[Xen-changelog] [xen stable-4.6] x86/traps: Fix handling of #DB exceptions in hypervisor context

[patchbot@xxxxxxx]

commit 3b9667632d52995bdf576e627bc8ea994d6941ee
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue May 8 18:28:03 2018 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue May 8 18:28:03 2018 +0100

    x86/traps: Fix handling of #DB exceptions in hypervisor context
    
    The WARN_ON() can be triggered by guest activities, and emits a full stack
    trace without rate limiting.  Swap it out for a ratelimited printk with just
    enough information to work out what is going on.
    
    Not all #DB exceptions are traps, so blindly continuing is not a safe action
    to take.  We don't let PV guests select these settings in the real %dr7 to
    begin with, but for added safety against unexpected situations, detect the
    fault cases and crash in an obvious manner.
    
    This is part of XSA-260 / CVE-2018-8897.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/traps.c | 44 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index b00149c756..eab5a3f23f 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -3716,16 +3716,44 @@ void do_debug(struct cpu_user_regs *regs)
                 regs->eflags &= ~X86_EFLAGS_TF;
             }
         }
-        else
+
+        /*
+         * Check for fault conditions.  General Detect, and instruction
+         * breakpoints are faults rather than traps, at which point attempting
+         * to ignore and continue will result in a livelock.
+         */
+        if ( dr6 & DR_GENERAL_DETECT )
         {
-            /*
-             * We ignore watchpoints when they trigger within Xen. This may
-             * happen when a buffer is passed to us which previously had a
-             * watchpoint set on it. No need to bump EIP; the only faulting
-             * trap is an instruction breakpoint, which can't happen to us.
-             */
-            WARN_ON(!search_exception_table(regs->eip));
+            printk(XENLOG_ERR "Hit General Detect in Xen context\n");
+            fatal_trap(regs);
+        }
+
+        if ( dr6 & (DR_TRAP3 | DR_TRAP2 | DR_TRAP1 | DR_TRAP0) )
+        {
+            unsigned int bp, dr7 = read_debugreg(7) >> DR_CONTROL_SHIFT;
+
+            for ( bp = 0; bp < 4; ++bp )
+            {
+                if ( (dr6 & (1u << bp)) && /* Breakpoint triggered? */
+                     ((dr7 & (3u << (bp * DR_CONTROL_SIZE))) == 0) /* Insn? */ 
)
+                {
+                    printk(XENLOG_ERR
+                           "Hit instruction breakpoint in Xen context\n");
+                    fatal_trap(regs);
+                }
+            }
         }
+
+        /*
+         * Whatever caused this #DB should be a trap.  Note it and continue.
+         * Guests can trigger this in certain corner cases, so ensure the
+         * message is ratelimited.
+         */
+        gprintk(XENLOG_WARNING,
+                "Hit #DB in Xen context: %04x:%p [%ps], stk %04x:%p, dr6 
%lx\n",
+                regs->cs, _p(regs->rip), _p(regs->rip),
+                regs->ss, _p(regs->rsp), dr6);
+
         goto out;
     }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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