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lock_vma_under_rcu() tries to guarantee that __anon_vma_prepare() can't
be called in the VMA-locked page fault path by ensuring that
vma->anon_vma is set.
However, this check happens before the VMA is locked, which means a
concurrent move_vma() can concurrently call unlink_anon_vmas(), which
disassociates the VMA's anon_vma.
This means we can get UAF in the following scenario:
THREAD 1 THREAD 2
======== ========
<page fault>
lock_vma_under_rcu()
rcu_read_lock()
mas_walk()
check vma->anon_vma
mremap() syscall
move_vma()
vma_start_write()
unlink_anon_vmas()
<syscall end>
handle_mm_fault()
__handle_mm_fault()
handle_pte_fault()
do_pte_missing()
do_anonymous_page()
anon_vma_prepare()
__anon_vma_prepare()
find_mergeable_anon_vma()
mas_walk() [looks up VMA X]
munmap() syscall (deletes VMA X)
reusable_anon_vma() [called on freed VMA X]
This is a security bug if you can hit it, although an attacker would
have to win two races at once where the first race window is only a few
instructions wide.
This patch is based on some previous discussion with Linus Torvalds on
the security list.
Cc: stable@vger.kernel.org
Fixes: 5e31275cc9 ("mm: add per-VMA lock and helper functions to control it")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 Restructured Text 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.