From 5577c003018abb1ad92dc4032cc71b1718a83dfa Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Fri, 10 Apr 2026 09:20:04 +0200 Subject: [PATCH] xen/privcmd: fix double free via VMA splitting privcmd_vm_ops defines .close (privcmd_close), but neither .may_split nor .open. When userspace does a partial munmap() on a privcmd mapping, the kernel splits the VMA via __split_vma(). Since may_split is NULL, the split is allowed. vm_area_dup() copies vm_private_data (a pages array allocated in alloc_empty_pages()) into the new VMA without any fixup, because there is no .open callback. Both VMAs now point to the same pages array. When the unmapped portion is closed, privcmd_close() calls: - xen_unmap_domain_gfn_range() - xen_free_unpopulated_pages() - kvfree(pages) The surviving VMA still holds the dangling pointer. When it is later destroyed, the same sequence runs again, which leads to a double free. Fix this issue by adding a .may_split callback denying the VMA split. This is XSA-487 / CVE-2026-31787 Fixes: d71f513985c2 ("xen: privcmd: support autotranslated physmap guests.") Reported-by: Atharva Vartak Suggested-by: Atharva Vartak Signed-off-by: Juergen Gross Reviewed-by: Jan Beulich --- drivers/xen/privcmd.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c index 15ba592236e8..725a49a0eee7 100644 --- a/drivers/xen/privcmd.c +++ b/drivers/xen/privcmd.c @@ -1620,6 +1620,12 @@ static void privcmd_close(struct vm_area_struct *vma) kvfree(pages); } +static int privcmd_may_split(struct vm_area_struct *area, unsigned long addr) +{ + /* Forbid splitting, avoids double free via privcmd_close(). */ + return -EINVAL; +} + static vm_fault_t privcmd_fault(struct vm_fault *vmf) { printk(KERN_DEBUG "privcmd_fault: vma=%p %lx-%lx, pgoff=%lx, uv=%p\n", @@ -1631,6 +1637,7 @@ static vm_fault_t privcmd_fault(struct vm_fault *vmf) static const struct vm_operations_struct privcmd_vm_ops = { .close = privcmd_close, + .may_split = privcmd_may_split, .fault = privcmd_fault }; -- 2.53.0