x86/fpu: Ensure shadow stack is active before "getting" registers

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

x86/fpu: Ensure shadow stack is active before "getting" registers

The x86 shadow stack support has its own set of registers. Those registers

are XSAVE-managed, but they are "supervisor state components" which means

that userspace can not touch them with XSAVE/XRSTOR. It also means that

they are not accessible from the existing ptrace ABI for XSAVE state.

Thus, there is a new ptrace get/set interface for it.

The regset code that ptrace uses provides an ->active() handler in

addition to the get/set ones. For shadow stack this ->active() handler

verifies that shadow stack is enabled via the ARCH_SHSTK_SHSTK bit in the

thread struct. The ->active() handler is checked from some call sites of

the regset get/set handlers, but not the ptrace ones. This was not

understood when shadow stack support was put in place.

As a result, both the set/get handlers can be called with

XFEATURE_CET_USER in its init state, which would cause get_xsave_addr() to

return NULL and trigger a WARN_ON(). The ssp_set() handler luckily has an

ssp_active() check to avoid surprising the kernel with shadow stack

behavior when the kernel is not ready for it (ARCH_SHSTK_SHSTK==0). That

check just happened to avoid the warning.

But the ->get() side wasn't so lucky. It can be called with shadow stacks

disabled, triggering the warning in practice, as reported by Christina

Schimpe:

WARNING: CPU: 5 PID: 1773 at arch/x86/kernel/fpu/regset.c:198 ssp_get+0x89/0xa0

[...]

Call Trace:

<TASK>

? show_regs+0x6e/0x80

? ssp_get+0x89/0xa0

? __warn+0x91/0x150

? ssp_get+0x89/0xa0

? report_bug+0x19d/0x1b0

? handle_bug+0x46/0x80

? exc_invalid_op+0x1d/0x80

? asm_exc_invalid_op+0x1f/0x30

? __pfx_ssp_get+0x10/0x10

? ssp_get+0x89/0xa0

? ssp_get+0x52/0xa0

__regset_get+0xad/0xf0

copy_regset_to_user+0x52/0xc0

ptrace_regset+0x119/0x140

ptrace_request+0x13c/0x850

? wait_task_inactive+0x142/0x1d0

? do_syscall_64+0x6d/0x90

arch_ptrace+0x102/0x300

[...]

Ensure that shadow stacks are active in a thread before looking them up

in the XSAVE buffer. Since ARCH_SHSTK_SHSTK and user_ssp[SHSTK_EN] are

set at the same time, the active check ensures that there will be

something to find in the XSAVE buffer.

[ dhansen: changelog/subject tweaks ]