[PATCH] SUNRPC: xprt: prevent shift-out-of-bounds

[Randy Dunlap]

Prevent a UBSAN shift-out-of-bounds warning:

UBSAN: shift-out-of-bounds in net/sunrpc/xprt.c:658:14
shift exponent 536871232 is too large for 64-bit type 'long unsigned int'

If to_exponential is true and to_retries is >= BITS_PER_LONG,
this will cause an invalid shift value when calculating the
new majortimeo value.
In this case, when to_retries >= BITS_PER_LONG, cap (new local) retry
at BITS_PER_LONG - 1 and continue.

Fixes: da953063bdce ("SUNRPC: Start the first major timeout calculation at task 
creation")
Link: https://syzkaller.appspot.com/bug?extid=ba2e91df8f74809417fa
Signed-off-by: Randy Dunlap <rdun...@infradead.org>
Reported-by: syzbot+ba2e91df8f7480941...@syzkaller.appspotmail.com
Cc: "J. Bruce Fields" <bfie...@fieldses.org>
Cc: Chuck Lever <chuck.le...@oracle.com>
Cc: linux-...@vger.kernel.org
Cc: Trond Myklebust <trond.mykleb...@hammerspace.com>
Cc: Anna Schumaker <anna.schuma...@netapp.com>
---
a shortcut if desired: (also drop local 'retry' & change it back to
                                'to->to_retries')
                if (to->to_retries >= BITS_PER_LONG)
                        goto set_maxval;
...
set_maxval:
                majortimeo = to->to_maxval;

 net/sunrpc/xprt.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- lnx-510.orig/net/sunrpc/xprt.c
+++ lnx-510/net/sunrpc/xprt.c
@@ -41,6 +41,7 @@
 #include <linux/module.h>
 
 #include <linux/types.h>
+#include <linux/bits.h>
 #include <linux/interrupt.h>
 #include <linux/workqueue.h>
 #include <linux/net.h>
@@ -593,10 +594,15 @@ static unsigned long xprt_calc_majortime
        const struct rpc_timeout *to = req->rq_task->tk_client->cl_timeout;
        unsigned long majortimeo = req->rq_timeout;
 
-       if (to->to_exponential)
-               majortimeo <<= to->to_retries;
-       else
+       if (to->to_exponential) {
+               unsigned int retry = to->to_retries;
+
+               if (retry >= BITS_PER_LONG)
+                       retry = BITS_PER_LONG - 1;
+               majortimeo <<= retry;
+       } else {
                majortimeo += to->to_increment * to->to_retries;
+       }
        if (majortimeo > to->to_maxval || majortimeo == 0)
                majortimeo = to->to_maxval;
        return majortimeo;