Subject: CVE-2021-46921: locking/qrwlock: Fix ordering in queued_write_lock_slowpath()

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

locking/qrwlock: Fix ordering in queued_write_lock_slowpath()

While this code is executed with the wait_lock held, a reader can
acquire the lock without holding wait_lock.  The writer side loops
checking the value with the atomic_cond_read_acquire(), but only truly
acquires the lock when the compare-and-exchange is completed
successfully which isn’t ordered. This exposes the window between the
acquire and the cmpxchg to an A-B-A problem which allows reads
following the lock acquisition to observe values speculatively before
the write lock is truly acquired.

We've seen a problem in epoll where the reader does a xchg while
holding the read lock, but the writer can see a value change out from
under it.

  Writer                                | Reader
  --------------------------------------------------------------------------------
  ep_scan_ready_list()                  |
  |- write_lock_irq()                   |
      |- queued_write_lock_slowpath()   |
	|- atomic_cond_read_acquire()   |
				        | read_lock_irqsave(&ep->lock, flags);
     --> (observes value before unlock) |  chain_epi_lockless()
     |                                  |    epi->next = xchg(&ep->ovflist, epi);
     |                                  | read_unlock_irqrestore(&ep->lock, flags);
     |                                  |
     |     atomic_cmpxchg_relaxed()     |
     |-- READ_ONCE(ep->ovflist);        |

A core can order the read of the ovflist ahead of the
atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire
semantics addresses this issue at which point the atomic_cond_read can
be switched to use relaxed semantics.

[peterz: use try_cmpxchg()]

The Linux kernel CVE team has assigned CVE-2021-46921 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.15 with commit b519b56e378e and fixed in 4.19.189 with commit 5902f9453a31
	Issue introduced in 4.15 with commit b519b56e378e and fixed in 5.4.115 with commit 82808cc02681
	Issue introduced in 4.15 with commit b519b56e378e and fixed in 5.10.33 with commit 82fa9ced35d8
	Issue introduced in 4.15 with commit b519b56e378e and fixed in 5.11.17 with commit d558fcdb1713
	Issue introduced in 4.15 with commit b519b56e378e and fixed in 5.12 with commit 84a24bf8c52e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-46921
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	kernel/locking/qrwlock.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e
	https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56
	https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a
	https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2
	https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896