Kernel space: the vmsplice() exploit
As this is being written, distributors are working quickly to ship kernel updates fixing the local root vulnerabilities in the vmsplice() system call. Unlike a number of other recent vulnerabilities which have required special situations (such as the presence of specific hardware) to exploit, these vulnerabilities are trivially exploited and the code to do so is circulating on the net. The author found himself wondering how such a wide hole could find its way into the core kernel code, so he set himself the task of figuring out just what was going on - a task which took rather longer than he had expected.
The splice() system call, remember, is a mechanism for creating data flow plumbing within the kernel. It can be used to join two file descriptors; the kernel will then read data from one of those descriptors and write it to the other in the most efficient way possible. So one can write a trivial file copy program which opens the source and destination files, then splices the two together. The vmsplice() variant connects a file descriptor (which must be a pipe) to a region of user memory; it is in this system call that the problems came to be.
The first step in understanding this vulnerability is that, in fact, it is three separate bugs.
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