Bug 2297494 (CVE-2024-40910)

Summary: CVE-2024-40910 kernel: ax25: Fix refcount imbalance on inbound connections
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: dfreiber, drow, jburrell, vkumar
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard:
Fixed In Version: kernel 6.1.95, kernel 6.6.35, kernel 6.9.6, kernel 6.10-rc3 Doc Type: If docs needed, set a value
Doc Text:
A vulnerability was found in the Linux kernel, related to the ax25 protocol. This issue can cause a reference count imbalance during the handling of inbound connections. The ax25_release() function incorrectly decrements the reference count of the associated ax.25 device using netdev_put() without a corresponding netdev_hold() in the connection acceptance path. This imbalance can lead to kernel crashes and memory errors, indicated by messages such as refcount_t: decrement hit 0; leaking memory and refcount_t: underflow; use-after-free.
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Description OSIDB Bzimport 2024-07-12 13:30:15 UTC 
In the Linux kernel, the following vulnerability has been resolved:

ax25: Fix refcount imbalance on inbound connections

When releasing a socket in ax25_release(), we call netdev_put() to
decrease the refcount on the associated ax.25 device. However, the
execution path for accepting an incoming connection never calls
netdev_hold(). This imbalance leads to refcount errors, and ultimately
to kernel crashes.

A typical call trace for the above situation will start with one of the
following errors:

    refcount_t: decrement hit 0; leaking memory.
    refcount_t: underflow; use-after-free.

And will then have a trace like:

    Call Trace:
    <TASK>
    ? show_regs+0x64/0x70
    ? __warn+0x83/0x120
    ? refcount_warn_saturate+0xb2/0x100
    ? report_bug+0x158/0x190
    ? prb_read_valid+0x20/0x30
    ? handle_bug+0x3e/0x70
    ? exc_invalid_op+0x1c/0x70
    ? asm_exc_invalid_op+0x1f/0x30
    ? refcount_warn_saturate+0xb2/0x100
    ? refcount_warn_saturate+0xb2/0x100
    ax25_release+0x2ad/0x360
    __sock_release+0x35/0xa0
    sock_close+0x19/0x20
    [...]

On reboot (or any attempt to remove the interface), the kernel gets
stuck in an infinite loop:

    unregister_netdevice: waiting for ax0 to become free. Usage count = 0

This patch corrects these issues by ensuring that we call netdev_hold()
and ax25_dev_hold() for new connections in ax25_accept(). This makes the
logic leading to ax25_accept() match the logic for ax25_bind(): in both
cases we increment the refcount, which is ultimately decremented in
ax25_release().