[xen stable-4.14] x86/hap: do not switch on log dirty for VRAM tracking

[patchbot@xxxxxxx]

commit abfa80967be95479ab53c8abc6194713b4f0ee0e
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Apr 5 15:08:40 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 5 15:08:40 2022 +0200

    x86/hap: do not switch on log dirty for VRAM tracking
    
    XEN_DMOP_track_dirty_vram possibly calls into paging_log_dirty_enable
    when using HAP mode, and it can interact badly with other ongoing
    paging domctls, as XEN_DMOP_track_dirty_vram is not holding the domctl
    lock.
    
    This was detected as a result of the following assert triggering when
    doing repeated migrations of a HAP HVM domain with a stubdom:
    
    Assertion 'd->arch.paging.log_dirty.allocs == 0' failed at paging.c:198
    ----[ Xen-4.17-unstable  x86_64  debug=y  Not tainted ]----
    CPU:    34
    RIP:    e008:[<ffff82d040314b3b>] 
arch/x86/mm/paging.c#paging_free_log_dirty_bitmap+0x606/0x6
    RFLAGS: 0000000000010206   CONTEXT: hypervisor (d0v23)
    [...]
    Xen call trace:
       [<ffff82d040314b3b>] R 
arch/x86/mm/paging.c#paging_free_log_dirty_bitmap+0x606/0x63a
       [<ffff82d040279f96>] S xsm/flask/hooks.c#domain_has_perm+0x5a/0x67
       [<ffff82d04031577f>] F paging_domctl+0x251/0xd41
       [<ffff82d04031640c>] F paging_domctl_continuation+0x19d/0x202
       [<ffff82d0403202fa>] F pv_hypercall+0x150/0x2a7
       [<ffff82d0403a729d>] F lstar_enter+0x12d/0x140
    
    Such assert triggered because the stubdom used
    XEN_DMOP_track_dirty_vram while dom0 was in the middle of executing
    XEN_DOMCTL_SHADOW_OP_OFF, and so log dirty become enabled while
    retiring the old structures, thus leading to new entries being
    populated in already clear slots.
    
    Fix this by not enabling log dirty for VRAM tracking, similar to what
    is done when using shadow instead of HAP. Call
    p2m_enable_hardware_log_dirty when enabling VRAM tracking in order to
    get some hardware assistance if available. As a side effect the memory
    pressure on the p2m pool should go down if only VRAM tracking is
    enabled, as the dirty bitmap is no longer allocated.
    
    Note that paging_log_dirty_range (used to get the dirty bitmap for
    VRAM tracking) doesn't use the log dirty bitmap, and instead relies on
    checking whether each gfn on the range has been switched from
    p2m_ram_logdirty to p2m_ram_rw in order to account for dirty pages.
    
    This is CVE-2022-26356 / XSA-397.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 4f4db53784d912c4f409a451c36ebfd4754e0a42
    master date: 2022-04-05 14:11:30 +0200
---
 xen/arch/x86/mm/hap/hap.c    | 11 ++++-------
 xen/arch/x86/mm/paging.c     |  2 +-
 xen/include/asm-x86/paging.h |  3 ---
 3 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 7f84d0c6ea..1349de01d4 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -69,13 +69,6 @@ int hap_track_dirty_vram(struct domain *d,
     {
         int size = (nr + BITS_PER_BYTE - 1) / BITS_PER_BYTE;
 
-        if ( !paging_mode_log_dirty(d) )
-        {
-            rc = paging_log_dirty_enable(d, false);
-            if ( rc )
-                goto out;
-        }
-
         rc = -ENOMEM;
         dirty_bitmap = vzalloc(size);
         if ( !dirty_bitmap )
@@ -107,6 +100,10 @@ int hap_track_dirty_vram(struct domain *d,
 
             paging_unlock(d);
 
+            domain_pause(d);
+            p2m_enable_hardware_log_dirty(d);
+            domain_unpause(d);
+
             if ( oend > ostart )
                 p2m_change_type_range(d, ostart, oend,
                                       p2m_ram_logdirty, p2m_ram_rw);
diff --git a/xen/arch/x86/mm/paging.c b/xen/arch/x86/mm/paging.c
index 7c265fb5f3..7662db1ad4 100644
--- a/xen/arch/x86/mm/paging.c
+++ b/xen/arch/x86/mm/paging.c
@@ -209,7 +209,7 @@ static int paging_free_log_dirty_bitmap(struct domain *d, 
int rc)
     return rc;
 }
 
-int paging_log_dirty_enable(struct domain *d, bool log_global)
+static int paging_log_dirty_enable(struct domain *d, bool log_global)
 {
     int ret;
 
diff --git a/xen/include/asm-x86/paging.h b/xen/include/asm-x86/paging.h
index abb9a7e75c..2ca6325c5f 100644
--- a/xen/include/asm-x86/paging.h
+++ b/xen/include/asm-x86/paging.h
@@ -160,9 +160,6 @@ void paging_log_dirty_range(struct domain *d,
                             unsigned long nr,
                             uint8_t *dirty_bitmap);
 
-/* enable log dirty */
-int paging_log_dirty_enable(struct domain *d, bool log_global);
-
 /* log dirty initialization */
 void paging_log_dirty_init(struct domain *d, const struct log_dirty_ops *ops);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.14