Revision 8aef18845266f5c05904c610088f2d1ed58f6be3 authored by Al Viro on 16 June 2011, 14:10:06 UTC, committed by Al Viro on 16 June 2011, 15:28:16 UTC
[Kudos to dhowells for tracking that crap down] If two processes attempt to cause automounting on the same mountpoint at the same time, the vfsmount holding the mountpoint will be left with one too few references on it, causing a BUG when the kernel tries to clean up. The problem is that lock_mount() drops the caller's reference to the mountpoint's vfsmount in the case where it finds something already mounted on the mountpoint as it transits to the mounted filesystem and replaces path->mnt with the new mountpoint vfsmount. During a pathwalk, however, we don't take a reference on the vfsmount if it is the same as the one in the nameidata struct, but do_add_mount() doesn't know this. The fix is to make sure we have a ref on the vfsmount of the mountpoint before calling do_add_mount(). However, if lock_mount() doesn't transit, we're then left with an extra ref on the mountpoint vfsmount which needs releasing. We can handle that in follow_managed() by not making assumptions about what we can and what we cannot get from lookup_mnt() as the current code does. The callers of follow_managed() expect that reference to path->mnt will be grabbed iff path->mnt has been changed. follow_managed() and follow_automount() keep track of whether such reference has been grabbed and assume that it'll happen in those and only those cases that'll have us return with changed path->mnt. That assumption is almost correct - it breaks in case of racing automounts and in even harder to hit race between following a mountpoint and a couple of mount --move. The thing is, we don't need to make that assumption at all - after the end of loop in follow_manage() we can check if path->mnt has ended up unchanged and do mntput() if needed. The BUG can be reproduced with the following test program: #include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <unistd.h> #include <sys/wait.h> int main(int argc, char **argv) { int pid, ws; struct stat buf; pid = fork(); stat(argv[1], &buf); if (pid > 0) wait(&ws); return 0; } and the following procedure: (1) Mount an NFS volume that on the server has something else mounted on a subdirectory. For instance, I can mount / from my server: mount warthog:/ /mnt -t nfs4 -r On the server /data has another filesystem mounted on it, so NFS will see a change in FSID as it walks down the path, and will mark /mnt/data as being a mountpoint. This will cause the automount code to be triggered. !!! Do not look inside the mounted fs at this point !!! (2) Run the above program on a file within the submount to generate two simultaneous automount requests: /tmp/forkstat /mnt/data/testfile (3) Unmount the automounted submount: umount /mnt/data (4) Unmount the original mount: umount /mnt At this point the kernel should throw a BUG with something like the following: BUG: Dentry ffff880032e3c5c0{i=2,n=} still in use (1) [unmount of nfs4 0:12] Note that the bug appears on the root dentry of the original mount, not the mountpoint and not the submount because sys_umount() hasn't got to its final mntput_no_expire() yet, but this isn't so obvious from the call trace: [<ffffffff8117cd82>] shrink_dcache_for_umount+0x69/0x82 [<ffffffff8116160e>] generic_shutdown_super+0x37/0x15b [<ffffffffa00fae56>] ? nfs_super_return_all_delegations+0x2e/0x1b1 [nfs] [<ffffffff811617f3>] kill_anon_super+0x1d/0x7e [<ffffffffa00d0be1>] nfs4_kill_super+0x60/0xb6 [nfs] [<ffffffff81161c17>] deactivate_locked_super+0x34/0x83 [<ffffffff811629ff>] deactivate_super+0x6f/0x7b [<ffffffff81186261>] mntput_no_expire+0x18d/0x199 [<ffffffff811862a8>] mntput+0x3b/0x44 [<ffffffff81186d87>] release_mounts+0xa2/0xbf [<ffffffff811876af>] sys_umount+0x47a/0x4ba [<ffffffff8109e1ca>] ? trace_hardirqs_on_caller+0x1fd/0x22f [<ffffffff816ea86b>] system_call_fastpath+0x16/0x1b as do_umount() is inlined. However, you can see release_mounts() in there. Note also that it may be necessary to have multiple CPU cores to be able to trigger this bug. Tested-by: Jeff Layton <jlayton@redhat.com> Tested-by: Ian Kent <raven@themaw.net> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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api-intro.txt
Scatterlist Cryptographic API
INTRODUCTION
The Scatterlist Crypto API takes page vectors (scatterlists) as
arguments, and works directly on pages. In some cases (e.g. ECB
mode ciphers), this will allow for pages to be encrypted in-place
with no copying.
One of the initial goals of this design was to readily support IPsec,
so that processing can be applied to paged skb's without the need
for linearization.
DETAILS
At the lowest level are algorithms, which register dynamically with the
API.
'Transforms' are user-instantiated objects, which maintain state, handle all
of the implementation logic (e.g. manipulating page vectors) and provide an
abstraction to the underlying algorithms. However, at the user
level they are very simple.
Conceptually, the API layering looks like this:
[transform api] (user interface)
[transform ops] (per-type logic glue e.g. cipher.c, compress.c)
[algorithm api] (for registering algorithms)
The idea is to make the user interface and algorithm registration API
very simple, while hiding the core logic from both. Many good ideas
from existing APIs such as Cryptoapi and Nettle have been adapted for this.
The API currently supports five main types of transforms: AEAD (Authenticated
Encryption with Associated Data), Block Ciphers, Ciphers, Compressors and
Hashes.
Please note that Block Ciphers is somewhat of a misnomer. It is in fact
meant to support all ciphers including stream ciphers. The difference
between Block Ciphers and Ciphers is that the latter operates on exactly
one block while the former can operate on an arbitrary amount of data,
subject to block size requirements (i.e., non-stream ciphers can only
process multiples of blocks).
Support for hardware crypto devices via an asynchronous interface is
under development.
Here's an example of how to use the API:
#include <linux/crypto.h>
#include <linux/err.h>
#include <linux/scatterlist.h>
struct scatterlist sg[2];
char result[128];
struct crypto_hash *tfm;
struct hash_desc desc;
tfm = crypto_alloc_hash("md5", 0, CRYPTO_ALG_ASYNC);
if (IS_ERR(tfm))
fail();
/* ... set up the scatterlists ... */
desc.tfm = tfm;
desc.flags = 0;
if (crypto_hash_digest(&desc, sg, 2, result))
fail();
crypto_free_hash(tfm);
Many real examples are available in the regression test module (tcrypt.c).
DEVELOPER NOTES
Transforms may only be allocated in user context, and cryptographic
methods may only be called from softirq and user contexts. For
transforms with a setkey method it too should only be called from
user context.
When using the API for ciphers, performance will be optimal if each
scatterlist contains data which is a multiple of the cipher's block
size (typically 8 bytes). This prevents having to do any copying
across non-aligned page fragment boundaries.
ADDING NEW ALGORITHMS
When submitting a new algorithm for inclusion, a mandatory requirement
is that at least a few test vectors from known sources (preferably
standards) be included.
Converting existing well known code is preferred, as it is more likely
to have been reviewed and widely tested. If submitting code from LGPL
sources, please consider changing the license to GPL (see section 3 of
the LGPL).
Algorithms submitted must also be generally patent-free (e.g. IDEA
will not be included in the mainline until around 2011), and be based
on a recognized standard and/or have been subjected to appropriate
peer review.
Also check for any RFCs which may relate to the use of specific algorithms,
as well as general application notes such as RFC2451 ("The ESP CBC-Mode
Cipher Algorithms").
It's a good idea to avoid using lots of macros and use inlined functions
instead, as gcc does a good job with inlining, while excessive use of
macros can cause compilation problems on some platforms.
Also check the TODO list at the web site listed below to see what people
might already be working on.
BUGS
Send bug reports to:
linux-crypto@vger.kernel.org
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
David S. Miller <davem@redhat.com>
FURTHER INFORMATION
For further patches and various updates, including the current TODO
list, see:
http://gondor.apana.org.au/~herbert/crypto/
AUTHORS
James Morris
David S. Miller
Herbert Xu
CREDITS
The following people provided invaluable feedback during the development
of the API:
Alexey Kuznetzov
Rusty Russell
Herbert Valerio Riedel
Jeff Garzik
Michael Richardson
Andrew Morton
Ingo Oeser
Christoph Hellwig
Portions of this API were derived from the following projects:
Kerneli Cryptoapi (http://www.kerneli.org/)
Alexander Kjeldaas
Herbert Valerio Riedel
Kyle McMartin
Jean-Luc Cooke
David Bryson
Clemens Fruhwirth
Tobias Ringstrom
Harald Welte
and;
Nettle (http://www.lysator.liu.se/~nisse/nettle/)
Niels Möller
Original developers of the crypto algorithms:
Dana L. How (DES)
Andrew Tridgell and Steve French (MD4)
Colin Plumb (MD5)
Steve Reid (SHA1)
Jean-Luc Cooke (SHA256, SHA384, SHA512)
Kazunori Miyazawa / USAGI (HMAC)
Matthew Skala (Twofish)
Dag Arne Osvik (Serpent)
Brian Gladman (AES)
Kartikey Mahendra Bhatt (CAST6)
Jon Oberheide (ARC4)
Jouni Malinen (Michael MIC)
NTT(Nippon Telegraph and Telephone Corporation) (Camellia)
SHA1 algorithm contributors:
Jean-Francois Dive
DES algorithm contributors:
Raimar Falke
Gisle Sælensminde
Niels Möller
Blowfish algorithm contributors:
Herbert Valerio Riedel
Kyle McMartin
Twofish algorithm contributors:
Werner Koch
Marc Mutz
SHA256/384/512 algorithm contributors:
Andrew McDonald
Kyle McMartin
Herbert Valerio Riedel
AES algorithm contributors:
Alexander Kjeldaas
Herbert Valerio Riedel
Kyle McMartin
Adam J. Richter
Fruhwirth Clemens (i586)
Linus Torvalds (i586)
CAST5 algorithm contributors:
Kartikey Mahendra Bhatt (original developers unknown, FSF copyright).
TEA/XTEA algorithm contributors:
Aaron Grothe
Michael Ringe
Khazad algorithm contributors:
Aaron Grothe
Whirlpool algorithm contributors:
Aaron Grothe
Jean-Luc Cooke
Anubis algorithm contributors:
Aaron Grothe
Tiger algorithm contributors:
Aaron Grothe
VIA PadLock contributors:
Michal Ludvig
Camellia algorithm contributors:
NTT(Nippon Telegraph and Telephone Corporation) (Camellia)
Generic scatterwalk code by Adam J. Richter <adam@yggdrasil.com>
Please send any credits updates or corrections to:
Herbert Xu <herbert@gondor.apana.org.au>
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