From: Jan Beulich <jbeulich@suse.com>
Subject: xenoprof: limit consumption of shared buffer data

Since a shared buffer can be written to by the guest, we may only read
the head and tail pointers from there (all other fields should only ever
be written to). Furthermore, for any particular operation the two values
must be read exactly once, with both checks and consumption happening
with the thus read values. (The backtrace related xenoprof_buf_space()
use in xenoprof_log_event() is an exception: The values used there get
re-checked by every subsequent xenoprof_add_sample().)

Since that code needed touching, also fix the double increment of the
lost samples count in case the backtrace related xenoprof_add_sample()
invocation in xenoprof_log_event() fails.

Where code is being touched anyway, add const as appropriate, but take
the opportunity to entirely drop the now unused domain parameter of
xenoprof_buf_space().

This is part of XSA-313.


Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>


--- a/xen/include/xen/lib.h
+++ b/xen/include/xen/lib.h
@@ -58,6 +58,11 @@ do {                                    
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]) + __must_be_array(x))
 
+#define __ACCESS_ONCE(x) ({                             \
+            (void)(typeof(x))0; /* Scalar typecheck. */ \
+            (volatile typeof(x) *)&(x); })
+#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
+
 #define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m)))
 #define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m))
 
--- a/xen/common/xenoprof.c
+++ b/xen/common/xenoprof.c
@@ -471,25 +471,22 @@ static int add_passive_list(XEN_GUEST_HA
 
 
 /* Get space in the buffer */
-static int xenoprof_buf_space(struct domain *d, xenoprof_buf_t * buf, int size)
+static int xenoprof_buf_space(int head, int tail, int size)
 {
-    int head, tail;
-
-    head = xenoprof_buf(d, buf, event_head);
-    tail = xenoprof_buf(d, buf, event_tail);
-
     return ((tail > head) ? 0 : size) + tail - head - 1;
 }
 
 /* Check for space and add a sample. Return 1 if successful, 0 otherwise. */
-static int xenoprof_add_sample(struct domain *d, xenoprof_buf_t *buf,
+static int xenoprof_add_sample(const struct domain *d,
+                               const struct xenoprof_vcpu *v,
                                uint64_t eip, int mode, int event)
 {
+    xenoprof_buf_t *buf = v->buffer;
     int head, tail, size;
 
     head = xenoprof_buf(d, buf, event_head);
     tail = xenoprof_buf(d, buf, event_tail);
-    size = xenoprof_buf(d, buf, event_size);
+    size = v->event_size;
     
     /* make sure indexes in shared buffer are sane */
     if ( (head < 0) || (head >= size) || (tail < 0) || (tail >= size) )
@@ -498,7 +495,7 @@ static int xenoprof_add_sample(struct do
         return 0;
     }
 
-    if ( xenoprof_buf_space(d, buf, size) > 0 )
+    if ( xenoprof_buf_space(head, tail, size) > 0 )
     {
         xenoprof_buf(d, buf, event_log[head].eip) = eip;
         xenoprof_buf(d, buf, event_log[head].mode) = mode;
@@ -522,7 +519,6 @@ static int xenoprof_add_sample(struct do
 int xenoprof_add_trace(struct vcpu *vcpu, uint64_t pc, int mode)
 {
     struct domain *d = vcpu->domain;
-    xenoprof_buf_t *buf = d->xenoprof->vcpu[vcpu->vcpu_id].buffer;
 
     /* Do not accidentally write an escape code due to a broken frame. */
     if ( pc == XENOPROF_ESCAPE_CODE )
@@ -531,7 +527,8 @@ int xenoprof_add_trace(struct vcpu *vcpu
         return 0;
     }
 
-    return xenoprof_add_sample(d, buf, pc, mode, 0);
+    return xenoprof_add_sample(d, &d->xenoprof->vcpu[vcpu->vcpu_id],
+                               pc, mode, 0);
 }
 
 void xenoprof_log_event(struct vcpu *vcpu, const struct cpu_user_regs *regs,
@@ -562,17 +559,22 @@ void xenoprof_log_event(struct vcpu *vcp
     /* Provide backtrace if requested. */
     if ( backtrace_depth > 0 )
     {
-        if ( (xenoprof_buf_space(d, buf, v->event_size) < 2) ||
-             !xenoprof_add_sample(d, buf, XENOPROF_ESCAPE_CODE, mode, 
-                                  XENOPROF_TRACE_BEGIN) )
+        if ( xenoprof_buf_space(xenoprof_buf(d, buf, event_head),
+                                xenoprof_buf(d, buf, event_tail),
+                                v->event_size) < 2 )
         {
             xenoprof_buf(d, buf, lost_samples)++;
             lost_samples++;
             return;
         }
+
+        /* xenoprof_add_sample() will increment lost_samples on failure */
+        if ( !xenoprof_add_sample(d, v, XENOPROF_ESCAPE_CODE, mode,
+                                  XENOPROF_TRACE_BEGIN) )
+            return;
     }
 
-    if ( xenoprof_add_sample(d, buf, pc, mode, event) )
+    if ( xenoprof_add_sample(d, v, pc, mode, event) )
     {
         if ( is_active(vcpu->domain) )
             active_samples++;
--- a/xen/include/xen/xenoprof.h
+++ b/xen/include/xen/xenoprof.h
@@ -55,12 +55,12 @@ struct xenoprof {
 
 #ifndef CONFIG_COMPAT
 #define XENOPROF_COMPAT(x) 0
-#define xenoprof_buf(d, b, field) ((b)->field)
+#define xenoprof_buf(d, b, field) ACCESS_ONCE((b)->field)
 #else
 #define XENOPROF_COMPAT(x) ((x)->is_compat)
-#define xenoprof_buf(d, b, field) (*(!(d)->xenoprof->is_compat ? \
-                                       &(b)->native.field : \
-                                       &(b)->compat.field))
+#define xenoprof_buf(d, b, field) ACCESS_ONCE(*(!(d)->xenoprof->is_compat \
+                                                ? &(b)->native.field \
+                                                : &(b)->compat.field))
 #endif
 
 struct domain;
