[xen stable-4.12] xen/common: event_channel: Don't ignore error in get_free_port()

[patchbot@xxxxxxx]

commit a634229ecf67e3e06c5dc26701d0561c4e4698d7
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Tue Jul 7 15:08:59 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jul 7 15:08:59 2020 +0200

    xen/common: event_channel: Don't ignore error in get_free_port()
    
    Currently, get_free_port() is assuming that the port has been allocated
    when evtchn_allocate_port() is not return -EBUSY.
    
    However, the function may return an error when:
        - We exhausted all the event channels. This can happen if the limit
        configured by the administrator for the guest ('max_event_channels'
        in xl cfg) is higher than the ABI used by the guest. For instance,
        if the guest is using 2L, the limit should not be higher than 4095.
        - We cannot allocate memory (e.g Xen has not more memory).
    
    Users of get_free_port() (such as EVTCHNOP_alloc_unbound) will validly
    assuming the port was valid and will next call evtchn_from_port(). This
    will result to a crash as the memory backing the event channel structure
    is not present.
    
    Fixes: 368ae9a05fe ("xen/pvshim: forward evtchn ops between L0 Xen and L2 
DomU")
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 2e9c2bc292231823a3a021d2e0a9f1956bf00b3c
    master date: 2020-07-07 14:35:36 +0200
---
 xen/common/event_channel.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index e86e2bfab0..a8d182b584 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -195,10 +195,10 @@ static int get_free_port(struct domain *d)
     {
         int rc = evtchn_allocate_port(d, port);
 
-        if ( rc == -EBUSY )
-            continue;
-
-        return port;
+        if ( rc == 0 )
+            return port;
+        else if ( rc != -EBUSY )
+            return rc;
     }
 
     return -ENOSPC;
--
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