User: Password:
Subscribe / Log in / New account

Re: [patch 0/5] Support for sanitization flag in low-level page allocator

From:  "Larry H." <>
To:  Alan Cox <>
Subject:  Re: [patch 0/5] Support for sanitization flag in low-level page allocator
Date:  Fri, 22 May 2009 11:03:51 -0700
Message-ID:  <>
Cc:  Ingo Molnar <>, Rik van Riel <>,, Linus Torvalds <>,, Ingo Molnar <>,
Archive-link:  Article

On 14:39 Fri 22 May     , Alan Cox wrote:
> > > performance point of view: we _dont_ want to clear the full stack 
> > > page for every kernel thread exiting.
> > 
> > Burning the stack there is beyond overkill.
> Yet most of our historic leaks have been padding bytes in stack based
> structures. Your position seems very inconsistent.

Alright, I think I had enough of the theoretical mumbo jumbo, with all
due respect. Let's get on with the show.

I'm going to present a very short analysis for different historic leaks
which had little to do with 'padding bytes in stack', but more like
arbitrary kernel memory leaked to userland, or written to disk, or sent
over the network. If by the end of this message you still
believe my position is remotely inconsistent, I'll have to politely
request you to back it up with something that can be technically and
empirically proven from both programmer and security perspectives.

1. CVE-2005-0400 aka the infamous ext2_make_empty() disaster

The ext2 code before was affected by an uninitialized variable
usage vulnerability which lead to 4072 bytes worth of kernel memory
being leaked to disk, when creating a block for a new directory entry.
The affected function was ext2_make_empty() and it was fixed by adding a
memset call to zero the memory.

 594        kaddr = kmap_atomic(page, KM_USER0);
 595       memset(kaddr, 0, chunk_size);
 596        de = (struct ext2_dir_entry_2 *)kaddr;
 597        de->name_len = 1;
 598        de->rec_len = cpu_to_le16(EXT2_DIR_REC_LEN(1));

 594        kaddr = kmap_atomic(page, KM_USER0);
 595        de = (struct ext2_dir_entry_2 *)kaddr;
 596        de->name_len = 1;
 597        de->rec_len = cpu_to_le16(EXT2_DIR_REC_LEN(1));
 598        memcpy (de->name, ".\0\0", 4);

An atomic call to kmap(). This lead to widespread searching for online
ext2 images and general hilarity. And it was a longstanding issue in
the kernel, too.

2. CVE-2009-0787 aka ecryptfs_write_metadata_to_contents() leak
(commit 8faece5f906725c10e7a1f6caf84452abadbdc7b)

The ecryptfs function ecryptfs_write_metadata_to_contents() leaked up to
an entire page to userland. An incorrect size was used during the copy
operation, leading to more bytes being copied, hence the leak.

+       virt_len = crypt_stat->num_header_bytes_at_front;
+       order = get_order(virt_len);
        /* Released in this function */
-       virt = (char *)get_zeroed_page(GFP_KERNEL);
+       virt = (char *)ecryptfs_get_zeroed_pages(GFP_KERNEL,

3. CVE-2002-0046 aka information leak over ICMP TTL Exceeded responses

Series of fragmented ICMP packets that generate an ICMP TTL
Exceeded response would include 20 bytes of arbitrary kernel memory,
sent back to the attacker. I didn't bother digging for the patch. But
you bet it has to do with kmallocated skb buffers (take a look at

4. CVE-2007-6417 aka shmem_getpage() tmpfs leak

An issue related with tmpfs, users were able to obtain kernel memory
because the shmem_getpage() didn't always zero the memory when reusing
an allocated page. The vulnerability was present from 2.6.11 through

@@ -1306,6 +1306,7 @@ repeat:
+		clear_highpage(filepage);

If the caller provided the page already allocated, the GFP_ZERO
allocation never happened, and the page was never cleared. Interesting
issue since my patch basically ensures this doesn't happen. Nevermind.

5. CVE-2008-4113 aka sctp_getsockopt_hmac_ident() leak (<
(commit d97240552cd98c4b07322f30f66fd9c3ba4171de)
(exploit by Jon Oberheide at

In kernels before with SCTP and the SCTP-AUTH extension
enabled, an unprivileged local can leak arbitrary kernel memory abusing
an unbounded (due to incorrect length check) copy in the
sctp_getsockopt_hmac_ident() function. The data copied comes from a
kmallocated object (the struct sctp_association *asoc). This could be
exploited with a SCTP_HMAC_IDENT IOCTL request (through sctp_getsockopt).

From the exploit:
  *   If SCTP AUTH is enabled (net.sctp.auth_enable = 1), this exploit
  *   allow an  unprivileged user to dump an arbitrary amount (DUMP_SIZE) of
  *   kernel memory out to a file (DUMP_FILE). If SCTP AUTH is not enabled, the
  *   exploit will trigger a kernel OOPS.

It's worth noting that the commit title and description don't reveal the
true nature of the bug (a perfectly exploitable vulnerability, platform
independent like most other information leaks):
"sctp: fix random memory dereference with SCTP_HMAC_IDENT option."

At least it's not entirely deceitful. It's definitely dereferencing
"random memory".

6. CVE-2007-1000 aka ipv6_getsockopt_sticky() leak (<
(commit 286930797d74b2c9a5beae84836044f6a836235f)
(exploit at

The bug was initially assumed to be a simple NULL pointer dereference by
Chris Wright... but since kernel and userland address space coexist in
x86 and other architectures, this is an exploitable condition which
was used to leak kernel memory to userland after a page was allocated at
NULL by the exploit abusing the issue.


Further examples could be found in the commit logs or mining other places.
Also, this is the tip of the iceberg. Whatever is lurking deep inside the
kernel sources right now will only be deterred with my patch and any future
modifications that cover corner cases.

The following file contains a list of CVE numbers correlated with
commits, which comes handy to look for more examples:

I've saved a backup copy in case it goes offline and will put it
somewhere accessible for people on the list in such a case.

My intention here is to make the kernel more secure, not proving you
wrong or right.

You are a smart fellow and I respect your technical and kernel development
acumen. Smart people don't waste their time on meaningless banter.

I'll have the modified patches ready in an hour or so, hopefully.


To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to  For more info on Linux MM,
see: .
Don't email: <a href=mailto:""> </a>

(Log in to post comments)

Copyright © 2009, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds