[xen stable-4.18] compat/memory: avoid UB shifts in XENMEM_exchange handling

[patchbot@xxxxxxx]

commit d980886fd7f1626d42f766fbb0868a2c35813ae7
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Apr 29 12:10:30 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 29 12:10:30 2025 +0200

    compat/memory: avoid UB shifts in XENMEM_exchange handling
    
    Add an early basic check, yielding the same error code as the more
    thorough one the main handler would produce.
    
    Fixes: b8a7efe8528a ("Enable compatibility mode operation for 
HYPERVISOR_memory_op")
    Reported-by: Manuel Andreas <manuel.andreas@xxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 560c51be8f6a88cde43c0a7c8be60158b5725982
    master date: 2025-04-22 11:25:23 +0200
---
 xen/common/compat/memory.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/common/compat/memory.c b/xen/common/compat/memory.c
index 45e5fb0e5d..e3eb3c01a1 100644
--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -161,6 +161,11 @@ int compat_memory_op(unsigned int cmd, 
XEN_GUEST_HANDLE_PARAM(void) arg)
             if ( copy_from_guest(&cmp.xchg, arg, 1) )
                 return -EFAULT;
 
+            /* Early coarse check, as max_order() isn't available here. */
+            if ( cmp.xchg.in.extent_order >= 32 ||
+                 cmp.xchg.out.extent_order >= 32 )
+                return -EPERM;
+
             order_delta = cmp.xchg.out.extent_order - cmp.xchg.in.extent_order;
             /* Various sanity checks. */
             if ( (cmp.xchg.nr_exchanged > cmp.xchg.in.nr_extents) ||
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18