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    Subject[PATCH 4.14 27/29] xen/arm: Fix race in RB-tree based P2M accounting
    Date
    From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

    commit b75cd218274e01d026dc5240e86fdeb44bbed0c8 upstream.

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm/xen/p2m.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    --- a/arch/arm/xen/p2m.c
    +++ b/arch/arm/xen/p2m.c
    @@ -61,11 +61,12 @@ 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 &&
    @@ -151,10 +152,11 @@ bool __set_phys_to_machine_multi(unsigne
    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 &&

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