[xen stable-4.18] x86/time: Fix UBSAN failure in __update_vcpu_system_time()

[patchbot@xxxxxxx]

commit 6f9285975b1c6e82889a6118503ca367e3aa78fd
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Nov 1 20:19:52 2023 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Nov 2 21:34:37 2023 +0000

    x86/time: Fix UBSAN failure in __update_vcpu_system_time()
    
    As reported:
    
      (XEN) 
================================================================================
      (XEN) UBSAN: Undefined behaviour in arch/x86/time.c:1542:32
      (XEN) member access within null pointer of type 'union vcpu_info_t'
      (XEN) ----[ Xen-4.19-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
      ...
      (XEN) Xen call trace:
      (XEN)    [<ffff82d040345036>] R 
common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xd2
      (XEN)    [<ffff82d0403456e8>] F __ubsan_handle_type_mismatch+0x133/0x49b
      (XEN)    [<ffff82d040345b4a>] F __ubsan_handle_type_mismatch_v1+0xfa/0xfc
      (XEN)    [<ffff82d040623356>] F 
arch/x86/time.c#__update_vcpu_system_time+0x212/0x30f
      (XEN)    [<ffff82d040623461>] F update_vcpu_system_time+0xe/0x10
      (XEN)    [<ffff82d04062389d>] F 
arch/x86/time.c#local_time_calibration+0x1f7/0x523
      (XEN)    [<ffff82d0402a64b5>] F common/softirq.c#__do_softirq+0x1f4/0x31a
      (XEN)    [<ffff82d0402a67ad>] F do_softirq+0x13/0x15
      (XEN)    [<ffff82d0405a95dc>] F arch/x86/domain.c#idle_loop+0x2e0/0x367
      (XEN)
      (XEN) 
================================================================================
    
    GCC 10 at least doesn't consider it valid to derive a pointer from 
vcpu_info()
    prior to checking that the underlying map pointer is good.
    
    Reorder actions so the map pointer is checked first.
    
    Fixes: 20279afd7323 ("x86: split populating of struct vcpu_time_info into a 
separate function")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-acked-by: Henry Wang <Henry.Wang@xxxxxxx>
    (cherry picked from commit 801b804945bb7ccbd760d25637b720d8aac7e004)
---
 xen/arch/x86/time.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index d0b0986509..6d33edd0ad 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1539,12 +1539,14 @@ static void collect_time_info(const struct vcpu *v,
 
 static void __update_vcpu_system_time(struct vcpu *v, int force)
 {
-    struct vcpu_time_info *u = &vcpu_info(v, time), _u;
+    struct vcpu_time_info *u, _u;
     const struct domain *d = v->domain;
 
     if ( !v->vcpu_info_area.map )
         return;
 
+    u = &vcpu_info(v, time);
+
     collect_time_info(v, &_u);
 
     /* Don't bother unless timestamp record has changed or we are forced. */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18