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Florian Westphal a16871c783 xfrm: state: fix out-of-bounds read during lookup 
[ Upstream commit e952837f3d ]

lookup and resize can run in parallel.

The xfrm_state_hash_generation seqlock ensures a retry, but the hash
functions can observe a hmask value that is too large for the new hlist
array.

rehash does:
  rcu_assign_pointer(net->xfrm.state_bydst, ndst) [..]
  net->xfrm.state_hmask = nhashmask;

While state lookup does:
  h = xfrm_dst_hash(net, daddr, saddr, tmpl->reqid, encap_family);
  hlist_for_each_entry_rcu(x, net->xfrm.state_bydst + h, bydst) {

This is only safe in case the update to state_bydst is larger than
net->xfrm.xfrm_state_hmask (or if the lookup function gets
serialized via state spinlock again).

Fix this by prefetching state_hmask and the associated pointers.
The xfrm_state_hash_generation seqlock retry will ensure that the pointer
and the hmask will be consistent.

The existing helpers, like xfrm_dst_hash(), are now unsafe for RCU side,
add lockdep assertions to document that they are only safe for insert
side.

xfrm_state_lookup_byaddr() uses the spinlock rather than RCU.
AFAICS this is an oversight from back when state lookup was converted to
RCU, this lock should be replaced with RCU in a future patch.

Reported-by: syzbot+5f9f31cb7d985f584d8e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/CACT4Y+azwfrE3uz6A5ZErov5YN2LYBN5KrsymBerT36VU8qzBA@mail.gmail.com/
Diagnosed-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: c2f672fc94 ("xfrm: state lookup can be lockless")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-02-08 09:58:00 +01:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.