[PATCH 1/6] xen/trace: Don't over-read trace objects

[Andrew Cooper <andrew.cooper3@citrix.com>]

In the case that 'extra' isn't a multiple of uint32_t, the calculation rounds
the number of bytes up, causing later logic to read unrelated bytes beyond the
end of the object.

Also, asserting that the object is within TRACE_EXTRA_MAX, but truncating it
in release builds is rude.

Instead, reject any out-of-spec records, leaving enough of a message to
identify the faulty caller.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: George Dunlap <George.Dunlap@eu.citrix.com>
CC: Ian Jackson <iwj@xenproject.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Wei Liu <wl@xen.org>
CC: Julien Grall <julien@xen.org>
CC: Dario Faggioli <dfaggioli@suse.com>

I've eyeballed the code and can't spot any problematic callers, but I came
very close to accidentally introducing some when trying to fix the stack
rubble leaks in subsequent patches.
---
 xen/common/trace.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/xen/common/trace.c b/xen/common/trace.c
index a2a389a1c7c3..25af6e1bd25e 100644
--- a/xen/common/trace.c
+++ b/xen/common/trace.c
@@ -686,22 +686,21 @@ void __trace_var(u32 event, bool_t cycles, unsigned int extra,
     unsigned long flags;
     u32 bytes_to_tail, bytes_to_wrap;
     unsigned int rec_size, total_size;
-    unsigned int extra_word;
     bool_t started_below_highwater;
 
     if( !tb_init_done )
         return;
 
-    /* Convert byte count into word count, rounding up */
-    extra_word = (extra / sizeof(u32));
-    if ( (extra % sizeof(u32)) != 0 )
-        extra_word++;
-    
-    ASSERT(extra_word <= TRACE_EXTRA_MAX);
-    extra_word = min_t(int, extra_word, TRACE_EXTRA_MAX);
-
-    /* Round size up to nearest word */
-    extra = extra_word * sizeof(u32);
+    /*
+     * Trace records require extra data which is an exact multiple of
+     * uint32_t.  Reject out-of-spec records.  Any failure here is an error in
+     * the caller.
+     */
+    if ( extra % sizeof(uint32_t) ||
+         extra / sizeof(uint32_t) > TRACE_EXTRA_MAX )
+        return printk_once(XENLOG_WARNING
+                           "Trace event %#x bad size %u, discarding\n",
+                           event, extra);
 
     if ( (tb_event_mask & event) == 0 )
         return;
-- 
2.11.0