CVE-2022-50579 — arm64: ftrace: fix module PLTs with mcount

Affected: Linux

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

arm64: ftrace: fix module PLTs with mcount

Li Huafei reports that mcount-based ftrace with module PLTs was broken by commit:

a6253579977e4c6f ("arm64: ftrace: consistently handle PLTs.")

When a module PLTs are used and a module is loaded sufficiently far away from the kernel, we'll create PLTs for any branches which are out-of-range. These are separate from the special ftrace trampoline PLTs, which the module PLT code doesn't directly manipulate.

When mcount is in use this is a problem, as each mcount callsite in a module will be initialized to point to a module PLT, but since commit a6253579977e4c6f ftrace_make_nop() will assume that the callsite has been initialized to point to the special ftrace trampoline PLT, and ftrace_find_callable_addr() rejects other cases.

This means that when ftrace tries to initialize a callsite via ftrace_make_nop(), the call to ftrace_find_callable_addr() will find that the `_mcount` stub is out-of-range and is not handled by the ftrace PLT, resulting in a splat:

| ftrace_test: loading out-of-tree module taints kernel. | ftrace: no module PLT for _mcount | ------------[ ftrace bug ]------------ | ftrace failed to modify | [<ffff800029180014>] 0xffff800029180014 | actual: 44:00:00:94 | Initializing ftrace call sites | ftrace record flags: 2000000 | (0) | expected tramp: ffff80000802eb3c | ------------[ cut here ]------------ | WARNING: CPU: 3 PID: 157 at kernel/tra…

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CVE-2022-50579 — arm64: ftrace: fix module PLTs with mcount