vfio/type1: prevent underflow of locked_vm via exec()

In the Linux kernel, the following vulnerability has been resolved: vfio/type1: prevent underflow of locked_vm via exec() When a vfio container is preserved across exec, the task does not change, but it gets a new mm with locked_vm=0, and loses the count from existing dma mappings. If the user later unmaps a dma mapping, locked_vm underflows to a large unsigned value, and a subsequent dma map request fails with ENOMEM in __account_locked_vm. To avoid underflow, grab and save the mm at the time a dma is mapped. Use that mm when adjusting locked_vm, rather than re-acquiring the saved task's mm, which may have changed. If the saved mm is dead, do nothing. locked_vm is incremented for existing mappings in a subsequent patch.

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Linux kernel 安全漏洞

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于vfio/type1容器在执行exec时未正确处理locked_vm计数，可能导致locked_vm下溢为极大无符号值。

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