Input: touchwin - reset the packet index on every complete packet

Overview

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

Input: touchwin - reset the packet index on every complete packet

tw_interrupt() accumulates each non-zero serial byte into a fixed three-byte buffer with a running index that is only reset once a full packet has been received and the device's two Y bytes agree:

tw->data[tw->idx++] = data;
if (tw->idx == TW_LENGTH && tw->data[1] == tw->data[2]) {
	...
	tw->idx = 0;
}

The reset is gated on tw->data[1] == tw->data[2], a value the device controls. A malicious, malfunctioning or counterfeit Touchwindow peripheral can stream non-zero bytes whose 2nd and 3rd bytes differ: the index reaches TW_LENGTH without the equality holding, is never reset, and keeps growing, so tw->data[tw->idx++] walks off the end of the three-byte array and the rest of the heap-allocated struct tw, one attacker-chosen byte at a time -- an unbounded, device-driven heap out-of-bounds write.

Reset the index on every completed packet and report an event only when the two Y bytes match, like the other serio touchscreen drivers do.

Affected

• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < ed9b66905407eb3d02df1aaeed82eb7a7f0eb508
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 044167cba2384bcd783547ad5e30ecd292b30919
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 6c9f29f128dd4057404838259af4c645318487e1
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 431ad239f2924dff337c3fccb9246597c1b63185
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 70e4248793762df9832fd4fc2fc6ac7924572c36
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 3e6f007b43e2fc6546e21fa74ee62c38984a6672
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < a8d87184576c889759e3aab899799a482f1e1a5b
• Linux >= 11ea3173d5f2de71d037ef58ac43395795fed2bc < 478cdd736f2ce3114f90e775d7358136d3977b94
• Linux 2.6.19

Remediation

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