Out-of-bounds access in Linux kernel ALSA ctxfi ct_vm_map()
CVE-2026-31602 is a local Linux kernel vulnerability in the ALSA ctxfi driver (snd_ctxfi). A prior change, commit 391e69143d0a, increased CT_PTP_NUM from 1 to 4 to support 256 playback streams, but the additional PTP pages were not handled correctly by the CT20K2 hardware path. Specifically, ct_vm_map() always uses page table entries from vm->ptp[0].area regardless of the CT_PTP_NUM setting. On AMD64, a single PTP covers 512 PTEs, equivalent to 2 MB of mapped space. When aggregate memory allocations exceed that limit, ct_vm_map() accesses beyond the allocated page table space, leading to an out-of-bounds memory access and a kernel page fault during ALSA playback preparation paths. The upstream fix resolves the issue by reverting CT_PTP_NUM to 1 rather than attempting to use multiple PTP pages without broader virtual memory allocation refactoring.
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