[Xen-devel] Re: [PATCH 5/6] trace: fix security issues

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 01.07.10 at 12:15, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Thu, Jul 1, 2010 at 9:49 AM, Jan Beulich <JBeulich@xxxxxxxxxx> wrote:
>> Ordering isn't a problem here, but enforcing the read-once
>> requirement can be done either way afaict.
> 
> I see.  Yeah, I agree with Keir; memory barriers are the right way to
> solve this problem.
> 
> Can you work up a patch add in the appropriate memory barriers?

Here we go - I suppose the patch will apply cleanly on top of your
queue.

Subject: trace: insert compiler memory barriers

This is to ensure fields shared writably with Dom0 get read only once
for any consistency checking followed by actual calculations.

I realized there was another multiple-read issue, a fix for which is
also included (which at once simplifies __insert_record()).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/xen/common/trace.c        2010-07-01 10:05:54.000000000 +0200
+++ b/xen/common/trace.c        2010-07-01 14:01:47.000000000 +0200
@@ -437,11 +437,13 @@ static inline bool_t bogus(u32 prod, u32
 static inline u32 calc_unconsumed_bytes(const struct t_buf *buf)
 {
     u32 prod = buf->prod, cons = buf->cons;
-    s32 x = prod - cons;
+    s32 x;
 
+    barrier(); /* must read buf->prod and buf->cons only once */
     if ( bogus(prod, cons) )
         return data_size;
 
+    x = prod - cons;
     if ( x < 0 )
         x += 2*data_size;
 
@@ -453,12 +455,14 @@ static inline u32 calc_unconsumed_bytes(
 
 static inline u32 calc_bytes_to_wrap(const struct t_buf *buf)
 {
-    u32 prod = buf->prod;
-    s32 x = data_size - prod;
+    u32 prod = buf->prod, cons = buf->cons;
+    s32 x;
 
-    if ( bogus(prod, buf->cons) )
+    barrier(); /* must read buf->prod and buf->cons only once */
+    if ( bogus(prod, cons) )
         return 0;
 
+    x = data_size - prod;
     if ( x <= 0 )
         x += data_size;
 
@@ -473,11 +477,14 @@ static inline u32 calc_bytes_avail(const
     return data_size - calc_unconsumed_bytes(buf);
 }
 
-static inline struct t_rec *next_record(const struct t_buf *buf)
+static inline struct t_rec *next_record(const struct t_buf *buf,
+                                        uint32_t *next)
 {
-    u32 x = buf->prod;
+    u32 x = buf->prod, cons = buf->cons;
 
-    if ( !tb_init_done || bogus(x, buf->cons) )
+    barrier(); /* must read buf->prod and buf->cons only once */
+    *next = x;
+    if ( !tb_init_done || bogus(x, cons) )
         return NULL;
 
     if ( x >= data_size )
@@ -504,23 +511,21 @@ static inline void __insert_record(volat
     BUG_ON(local_rec_size != rec_size);
     BUG_ON(extra & 3);
 
+    rec = next_record(buf, &next);
+    if ( !rec )
+        return;
     /* Double-check once more that we have enough space.
      * Don't bugcheck here, in case the userland tool is doing
      * something stupid. */
-    next = calc_bytes_avail(buf);
-    if ( next < rec_size )
+    if ( (unsigned char *)rec + rec_size > this_cpu(t_data) + data_size )
     {
         if ( printk_ratelimit() )
             printk(XENLOG_WARNING
-                   "%s: avail=%u (size=%08x prod=%08x cons=%08x) rec=%u\n",
-                   __func__, next, data_size, buf->prod, buf->cons, rec_size);
+                   "%s: size=%08x prod=%08x cons=%08x rec=%u\n",
+                   __func__, data_size, next, buf->cons, rec_size);
         return;
     }
-    rmb();
 
-    rec = next_record(buf);
-    if ( !rec )
-        return;
     rec->event = event;
     rec->extra_u32 = extra_word;
     dst = (unsigned char *)rec->u.nocycles.extra_u32;
@@ -537,9 +542,6 @@ static inline void __insert_record(volat
 
     wmb();
 
-    next = buf->prod;
-    if ( bogus(next, buf->cons) )
-        return;
     next += rec_size;
     if ( next >= 2*data_size )
         next -= 2*data_size;

Attachment: trace-barriers.patch
Description: Text document

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