEL_map - PAGE_OFFSET);

                /* carry flag will be set if starting x was >= PAGE_OFFSET */
                VIRTUAL_BUG_ON((x > y) || !phys_addr_valid(x));

        return x;

These are well-established denial of service issues that CVEs have been
assigned for in the past.

I've dug into the USB case a bit more since reading your mail and Greg's.
It has changed apparently since I originally wrote KSTACKOVERFLOW in
2014.  Here's what seems to be the relevant commit:

commit 29d2fef8be1165a26984a94fbcf81d68c1442fc5
Author: Dan Williams <>
Date:   Thu May 8 19:25:56 2014 +0300

    usb: catch attempts to submit urbs with a vmalloc'd transfer buffer
    Save someone else the debug cycles of figuring out why a driver's
    transfer request is failing or causing undefined system behavior.
    Buffers submitted for dma must come from GFP allocated / DMA-able
    Return -EAGAIN matching the return value for dma_mapping_error() cases.
    Acked-by: Alan Stern <>
    Cc: Sarah Sharp <>
    Cc: Mathias Nyman <>
    Signed-off-by: Dan Williams <>
    Signed-off-by: Mathias Nyman <>
    Signed-off-by: Greg Kroah-Hartman <>

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 9c4e292..adddc66 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1502,6 +1502,9 @@ int usb_hcd_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
                                        ret = -EAGAIN;
                                        urb->transfer_flags |= URB_DMA_MAP_PAGE;
+                       } else if (is_vmalloc_addr(urb->transfer_buffer)) {
+                               WARN_ONCE(1, "transfer buffer not dma capable\n");
+                               ret = -EAGAIN;
                        } else {
                                urb->transfer_dma = dma_map_single(

So it wasn't at all the case that the transfer would simply be rejected, 
certainly not since the 2.2 days.  Prior to this patch (assuming 
CONFIG_VMAP_STACK existed back then, which it didn't) you can see it 
would have hit the dma_map_single call, which would call virt_to_page 
which will end up in __phys_addr() and hit the BUG() on 
CONFIG_DEBUG_VIRTUAL. My first version of KSTACKOVERFLOW seems to have 
been for 3.14 which didn't carry this patch.  When I ported to 3.15, I 
had already redesigned KSTACKOVERFLOW to avoid triggering DoS conditions 
in buggy drivers/crypto code since it was clear the long-held no DMA on 
stack policy was never enforced across the board, so we would have never 
seen any USB-related issues again to have ever seen that WARN().

For the CVE dispute/reject process, Kurt Seifried can tell you all
about it.  Give him a couple weeks, since
is still up, the CVE still apparently is not rejected, and he is
very busy taking pictures of his dinner.  I am certain he will
treat a member of upstream Linux the same as I've been treated,
as he is a very professional and equitable person.

That said, I'm happy to be wrong about the USB case (and to clear
up Greg's misconception about it), and I hope the above info/history
makes clear where my assumption went wrong.  I'll gladly work with you
to get the USB-related CVEs rejected that you were saved from via
the above commit.


On Sun, Jun 25, 2017 at 08:49:43PM -0700, Andy Lutomirski wrote:
> As the author of the CONFIG_VMAP_STACK patches, I'm a bit confused
> here.  There have been quite a few bugs in which some code passes a
> stack buffer to either sg_set_buf(), etc. or to the usb core.  The
> former seem to all be crypto users.
> As I understand it, the supposed vulnerability is that, if you can
> force the buffer to span a page boundary, the kernel or device will
> instead hit the physical page following the the first page of the
> buffer, which is likely to be the wrong page.  This causes corruption
> and maybe code execution.
> Naively, this failure mode occurs because __pa (or virt_to_phys() or
> virt_to_page() or whatever interface gets used) will return the PA of
> the *beginning* of the buffer, but the next virtual page may not be
> the next physical page.  But this makes no sense -- __pa and friends
> don't have that effect when called on addresses in vmap space.
> So I tried to refresh my memory of what actually happened.  (I looked
> into this when I wrote CONFIG_VMAP_STACK.)  __pa() and friends return
> garbage when called on a vmap address.  (I think it's likely to be a
> totally bogus PA that won't even correspond to a real physical page of
> memory.)  The tricky but is that it's *invertable* garbage.  When
> these buffers are passed to synchronous crypto APIs, the crypto core
> calls sg_virt(), which inverts the transformation and returns a valid
> virtual address of the page.  But this is the original VA and points
> to the vmap space where the buffer is genuinely contiguous.
> IOW, for most synchronous crypto, using sg_set_buf() on a stack
> address is utterly bogus, but it works correctly.  Ick.
> I haven't checked what USB does, but I suspect it's a wildly
> out-of-bounds DMA transfer that's more likely to result in a
> straight-up abort than easily exploitable corruption.
> So could someone all these CVEs, please?
> --Andy

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