Backtrace after invalid XRSTOR after "x86/fault: BUG() when uaccess helpers fault on kernel addresses"

[Sebastian Andrzej Siewior]

Commit 75045f77f7a7 ("x86/extable: Introduce _ASM_EXTABLE_UA for uaccess
fixups") made copy_user_to_xregs() -> XSTATE_OP() use _ASM_EXTABLE_UA.
Commit 9da3f2b74054 ("x86/fault: BUG() when uaccess helpers fault on
kernel addresses") then decided that a #GP is not good and has to be
reported loudly.

I had a TC which sets a few invalid bits in xstate which is used by
copy_user_to_xregs() on sig-return. Before that change I had:
| sig-xstate-bum[2253] bad frame in rt_sigreturn frame:0000000056078134 ip:7f9da336c227 sp:7ffc871325e8 orax:ffffffffffffffff in  libc-2.27.so[7f9da3358000+146000]

after those two patches are applied:
|BUG: GPF in non-whitelisted uaccess (non-canonical address?)
|general protection fault: 0000 [#1] PREEMPT SMP NOPTI
|CPU: 26 PID: 2236 Comm: sig-xstate-bum Not tainted 4.20.0-rc3 #45
|RIP: 0010:__fpu__restore_sig+0x1c1/0x540
|Code: 02 00 00 48 8b 95 58 ff ff ff 48 f7 d2 48 21 d0 0f 85 6e 03 00 00 0f 01 cb 48 8b 85 58 ff ff ff 48 89 df 48 89 c2 48 c1 ea 20 <48> 0f ae 2f 31 db 0f 01 ca 85 db 0f 84 d7 00 00 00 4c 89 f7 bb ff
|Call Trace:
| fpu__restore_sig+0x28/0x40
| restore_sigcontext+0x13a/0x180
| __ia32_sys_rt_sigreturn+0xae/0x100
| do_syscall_64+0x4f/0x100
| entry_SYSCALL_64_after_hwframe+0x44/0xa9
|RIP: 0033:0x7f9b06aea227
|---[ end trace a45ac23b593e9ab0 ]---

The expected behaviour would that `xrstor' performs a #GP and this does
not a produce a backtrace like that and copy_user_to_fxregs() returns an
error.
copy_user_to_fxregs() / user_insn() does not have this behaviour and
that also might generate a #GP (if invalid bits are set in MCSR).
What do we do?

Sebastian