[Xen-changelog] [xen stable-4.12] x86/livepatch: Prevent patching with active waitqueues

[patchbot@xxxxxxx]

commit 1d64dc701ffbb470cae44a3093b2bdc29ff230ef
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Nov 25 16:05:48 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Nov 25 16:05:48 2019 +0100

    x86/livepatch: Prevent patching with active waitqueues
    
    The safety of livepatching depends on every stack having been unwound, but
    there is one corner case where this is not true.  The Sharing/Paging/Monitor
    infrastructure may use waitqueues, which copy the stack frame sideways and
    longjmp() to a different vcpu.
    
    This case is rare, and can be worked around by pausing the offending
    domain(s), waiting for their rings to drain, then performing a livepatch.
    
    In the case that there is an active waitqueue, fail the livepatch attempt 
with
    -EBUSY, which is preforable to the fireworks which occur from trying to 
unwind
    the old stack frame at a later point.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
    Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
    master commit: ca4cd3668237d50a0b33b48e7de7f93d9475120d
    master date: 2019-11-22 17:05:43 +0000
---
 xen/arch/arm/livepatch.c    |  5 +++++
 xen/arch/x86/livepatch.c    | 36 ++++++++++++++++++++++++++++++++++++
 xen/common/livepatch.c      |  8 ++++++++
 xen/include/xen/livepatch.h |  1 +
 4 files changed, 50 insertions(+)

diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
index 279d52cc6c..7184535c85 100644
--- a/xen/arch/arm/livepatch.c
+++ b/xen/arch/arm/livepatch.c
@@ -18,6 +18,11 @@
 
 void *vmap_of_xen_text;
 
+int arch_livepatch_safety_check(void)
+{
+    return 0;
+}
+
 int arch_livepatch_quiesce(void)
 {
     mfn_t text_mfn;
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index 406eb910cc..fd98d2bf03 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -10,10 +10,46 @@
 #include <xen/vmap.h>
 #include <xen/livepatch_elf.h>
 #include <xen/livepatch.h>
+#include <xen/sched.h>
 
 #include <asm/nmi.h>
 #include <asm/livepatch.h>
 
+static bool has_active_waitqueue(const struct vm_event_domain *ved)
+{
+    /* ved may be xzalloc()'d without INIT_LIST_HEAD() yet. */
+    return (ved && !list_head_is_null(&ved->wq.list) &&
+            !list_empty(&ved->wq.list));
+}
+
+/*
+ * x86's implementation of waitqueue violates the livepatching safey principle
+ * of having unwound every CPUs stack before modifying live content.
+ *
+ * Search through every domain and check that no vCPUs have an active
+ * waitqueue.
+ */
+int arch_livepatch_safety_check(void)
+{
+    struct domain *d;
+
+    for_each_domain ( d )
+    {
+        if ( has_active_waitqueue(d->vm_event_share) )
+            goto fail;
+        if ( has_active_waitqueue(d->vm_event_paging) )
+            goto fail;
+        if ( has_active_waitqueue(d->vm_event_monitor) )
+            goto fail;
+    }
+
+    return 0;
+
+ fail:
+    printk(XENLOG_ERR LIVEPATCH "%pd found with active waitqueue\n", d);
+    return -EBUSY;
+}
+
 int arch_livepatch_quiesce(void)
 {
     /* Disable WP to allow changes to read-only pages. */
diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index d6eaae6d3b..d69fdb3666 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -1060,6 +1060,14 @@ static int apply_payload(struct payload *data)
     unsigned int i;
     int rc;
 
+    rc = arch_livepatch_safety_check();
+    if ( rc )
+    {
+        printk(XENLOG_ERR LIVEPATCH "%s: Safety checks failed: %d\n",
+               data->name, rc);
+        return rc;
+    }
+
     printk(XENLOG_INFO LIVEPATCH "%s: Applying %u functions\n",
             data->name, data->nfuncs);
 
diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
index 1b1817ca0d..69ede75d20 100644
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -104,6 +104,7 @@ static inline int livepatch_verify_distance(const struct 
livepatch_func *func)
  * These functions are called around the critical region patching live code,
  * for an architecture to take make appropratie global state adjustments.
  */
+int arch_livepatch_safety_check(void);
 int arch_livepatch_quiesce(void);
 void arch_livepatch_revive(void);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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