CVE-2026-98586: sctp: hold asoc or transport before mod_timer() in timer handlers
In the Linux kernel, the following vulnerability has been resolved:
sctp: hold asoc or transport before modtimer() in timer handlers
Take the association or transport reference before rearming a timer in the timer handlers.
The existing code calls modtimer() before taking the reference needed by the rearmed timer without holding the sock lock. This creates a race with timer cleanup: if the timer is deleted after modtimer() returns but before the reference is taken, the cleanup path can drop the timer's reference and destroy the transport or association. The timer handler then takes a reference on the already freed object and eventually drops it, causing a refcount underflow.
Hold the object before modtimer() and drop the reference if modtimer() reports that the timer was already pending in timer handlers. Apply the same ordering to the proto-unreachable path, which can rearm a transport timer outside the timer handlers without holding the sock lock.
Affected Software
Remediation
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Fixed in 6.6.159.1-1 - Upgrade
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In SCTP timer handlers and the proto-unreachable path, take the association or transport reference before calling mod_timer(); if mod_timer() reports that the timer was already pending, drop the reference.
Event History
Frequently Asked Questions
What conditions are required to trigger this issue?
The race requires SCTP timer handling or the proto-unreachable path to rearm an association or transport timer while timer cleanup can delete that timer. It occurs because the existing ordering rearms the timer before obtaining the reference required by the rearmed timer, without holding the socket lock.
What is the consequence if the race occurs?
Timer cleanup can release the timer's reference and destroy the SCTP transport or association before the timer handler obtains its reference. The handler can then reference an already freed object and later cause a refcount underflow when dropping that reference.
Is the issue limited to normal timer handlers?
No. The affected ordering also exists in the proto-unreachable path, which can rearm a transport timer outside the timer handlers and without holding the socket lock.
What change resolves the race?
The fix obtains a reference to the association or transport before calling mod_timer(). If mod_timer() indicates that the timer was already pending, that extra reference is dropped.