perf: Fix sample vs do_exit()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: perf: Fix sample vs do_exit() Baisheng Gao reported an ARM64 crash, which Mark decoded as being a synchronous external abort -- most likely due to trying to access MMIO in bad ways. The crash further shows perf trying to do a user stack sample while in exit_mmap()'s tlb_finish_mmu() -- i.e. while tearing down the address space it is trying to access. It turns out that we stop perf after we tear down the userspace mm; a receipie for disaster, since perf likes to access userspace for various reasons. Flip this order by moving up where we stop perf in do_exit(). Additionally, harden PERF_SAMPLE_CALLCHAIN and PERF_SAMPLE_STACK_USER to abort when the current task does not have an mm (exit_mm() makes sure to set current->mm = NULL; before commencing with the actual teardown). Such that CPU wide events don't trip on this same problem.

References

• https://git.kernel.org/stable/c/7b8f3c72175c6a63a95cf2e219f8b78e2baad34e
• https://git.kernel.org/stable/c/507c9a595bad3abd107c6a8857d7fd125d89f386
• https://git.kernel.org/stable/c/a9f6aab7910a0ef2895797f15c947f6d1053160f
• https://git.kernel.org/stable/c/975ffddfa2e19823c719459d2364fcaa17673964
• https://git.kernel.org/stable/c/2ee6044a693735396bb47eeaba1ac3ae26c1c99b
• https://git.kernel.org/stable/c/456019adaa2f5366b89c868dea9b483179bece54
• https://git.kernel.org/stable/c/7311970d07c4606362081250da95f2c7901fc0db
• https://git.kernel.org/stable/c/4f6fc782128355931527cefe3eb45338abd8ab39