[xen master] xen/sched: Fix UB shift in compat_set_timer_op()

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commit ae6d4fd876765e6d623eec67d14f5d0464be09cb
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Jan 30 20:44:34 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Feb 1 19:52:44 2024 +0000

    xen/sched: Fix UB shift in compat_set_timer_op()
    
    Tamas reported this UBSAN failure from fuzzing:
    
      (XEN) 
================================================================================
      (XEN) UBSAN: Undefined behaviour in common/sched/compat.c:48:37
      (XEN) left shift of negative value -2147425536
      (XEN) ----[ Xen-4.19-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
      ...
      (XEN) Xen call trace:
      (XEN)    [<ffff82d040307c1c>] R ubsan.c#ubsan_epilogue+0xa/0xd9
      (XEN)    [<ffff82d040308afb>] F 
__ubsan_handle_shift_out_of_bounds+0x11a/0x1c5
      (XEN)    [<ffff82d040307758>] F compat_set_timer_op+0x41/0x43
      (XEN)    [<ffff82d04040e4cc>] F hvm_do_multicall_call+0x77f/0xa75
      (XEN)    [<ffff82d040519462>] F arch_do_multicall_call+0xec/0xf1
      (XEN)    [<ffff82d040261567>] F do_multicall+0x1dc/0xde3
      (XEN)    [<ffff82d04040d2b3>] F hvm_hypercall+0xa00/0x149a
      (XEN)    [<ffff82d0403cd072>] F vmx_vmexit_handler+0x1596/0x279c
      (XEN)    [<ffff82d0403d909b>] F vmx_asm_vmexit_handler+0xdb/0x200
    
    Left-shifting any negative value is strictly undefined behaviour in C, and
    the two parameters here come straight from the guest.
    
    The fuzzer happened to choose lo 0xf, hi 0x8000e300.
    
    Switch everything to be unsigned values, making the shift well defined.
    
    As GCC documents:
    
      As an extension to the C language, GCC does not use the latitude given in
      C99 and C11 only to treat certain aspects of signed '<<' as undefined.
      However, -fsanitize=shift (and -fsanitize=undefined) will diagnose such
      cases.
    
    this was deemed not to need an XSA.
    
    Note: The unsigned -> signed conversion for do_set_timer_op()'s s_time_t
    parameter is also well defined.  C makes it implementation defined, and GCC
    defines it as reduction modulo 2^N to be within range of the new type.
    
    Fixes: 2942f45e09fb ("Enable compatibility mode operation for 
HYPERVISOR_sched_op and HYPERVISOR_set_timer_op.")
    Reported-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/sched/compat.c    | 4 ++--
 xen/include/hypercall-defs.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/common/sched/compat.c b/xen/common/sched/compat.c
index d718e450d4..dd97593630 100644
--- a/xen/common/sched/compat.c
+++ b/xen/common/sched/compat.c
@@ -43,9 +43,9 @@ static int compat_poll(struct compat_sched_poll *compat)
 
 #include "core.c"
 
-int compat_set_timer_op(uint32_t lo, int32_t hi)
+int compat_set_timer_op(uint32_t lo, uint32_t hi)
 {
-    return do_set_timer_op(((s64)hi << 32) | lo);
+    return do_set_timer_op(((uint64_t)hi << 32) | lo);
 }
 
 #endif /* __COMMON_SCHED_COMPAT_C__ */
diff --git a/xen/include/hypercall-defs.c b/xen/include/hypercall-defs.c
index 6d361ddfce..47c093acc8 100644
--- a/xen/include/hypercall-defs.c
+++ b/xen/include/hypercall-defs.c
@@ -134,7 +134,7 @@ xenoprof_op(int op, void *arg)
 
 #ifdef CONFIG_COMPAT
 prefix: compat
-set_timer_op(uint32_t lo, int32_t hi)
+set_timer_op(uint32_t lo, uint32_t hi)
 multicall(multicall_entry_compat_t *call_list, uint32_t nr_calls)
 memory_op(unsigned int cmd, void *arg)
 #ifdef CONFIG_IOREQ_SERVER
--
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