[PATCH] x86/tdx: Make seamcall/tdcall CET-compliant

[Nikolay Borisov <nik.borisov@suse.com>]

_seamcall/_ret/_saved_ret can be the target of indirect calls via
sc_retry_prerr/__seamcall_dirty_cache so on machines with CET enabled
such call chains result in a  splat and a BUG():

Missing ENDBR: __seamcall+0x0/0x50
------------[ cut here ]------------
kernel BUG at arch/x86/kernel/cet.c:132!
Oops: invalid opcode: 0000 [#1] SMP NOPTI
CPU: 195 UID: 0 PID: 3525 Comm: (udev-worker) Tainted: G                   n 6.12.0-160000.3.gccf23ce-default #1 PREEMPT(voluntary) SLFO-1.2 (unreleased) c9419bf0caf542825c59d4f407ef13fb3c33bc31
Tainted: [n]=NO_SUPPORT
Hardware name: Intel Corporation D50DNP/D50DNP, BIOS SE5C741.86B.01.02.0002.2408050237 08/05/2024
RIP: 0010:exc_control_protection+0x2c4/0x2d0
Code: d8 b9 09 00 00 00 48 8b 93 80 00 00 00 be 80 00 00 00 48 c7 c7 e5 1c a2 bd e8 c8 4a 34 ff 80 a3 8a 00 00 00 fb e9 11 fe ff ff <0f> 0b 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ff6bb6927112f988 EFLAGS: 00010002
RAX: 0000000000000022 RBX: ff6bb6927112f9b8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ff474944fafa6bc0 RDI: ff474944fafa6bc0
RBP: 0000000000000000 R08: 0000000000000000 R09: ff6bb6927112f778
R10: 0000000000000003 R11: ff474985fbd87e28 R12: 0000000000000003
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS:  00007f3b43ac3900(0000) GS:ff474944faf80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3b419ea000 CR3: 000000c0db9c6006 CR4: 0000000000f73ef0
PKRU: 55555554
Call Trace:
 <TASK>
 ? __die_body.cold+0x14/0x20
 ? die+0x2e/0x50
 ? do_trap+0xca/0x110
 ? do_error_trap+0x65/0x80
 ? exc_control_protection+0x2c4/0x2d0
 ? exc_invalid_op+0x50/0x70
 ? exc_control_protection+0x2c4/0x2d0
 ? asm_exc_invalid_op+0x1a/0x20
 ? exc_control_protection+0x2c4/0x2d0
 ? exc_control_protection+0x280/0x2d0
 asm_exc_control_protection+0x26/0x30
RIP: 0010:__seamcall+0x0/0x50
Code: 44 00 00 bf 07 00 00 00 e8 7d df cb 00 84 c0 74 02 0f 09 c3 cc cc cc cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 <55> 48 89 f8 48 8b 0e 48 8b 56 08 4c 8b 46 10 4c 8b 4e 18 4c 8b 56
RSP: 0018:ff6bb6927112fa68 EFLAGS: 00010283
RAX: ffffffffbc4c21a0 RBX: ff4749061045b200 RCX: 0000000000000005
RDX: ff6bb6927112fa78 RSI: ff6bb6927112fa78 RDI: 000000000000002d
RBP: 000000000000000a R08: 0000001f7d200000 R09: 0000000080000000
R10: 00b8b77a00000000 R11: 0000000000000400 R12: 8000020300000000
R13: 0000000000000010 R14: 0000000000000000 R15: 0000000000000005
 ? __pfx___seamcall+0x10/0x10
 do_seamcall+0x1a/0x40
 config_tdx_module.constprop.0+0x10a/0x197
 tdx_enable.cold+0x508/0x7e4
 tdx_bringup+0x1c1/0x1280 [kvm_intel f93f40ca63d984c168979eef9c8c2c660b2fd468]
 vt_init+0x1a/0x60 [kvm_intel f93f40ca63d984c168979eef9c8c2c660b2fd468]
 ? __pfx_vt_init+0x10/0x10 [kvm_intel f93f40ca63d984c168979eef9c8c2c660b2fd468]
 do_one_initcall+0x45/0x2f0
 do_init_module+0x90/0x250
 __do_sys_init_module+0x183/0x1c0
 do_syscall_64+0x7d/0x160
 ? __vm_munmap+0xc4/0x160
 ? syscall_exit_to_user_mode+0x32/0x1b0
 ? do_syscall_64+0x89/0x160
 ? sched_balance_trigger+0x66/0x360
 ? __count_memcg_events+0x53/0xf0
 ? handle_mm_fault+0xb9/0x2e0
 ? do_flush_tlb_all+0xe/0x20
 ? __flush_smp_call_function_queue+0x96/0x420
 ? __irq_exit_rcu+0x39/0xe0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f3b43d1a6be

Fix it by adding an ENBDR in TDX_MODULE_CALL macro to cover all
cases.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
---

The kernel this was observed is a SLE, however it contains the current upstream
TDX patches. And looking at the usptream code the problem persists there as well.

 arch/x86/virt/vmx/tdx/tdxcall.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
index 016a2a1ec1d6..a2137cd7a669 100644
--- a/arch/x86/virt/vmx/tdx/tdxcall.S
+++ b/arch/x86/virt/vmx/tdx/tdxcall.S
@@ -43,6 +43,7 @@
  * TDH.EXPORT.MEM.
  */
 .macro TDX_MODULE_CALL host:req ret=0 saved=0
+	ENDBR
 	FRAME_BEGIN

 	/* Move Leaf ID to RAX */
--
2.51.1