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Date: Mon, 8 Aug 2022 13:07:13 +0200
From: Solar Designer <>
Cc: David Bouman <>
Subject: Re: Linux: UaF due to concurrency issue in io_uring timeouts


Jayden and David have recently published a lengthy write-up on this
vulnerability, here:

and exploit here:


On Fri, Apr 22, 2022 at 06:02:58PM +0200, Salvatore Bonaccorso wrote:
> Hi David,
> On Fri, Apr 22, 2022 at 02:43:27AM +0200, David Bouman wrote:
> > Hello list,
> > 
> > We (Jayden Rivers and David Bouman) are disclosing a bug we found in the
> > Linux kernel's io_uring subsystem. We have written a local privilege
> > escalation PoC that can successfully elevate to system root from an
> > unprivileged process (in a container). We will be releasing a blog post
> > (including exploit code) in a week or two. It should be noted that unlike
> > many Linux vulnerabilities that have surfaced recently, triggering this one
> > does not require an attacker to have any kind of privileges (e.g. in a user
> > namespace). This leaves many systems vulnerable.
> > 
> > We are still looking for a CNA representative that can assign a CVE number
> > for this vulnerability; please contact us!
> > 
> > Kernel versions 5.10+ are affected, and linux-stable patches are already
> > pushed. The upstream patch commit is
> > e677edbcabee849bfdd43f1602bccbecf736a646 ("io_uring: fix race between
> > timeout flush and removal").
> > 
> > When the IORING_OP_TIMEOUT (T) and IORING_OP_LINK_TIMEOUT (LT) opcodes are
> > combined in a linked submission queue entry, and another request (B)
> > finishes, a race might occur: namely, when due to the completion of B, T is
> > cancelled (through the completion event count), and LT is canceled by its
> > hrtimer at the same time. Whilst T is still being cleaned up, LT is already
> > freed by a different execution context, and since they are linked, the
> > cleanup of T retains a dangling reference to the now-freed LT. Hence,
> > there's a use-after-free.
> > 
> > Exploitation-wise, the attacker can reallocate LT to another `struct
> > io_kiocb` and defer the UaF to e.g. a `struct file` (this is the technique
> > we will describe in aforementioned blog post).
> > 
> > The race window is quite tight and the scenario is complicated, so the race
> > can only be won very infrequently in our experience.
> > 
> > It is advised to upgrade your kernel to latest ASAP.
> > 
> > Greetings,
> > 
> > Jayden Rivers & David Bouman
> This has CVE-2022-29582 assigned.
> Regards,
> Salvatore

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