[Xen-devel] [PATCH for-4.13 0/2] xen/livepatch: Safety fixes

[Xen-devel] [PATCH for-4.13 0/2] xen/livepatch: Safety fixes

[Andrew Cooper]

This pair of patches is long overdue being posted upstream.  For several years
now, XenServer has shipped the 2nd patch as a safety check (seeing as we have
both livepatching and introspection), implemented with some return address
manipulation to turn a void load hook into one which can return -EBUSY.

Andrew Cooper (2):
  xen/livepatch: Add a return value to load hooks
  x86/livepatch: Prevent patching with active waitqueues

 xen/arch/arm/livepatch.c             |  5 +++++
 xen/arch/x86/livepatch.c             | 39 ++++++++++++++++++++++++++++++++++++
 xen/common/livepatch.c               | 37 ++++++++++++++++++++++++----------
 xen/include/xen/livepatch.h          |  1 +
 xen/include/xen/livepatch_payload.h  |  2 +-
 xen/test/livepatch/xen_hello_world.c | 12 ++++++++---
 6 files changed, 81 insertions(+), 15 deletions(-)

-- 
2.11.0


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[Xen-devel] [PATCH 1/2] xen/livepatch: Add a return value to load hooks

[Andrew Cooper]

One use of load hooks is to perform a safety check of the system in its
quiesced state.  Any non-zero return value from a load hook will abort the
apply attempt.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Juergen Gross <jgross@suse.com>

For several years, the following patch in the series has been shipped in every
XenServer livepatch, implemented as a void load hook which modifies its return
address to skip to the end of apply_payload().  This method is distinctly less
hacky.
---
 xen/common/livepatch.c               | 30 +++++++++++++++++++-----------
 xen/include/xen/livepatch_payload.h  |  2 +-
 xen/test/livepatch/xen_hello_world.c | 12 +++++++++---
 3 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index 7caa30c202..962647616a 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -1076,25 +1076,33 @@ static int apply_payload(struct payload *data)
      * temporarily disable the spin locks IRQ state checks.
      */
     spin_debug_disable();
-    for ( i = 0; i < data->n_load_funcs; i++ )
-        data->load_funcs[i]();
+    for ( i = 0; !rc && i < data->n_load_funcs; i++ )
+        rc = data->load_funcs[i]();
     spin_debug_enable();
 
+    if ( rc )
+        printk(XENLOG_ERR LIVEPATCH "%s: load_funcs[%u] failed: %d\n",
+               data->name, i, rc);
+
     ASSERT(!local_irq_is_enabled());
 
-    for ( i = 0; i < data->nfuncs; i++ )
-        arch_livepatch_apply(&data->funcs[i]);
+    if ( !rc )
+        for ( i = 0; i < data->nfuncs; i++ )
+            arch_livepatch_apply(&data->funcs[i]);
 
     arch_livepatch_revive();
 
-    /*
-     * We need RCU variant (which has barriers) in case we crash here.
-     * The applied_list is iterated by the trap code.
-     */
-    list_add_tail_rcu(&data->applied_list, &applied_list);
-    register_virtual_region(&data->region);
+    if ( !rc )
+    {
+        /*
+         * We need RCU variant (which has barriers) in case we crash here.
+         * The applied_list is iterated by the trap code.
+         */
+        list_add_tail_rcu(&data->applied_list, &applied_list);
+        register_virtual_region(&data->region);
+    }
 
-    return 0;
+    return rc;
 }
 
 static int revert_payload(struct payload *data)
diff --git a/xen/include/xen/livepatch_payload.h b/xen/include/xen/livepatch_payload.h
index 4a1a96d054..3788b52ddf 100644
--- a/xen/include/xen/livepatch_payload.h
+++ b/xen/include/xen/livepatch_payload.h
@@ -9,7 +9,7 @@
  * The following definitions are to be used in patches. They are taken
  * from kpatch.
  */
-typedef void livepatch_loadcall_t(void);
+typedef int livepatch_loadcall_t(void);
 typedef void livepatch_unloadcall_t(void);
 
 /*
diff --git a/xen/test/livepatch/xen_hello_world.c b/xen/test/livepatch/xen_hello_world.c
index 02f3f85dc0..d720001f07 100644
--- a/xen/test/livepatch/xen_hello_world.c
+++ b/xen/test/livepatch/xen_hello_world.c
@@ -16,9 +16,11 @@ static const char hello_world_patch_this_fnc[] = "xen_extra_version";
 extern const char *xen_hello_world(void);
 static unsigned int cnt;
 
-static void apply_hook(void)
+static int apply_hook(void)
 {
     printk(KERN_DEBUG "Hook executing.\n");
+
+    return 0;
 }
 
 static void revert_hook(void)
@@ -26,10 +28,14 @@ static void revert_hook(void)
     printk(KERN_DEBUG "Hook unloaded.\n");
 }
 
-static void  hi_func(void)
+static int hi_func(void)
 {
     printk(KERN_DEBUG "%s: Hi! (called %u times)\n", __func__, ++cnt);
+
+    return 0;
 };
+/* Safe to cast away the return value for this trivial case. */
+void (void_hi_func)(void) __attribute__((alias("hi_func")));
 
 static void check_fnc(void)
 {
@@ -43,7 +49,7 @@ LIVEPATCH_UNLOAD_HOOK(revert_hook);
 /* Imbalance here. Two load and three unload. */
 
 LIVEPATCH_LOAD_HOOK(hi_func);
-LIVEPATCH_UNLOAD_HOOK(hi_func);
+LIVEPATCH_UNLOAD_HOOK(void_hi_func);
 
 LIVEPATCH_UNLOAD_HOOK(check_fnc);
 
-- 
2.11.0


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[Xen-devel] [PATCH 2/2] x86/livepatch: Prevent patching with active waitqueues

[Andrew Cooper]

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@citrix.com>
---
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Juergen Gross <jgross@suse.com>

This fix wants backporting, and is long overdue for posting upstream.
---
 xen/arch/arm/livepatch.c    |  5 +++++
 xen/arch/x86/livepatch.c    | 39 +++++++++++++++++++++++++++++++++++++++
 xen/common/livepatch.c      |  7 +++++++
 xen/include/xen/livepatch.h |  1 +
 4 files changed, 52 insertions(+)

diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
index 00c5e2bc45..915e9d926a 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 c82cf53b9e..0f129fa6b2 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -14,6 +14,45 @@
 #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 )
+    {
+#ifdef CONFIG_MEM_SHARING
+        if ( has_active_waitqueue(d->vm_event_share) )
+            goto fail;
+#endif
+#ifdef CONFIG_MEM_PAGING
+        if ( has_active_waitqueue(d->vm_event_paging) )
+            goto fail;
+#endif
+        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 962647616a..27ee5bdeb7 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -1060,6 +1060,13 @@ static int apply_payload(struct payload *data)
     unsigned int i;
     int rc;
 
+    rc = apply_safety_checks();
+    if ( rc )
+    {
+        printk(XENLOG_ERR LIVEPATCH "%s: Safety checks failed\n", data->name);
+        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);
 
-- 
2.11.0


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[Xen-devel] [PATCH v1.5] x86/livepatch: Prevent patching with active waitqueues

[Andrew Cooper]

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@citrix.com>
---
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Juergen Gross <jgross@suse.com>

This fix wants backporting, and is long overdue for posting upstream.

v1.5:
 * Send out a non-stale patch this time.
---
 xen/arch/arm/livepatch.c    |  5 +++++
 xen/arch/x86/livepatch.c    | 40 ++++++++++++++++++++++++++++++++++++++++
 xen/common/livepatch.c      |  8 ++++++++
 xen/include/xen/livepatch.h |  1 +
 4 files changed, 54 insertions(+)

diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
index 00c5e2bc45..915e9d926a 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 c82cf53b9e..2749cbc5cf 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -10,10 +10,50 @@
 #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 )
+    {
+#ifdef CONFIG_MEM_SHARING
+        if ( has_active_waitqueue(d->vm_event_share) )
+            goto fail;
+#endif
+#ifdef CONFIG_MEM_PAGING
+        if ( has_active_waitqueue(d->vm_event_paging) )
+            goto fail;
+#endif
+        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 962647616a..8386e611f2 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);
 
-- 
2.11.0


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Re: [Xen-devel] [PATCH 1/2] xen/livepatch: Add a return value to load hooks

[Konrad Rzeszutek Wilk]

On Tue, Nov 05, 2019 at 07:43:16PM +0000, Andrew Cooper wrote:
> One use of load hooks is to perform a safety check of the system in its
> quiesced state.  Any non-zero return value from a load hook will abort the
> apply attempt.
> 

Shouldn't you also update the documentation (design doc?)

Thanks.
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Ross Lagerwall <ross.lagerwall@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> 
> For several years, the following patch in the series has been shipped in every
> XenServer livepatch, implemented as a void load hook which modifies its return
> address to skip to the end of apply_payload().  This method is distinctly less
> hacky.
> ---
>  xen/common/livepatch.c               | 30 +++++++++++++++++++-----------
>  xen/include/xen/livepatch_payload.h  |  2 +-
>  xen/test/livepatch/xen_hello_world.c | 12 +++++++++---
>  3 files changed, 29 insertions(+), 15 deletions(-)
> 
> diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
> index 7caa30c202..962647616a 100644
> --- a/xen/common/livepatch.c
> +++ b/xen/common/livepatch.c
> @@ -1076,25 +1076,33 @@ static int apply_payload(struct payload *data)
>       * temporarily disable the spin locks IRQ state checks.
>       */
>      spin_debug_disable();
> -    for ( i = 0; i < data->n_load_funcs; i++ )
> -        data->load_funcs[i]();
> +    for ( i = 0; !rc && i < data->n_load_funcs; i++ )
> +        rc = data->load_funcs[i]();
>      spin_debug_enable();
>  
> +    if ( rc )
> +        printk(XENLOG_ERR LIVEPATCH "%s: load_funcs[%u] failed: %d\n",
> +               data->name, i, rc);
> +
>      ASSERT(!local_irq_is_enabled());
>  
> -    for ( i = 0; i < data->nfuncs; i++ )
> -        arch_livepatch_apply(&data->funcs[i]);
> +    if ( !rc )
> +        for ( i = 0; i < data->nfuncs; i++ )
> +            arch_livepatch_apply(&data->funcs[i]);
>  
>      arch_livepatch_revive();
>  
> -    /*
> -     * We need RCU variant (which has barriers) in case we crash here.
> -     * The applied_list is iterated by the trap code.
> -     */
> -    list_add_tail_rcu(&data->applied_list, &applied_list);
> -    register_virtual_region(&data->region);
> +    if ( !rc )
> +    {
> +        /*
> +         * We need RCU variant (which has barriers) in case we crash here.
> +         * The applied_list is iterated by the trap code.
> +         */
> +        list_add_tail_rcu(&data->applied_list, &applied_list);
> +        register_virtual_region(&data->region);
> +    }
>  
> -    return 0;
> +    return rc;
>  }
>  
>  static int revert_payload(struct payload *data)
> diff --git a/xen/include/xen/livepatch_payload.h b/xen/include/xen/livepatch_payload.h
> index 4a1a96d054..3788b52ddf 100644
> --- a/xen/include/xen/livepatch_payload.h
> +++ b/xen/include/xen/livepatch_payload.h
> @@ -9,7 +9,7 @@
>   * The following definitions are to be used in patches. They are taken
>   * from kpatch.
>   */
> -typedef void livepatch_loadcall_t(void);
> +typedef int livepatch_loadcall_t(void);
>  typedef void livepatch_unloadcall_t(void);
>  
>  /*
> diff --git a/xen/test/livepatch/xen_hello_world.c b/xen/test/livepatch/xen_hello_world.c
> index 02f3f85dc0..d720001f07 100644
> --- a/xen/test/livepatch/xen_hello_world.c
> +++ b/xen/test/livepatch/xen_hello_world.c
> @@ -16,9 +16,11 @@ static const char hello_world_patch_this_fnc[] = "xen_extra_version";
>  extern const char *xen_hello_world(void);
>  static unsigned int cnt;
>  
> -static void apply_hook(void)
> +static int apply_hook(void)
>  {
>      printk(KERN_DEBUG "Hook executing.\n");
> +
> +    return 0;
>  }
>  
>  static void revert_hook(void)
> @@ -26,10 +28,14 @@ static void revert_hook(void)
>      printk(KERN_DEBUG "Hook unloaded.\n");
>  }
>  
> -static void  hi_func(void)
> +static int hi_func(void)
>  {
>      printk(KERN_DEBUG "%s: Hi! (called %u times)\n", __func__, ++cnt);
> +
> +    return 0;
>  };
> +/* Safe to cast away the return value for this trivial case. */
> +void (void_hi_func)(void) __attribute__((alias("hi_func")));
>  
>  static void check_fnc(void)
>  {
> @@ -43,7 +49,7 @@ LIVEPATCH_UNLOAD_HOOK(revert_hook);
>  /* Imbalance here. Two load and three unload. */
>  
>  LIVEPATCH_LOAD_HOOK(hi_func);
> -LIVEPATCH_UNLOAD_HOOK(hi_func);
> +LIVEPATCH_UNLOAD_HOOK(void_hi_func);
>  
>  LIVEPATCH_UNLOAD_HOOK(check_fnc);
>  
> -- 
> 2.11.0
> 

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Re: [Xen-devel] [PATCH 2/2] x86/livepatch: Prevent patching with active waitqueues

[Konrad Rzeszutek Wilk]

On Tue, Nov 05, 2019 at 07:43:17PM +0000, Andrew Cooper wrote:
> 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@citrix.com>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

> CC: Ross Lagerwall <ross.lagerwall@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> 
> This fix wants backporting, and is long overdue for posting upstream.
> ---
>  xen/arch/arm/livepatch.c    |  5 +++++
>  xen/arch/x86/livepatch.c    | 39 +++++++++++++++++++++++++++++++++++++++
>  xen/common/livepatch.c      |  7 +++++++
>  xen/include/xen/livepatch.h |  1 +
>  4 files changed, 52 insertions(+)
> 
> diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
> index 00c5e2bc45..915e9d926a 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 c82cf53b9e..0f129fa6b2 100644
> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -14,6 +14,45 @@
>  #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 )
> +    {
> +#ifdef CONFIG_MEM_SHARING
> +        if ( has_active_waitqueue(d->vm_event_share) )
> +            goto fail;
> +#endif
> +#ifdef CONFIG_MEM_PAGING
> +        if ( has_active_waitqueue(d->vm_event_paging) )
> +            goto fail;
> +#endif
> +        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 962647616a..27ee5bdeb7 100644
> --- a/xen/common/livepatch.c
> +++ b/xen/common/livepatch.c
> @@ -1060,6 +1060,13 @@ static int apply_payload(struct payload *data)
>      unsigned int i;
>      int rc;
>  
> +    rc = apply_safety_checks();
> +    if ( rc )
> +    {
> +        printk(XENLOG_ERR LIVEPATCH "%s: Safety checks failed\n", data->name);
> +        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);
>  
> -- 
> 2.11.0
> 

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Re: [Xen-devel] [PATCH 1/2] xen/livepatch: Add a return value to load hooks

[Jan Beulich]

On 05.11.2019 20:43, Andrew Cooper wrote:
> --- a/xen/common/livepatch.c
> +++ b/xen/common/livepatch.c
> @@ -1076,25 +1076,33 @@ static int apply_payload(struct payload *data)
>       * temporarily disable the spin locks IRQ state checks.
>       */
>      spin_debug_disable();
> -    for ( i = 0; i < data->n_load_funcs; i++ )
> -        data->load_funcs[i]();
> +    for ( i = 0; !rc && i < data->n_load_funcs; i++ )
> +        rc = data->load_funcs[i]();
>      spin_debug_enable();
>  
> +    if ( rc )
> +        printk(XENLOG_ERR LIVEPATCH "%s: load_funcs[%u] failed: %d\n",
> +               data->name, i, rc);

Is there a possible problem here if some of the load_funcs()
succeeded before one fails? Or are those required to not do
any state change to the system (which would need rolling back)?

Jan

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Re: [Xen-devel] [PATCH 2/2] x86/livepatch: Prevent patching with active waitqueues

[Ross Lagerwall]

On 11/5/19 7:43 PM, Andrew Cooper wrote:
> 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@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

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Re: [Xen-devel] [PATCH 1/2] xen/livepatch: Add a return value to load hooks

[Ross Lagerwall]

On 11/5/19 7:43 PM, Andrew Cooper wrote:
> One use of load hooks is to perform a safety check of the system in its
> quiesced state.  Any non-zero return value from a load hook will abort the
> apply attempt.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

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Re: [Xen-devel] [PATCH v1.5] x86/livepatch: Prevent patching with active waitqueues

[Jürgen Groß]

On 05.11.19 20:49, Andrew Cooper wrote:
> 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@citrix.com>

Release-acked-by: Juergen Gross <jgross@suse.com>


Juergen

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Re: [Xen-devel] [PATCH v1.5] x86/livepatch: Prevent patching with active waitqueues

[Sarah Newman]

On 11/5/19 11:49 AM, Andrew Cooper wrote:
> 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@citrix.com>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Ross Lagerwall <ross.lagerwall@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> 
> This fix wants backporting, and is long overdue for posting upstream.
> 
> v1.5:
>   * Send out a non-stale patch this time.
> ---
>   xen/arch/arm/livepatch.c    |  5 +++++
>   xen/arch/x86/livepatch.c    | 40 ++++++++++++++++++++++++++++++++++++++++
>   xen/common/livepatch.c      |  8 ++++++++
>   xen/include/xen/livepatch.h |  1 +
>   4 files changed, 54 insertions(+)
> 
> diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
> index 00c5e2bc45..915e9d926a 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 c82cf53b9e..2749cbc5cf 100644
> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -10,10 +10,50 @@
>   #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 )
> +    {
> +#ifdef CONFIG_MEM_SHARING
> +        if ( has_active_waitqueue(d->vm_event_share) )
> +            goto fail;
> +#endif
> +#ifdef CONFIG_MEM_PAGING
> +        if ( has_active_waitqueue(d->vm_event_paging) )'

Is this supposed to be CONFIG_HAS_MEM_PAGING?

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Re: [Xen-devel] [PATCH v1.5] x86/livepatch: Prevent patching with active waitqueues

[Sarah Newman]

On 11/5/19 11:49 AM, Andrew Cooper wrote:
> 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@citrix.com>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Ross Lagerwall <ross.lagerwall@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> 
> This fix wants backporting, and is long overdue for posting upstream.
> 
> v1.5:
>   * Send out a non-stale patch this time.
> ---
>   xen/arch/arm/livepatch.c    |  5 +++++
>   xen/arch/x86/livepatch.c    | 40 ++++++++++++++++++++++++++++++++++++++++
>   xen/common/livepatch.c      |  8 ++++++++
>   xen/include/xen/livepatch.h |  1 +
>   4 files changed, 54 insertions(+)
> 
> diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
> index 00c5e2bc45..915e9d926a 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 c82cf53b9e..2749cbc5cf 100644
> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -10,10 +10,50 @@
>   #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 )
> +    {
> +#ifdef CONFIG_MEM_SHARING
> +        if ( has_active_waitqueue(d->vm_event_share) )
> +            goto fail;
> +#endif
> +#ifdef CONFIG_MEM_PAGING
> +        if ( has_active_waitqueue(d->vm_event_paging) )
> +            goto fail;
> +#endif
> +        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 962647616a..8386e611f2 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;
> +    }
> +

Would it make sense to call arch_livepatch_safety_check from
arch_livepatch_quiesce rather than directly, so that
arch_livepatch_safety_check is also called from revert_payload?

--Sarah


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