KVM: x86: Use jmp to invoke kvm_spurious_fault() from .fixup

____kvm_handle_fault_on_reboot() provides a generic exception fixup
handler that is used to cleanly handle faults on VMX/SVM instructions
during reboot (or at least try to).  If there isn't a reboot in
progress, ____kvm_handle_fault_on_reboot() treats any exception as
fatal to KVM and invokes kvm_spurious_fault(), which in turn generates
a BUG() to get a stack trace and die.

When it was originally added by commit 4ecac3fd6d ("KVM: Handle
virtualization instruction #UD faults during reboot"), the "call" to
kvm_spurious_fault() was handcoded as PUSH+JMP, where the PUSH'd value
is the RIP of the faulting instructing.

The PUSH+JMP trickery is necessary because the exception fixup handler
code lies outside of its associated function, e.g. right after the
function.  An actual CALL from the .fixup code would show a slightly
bogus stack trace, e.g. an extra "random" function would be inserted
into the trace, as the return RIP on the stack would point to no known
function (and the unwinder will likely try to guess who owns the RIP).

Unfortunately, the JMP was replaced with a CALL when the macro was
reworked to not spin indefinitely during reboot (commit b7c4145ba2
"KVM: Don't spin on virt instruction faults during reboot").  This
causes the aforementioned behavior where a bogus function is inserted
into the stack trace, e.g. my builds like to blame free_kvm_area().

Revert the CALL back to a JMP.  The changelog for commit b7c4145ba2
("KVM: Don't spin on virt instruction faults during reboot") contains
nothing that indicates the switch to CALL was deliberate.  This is
backed up by the fact that the PUSH <insn RIP> was left intact.

Note that an alternative to the PUSH+JMP magic would be to JMP back
to the "real" code and CALL from there, but that would require adding
a JMP in the non-faulting path to avoid calling kvm_spurious_fault()
and would add no value, i.e. the stack trace would be the same.

Using CALL:

------------[ cut here ]------------
kernel BUG at /home/sean/go/src/kernel.org/linux/arch/x86/kvm/x86.c:356!
invalid opcode: 0000 [#1] SMP
CPU: 4 PID: 1057 Comm: qemu-system-x86 Not tainted 4.20.0-rc6+ #75
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:kvm_spurious_fault+0x5/0x10 [kvm]
Code: <0f> 0b 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 55 49 89 fd 41
RSP: 0018:ffffc900004bbcc8 EFLAGS: 00010046
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffffffffffff
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff888273fd8000 R08: 00000000000003e8 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000784 R12: ffffc90000371fb0
R13: 0000000000000000 R14: 000000026d763cf4 R15: ffff888273fd8000
FS:  00007f3d69691700(0000) GS:ffff888277800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055f89bc56fe0 CR3: 0000000271a5a001 CR4: 0000000000362ee0
Call Trace:
 free_kvm_area+0x1044/0x43ea [kvm_intel]
 ? vmx_vcpu_run+0x156/0x630 [kvm_intel]
 ? kvm_arch_vcpu_ioctl_run+0x447/0x1a40 [kvm]
 ? kvm_vcpu_ioctl+0x368/0x5c0 [kvm]
 ? kvm_vcpu_ioctl+0x368/0x5c0 [kvm]
 ? __set_task_blocked+0x38/0x90
 ? __set_current_blocked+0x50/0x60
 ? __fpu__restore_sig+0x97/0x490
 ? do_vfs_ioctl+0xa1/0x620
 ? __x64_sys_futex+0x89/0x180
 ? ksys_ioctl+0x66/0x70
 ? __x64_sys_ioctl+0x16/0x20
 ? do_syscall_64+0x4f/0x100
 ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
Modules linked in: vhost_net vhost tap kvm_intel kvm irqbypass bridge stp llc
---[ end trace 9775b14b123b1713 ]---

Using JMP:

------------[ cut here ]------------
kernel BUG at /home/sean/go/src/kernel.org/linux/arch/x86/kvm/x86.c:356!
invalid opcode: 0000 [#1] SMP
CPU: 6 PID: 1067 Comm: qemu-system-x86 Not tainted 4.20.0-rc6+ #75
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:kvm_spurious_fault+0x5/0x10 [kvm]
Code: <0f> 0b 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 55 49 89 fd 41
RSP: 0018:ffffc90000497cd0 EFLAGS: 00010046
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffffffffffff
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff88827058bd40 R08: 00000000000003e8 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000784 R12: ffffc90000369fb0
R13: 0000000000000000 R14: 00000003c8fc6642 R15: ffff88827058bd40
FS:  00007f3d7219e700(0000) GS:ffff888277900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3d64001000 CR3: 0000000271c6b004 CR4: 0000000000362ee0
Call Trace:
 vmx_vcpu_run+0x156/0x630 [kvm_intel]
 ? kvm_arch_vcpu_ioctl_run+0x447/0x1a40 [kvm]
 ? kvm_vcpu_ioctl+0x368/0x5c0 [kvm]
 ? kvm_vcpu_ioctl+0x368/0x5c0 [kvm]
 ? __set_task_blocked+0x38/0x90
 ? __set_current_blocked+0x50/0x60
 ? __fpu__restore_sig+0x97/0x490
 ? do_vfs_ioctl+0xa1/0x620
 ? __x64_sys_futex+0x89/0x180
 ? ksys_ioctl+0x66/0x70
 ? __x64_sys_ioctl+0x16/0x20
 ? do_syscall_64+0x4f/0x100
 ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
Modules linked in: vhost_net vhost tap kvm_intel kvm irqbypass bridge stp llc
---[ end trace f9daedb85ab3ddba ]---

Fixes: b7c4145ba2 ("KVM: Don't spin on virt instruction faults during reboot")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1501,7 +1501,7 @@ asmlinkage void kvm_spurious_fault(void);
 	"cmpb $0, kvm_rebooting \n\t"	      \
 	"jne 668b \n\t"      		      \
 	__ASM_SIZE(push) " $666b \n\t"	      \
-	"call kvm_spurious_fault \n\t"	      \
+	"jmp kvm_spurious_fault \n\t"	      \
 	".popsection \n\t" \
 	_ASM_EXTABLE(666b, 667b)