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From: Thomas Gleixner <>
To: Jay Lang <>
Cc:, Ingo Molnar <>,
	Borislav Petkov <>,, "H. Peter Anvin" <>,
	Christian Brauner <>,
	Andy Lutomirski <>,
	Juergen Gross <>, Jan Beulich <>,
	Benjamin Thiel <>,
	Matthew Garrett <>,
	"Peter Zijlstra \(Intel\)" <>,
	Brian Gerst <>,
	Marcelo Tosatti <>,
	yu kuai <>,
Subject: Re: [PATCH] x86/ioperm: fix a memory leak bug
Date: Thu, 28 May 2020 21:30:32 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


Jay Lang <> writes:

> In the copy_process() routine called by _do_fork(), failure to allocate
> a PID (or further along in the function) will trigger an invocation to
> exit_thread(). This is done to clean up from an earlier call to
> copy_thread_tls(). Naturally, the child task is passed into exit_thread(),
> however during the process, io_bitmap_exit() nullifies the parent's
> io_bitmap rather than the child's.
> As copy_thread_tls() has been called ahead of the failure, the reference
> count on the calling thread's io_bitmap is incremented as we would expect.
> However, io_bitmap_exit() doesn't accept any arguments, and thus assumes
> it should trash the current thread's io_bitmap reference rather than the
> child's. This is pretty sneaky in practice, because in all instances but
> this one, exit_thread() is called with respect to the current task and
> everything works out.
> A determined attacker can issue an appropriate ioctl (i.e. KDENABIO) to
> get a bitmap allocated, and force a clone3() syscall to fail by passing
> in a zeroed clone_args structure. The kernel handles the erroneous struct
> and the buggy code path is followed, and even though the parent's reference
> to the io_bitmap is trashed, the child still holds a reference and thus
> the structure will never be freed.

Nice catch! I'm sure I thought about that at some point and then forgot

> Fix this by tweaking io_bitmap_exit() and its subroutines to accept a
> task_struct argument which to operate on.

> This may not be the most elegant solution, but it mitigates the
> trigger described above on an x86_64 kernel.

It's well done and straight forward and comes with a well done change
log. Appreciated!



  reply	other threads:[~2020-05-28 19:30 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-24 16:27 Jay Lang
2020-05-28 19:30 ` Thomas Gleixner [this message]
2020-05-28 19:38 ` [tip: x86/urgent] x86/ioperm: Prevent a memory leak when fork fails tip-bot2 for Jay Lang

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