CVE-2026-53243 — rseq: Fix using an uninitialized stack variable in rseq_exit_user_update()

Affected: Linux

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

rseq: Fix using an uninitialized stack variable in rseq_exit_user_update()

There is an bug in which an uninitialized stack variable is used in rseq_exit_user_update() as reported by syzbot:

BUG: KMSAN: kernel-infoleak in rseq_set_ids_get_csaddr include/linux/rseq_entry.h:502 [inline]

The local variable:

struct rseq_ids ids = { .cpu_id = task_cpu(t), .mm_cid = task_mm_cid(t), .node_id = cpu_to_node(ids.cpu_id), };

According to the C standard, the evaluation order of expressions in an initializer list is indeterminately sequenced. The compiler (Clang, in this KMSAN build) evaluates `cpu_to_node(ids.cpu_id)` *before* `ids.cpu_id` is initialized with `task_cpu(t)`.

This is fixed by moving the assignment of ids.node_id outside the structure initialization.

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CVE-2026-53243 — rseq: Fix using an uninitialized stack variable in rseq_exit_user_update()