[xen staging] x86/APIC: handle overflow in TMICT calculation

[patchbot@xxxxxxx]

commit 4f14d97a620c3a005ad0604ae47ee6091281cda0
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Apr 16 10:29:54 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 16 10:29:54 2026 +0200

    x86/APIC: handle overflow in TMICT calculation
    
    With an expiry value on the order of 10 hours, and with a bus scale value
    of 256k (as supplied by qemu), the (signed) multiplication will be UB. As
    we've checked that the value is positive, we mean unsigned multiplication
    anyway. Yet let's play safe against even larger expiry and bus scale
    values, leveraging the compiler builtin that there is for this purpose.
    
    While there also drop the stray cast from the actual TMICT write.
    
    Fixes: 9062553a0dc1 ("added time and accurate timer support")
    Fixes: b95beb185810 ("x86: Clean up APIC local timer handling")
    Reported-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Tested-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
---
 xen/arch/x86/apic.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index d69ef9e16a..176f1498e3 100644
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1224,10 +1224,16 @@ int reprogram_timer(s_time_t timeout)
     }
 
     if ( timeout && ((expire = timeout - NOW()) > 0) )
-        apic_tmict = min_t(uint64_t, (bus_scale * expire) >> BUS_SCALE_SHIFT,
-                           UINT32_MAX);
+    {
+        unsigned long product;
+
+        apic_tmict = UINT32_MAX;
+        if ( !__builtin_umull_overflow(bus_scale, expire, &product) &&
+             (product >>= BUS_SCALE_SHIFT) < apic_tmict )
+            apic_tmict = product;
+    }
 
-    apic_write(APIC_TMICT, (unsigned long)apic_tmict);
+    apic_write(APIC_TMICT, apic_tmict);
 
     return apic_tmict || !timeout;
 }
--
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