sctp: hold asoc or transport before mod_timer() in timer handlers

Overview

In the Linux kernel, the following vulnerability has been resolved:

sctp: hold asoc or transport before mod_timer() in timer handlers

Take the association or transport reference before rearming a timer in the timer handlers.

The existing code calls mod_timer() 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 mod_timer() 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 mod_timer() and drop the reference if mod_timer() 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

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < f7dea7eaea20fe9b80f8f77ba44d02b4a17f8050
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < f9d929026d487a988bfa12c065f892e823e5b6ab
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 9d3d39da054476e7aa05cd877b93ca73a9a77434
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 27c51a9b55b227309161c903f9ecfbf022516bc2
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < d18cb8fd1b6ba95c85de644203cfe1b071ff0c0f
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 35bf3b23c9a871f44c1453d85f8332be4e088dda
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < cae23ae3f7887a1cf8a75da38edcebeef695040f
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.