riscv: fix race when vmap stack overflow

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: riscv: fix race when vmap stack overflow Currently, when detecting vmap stack overflow, riscv firstly switches to the so called shadow stack, then use this shadow stack to call the get_overflow_stack() to get the overflow stack. However, there's a race here if two or more harts use the same shadow stack at the same time. To solve this race, we introduce spin_shadow_stack atomic var, which will be swap between its own address and 0 in atomic way, when the var is set, it means the shadow_stack is being used; when the var is cleared, it means the shadow_stack isn't being used. [Palmer: Add AQ to the swap, and also some comments.]

References

• https://git.kernel.org/stable/c/ac00301adb19df54f2eae1efc4bad7447c0156ce
• https://git.kernel.org/stable/c/879fabc5a95401d9bce357e4b1d24ae4a360a81f
• https://git.kernel.org/stable/c/7e1864332fbc1b993659eab7974da9fe8bf8c128