xen/arm: Fix race in RB-tree based P2M accounting

During the PV driver life cycle the mappings are added to
the RB-tree by set_foreign_p2m_mapping(), which is called from
gnttab_map_refs() and are removed by clear_foreign_p2m_mapping()
which is called from gnttab_unmap_refs(). As both functions end
up calling __set_phys_to_machine_multi() which updates the RB-tree,
this function can be called concurrently.

There is already a "p2m_lock" to protect against concurrent accesses,
but the problem is that the first read of "phys_to_mach.rb_node"
in __set_phys_to_machine_multi() is not covered by it, so this might
lead to the incorrect mappings update (removing in our case) in RB-tree.

In my environment the related issue happens rarely and only when
PV net backend is running, the xen_add_phys_to_mach_entry() claims
that it cannot add new pfn <-> mfn mapping to the tree since it is
already exists which results in a failure when mapping foreign pages.

But there might be other bad consequences related to the non-protected
root reads such use-after-free, etc.

While at it, also fix the similar usage in __pfn_to_mfn(), so
initialize "struct rb_node *n" with the "p2m_lock" held in both
functions to avoid possible bad consequences.

This is CVE-2022-33744 / XSA-406.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/arch/arm/xen/p2m.c b/arch/arm/xen/p2m.c
index 84a1cea1f43b9a3331b35093e4c57cb78d670815..309648c17f48685c8a115bcb1e03dd0212a35f49 100644 (file)
--- a/arch/arm/xen/p2m.c
+++ b/arch/arm/xen/p2m.c
@@ -63,11 +63,12 @@ out:
 
 unsigned long __pfn_to_mfn(unsigned long pfn)
 {
-       struct rb_node *n = phys_to_mach.rb_node;
+       struct rb_node *n;
        struct xen_p2m_entry *entry;
        unsigned long irqflags;
 
        read_lock_irqsave(&p2m_lock, irqflags);
+       n = phys_to_mach.rb_node;
        while (n) {
                entry = rb_entry(n, struct xen_p2m_entry, rbnode_phys);
                if (entry->pfn <= pfn &&
@@ -152,10 +153,11 @@ bool __set_phys_to_machine_multi(unsigned long pfn,
        int rc;
        unsigned long irqflags;
        struct xen_p2m_entry *p2m_entry;
-       struct rb_node *n = phys_to_mach.rb_node;
+       struct rb_node *n;
 
        if (mfn == INVALID_P2M_ENTRY) {
                write_lock_irqsave(&p2m_lock, irqflags);
+               n = phys_to_mach.rb_node;
                while (n) {
                        p2m_entry = rb_entry(n, struct xen_p2m_entry, rbnode_phys);
                        if (p2m_entry->pfn <= pfn &&