[PATCH 1/1] Fix int1 recursion when no perf_bp_event is registered

[Jeff Merkey]

If an int1 hardware breakpoint exception is triggered, but no perf bp
pevent block was registered from arch_install_hw_breakpoint, the
system will hard hang with the CPU stuck constantly re-interrupting at
the same execution address because the resume flag never gets set, and
the NOTIFY_DONE state prevents other int1 handlers, including the
default handler in do_debug, from running to handle the condition.
Can be reproduced by writing a program that sets an execute breakpoint
at schedule() without calling arch_install_hw_breakpoint.

The proposed fix checks the dr7 register and sets the resume flag in
pt->regs if it determines an executed breakpoint was triggered just in
case the check lower down fails.  I have seen this bug and its a bug.

Signed-off-by:  jeffmerkey@xxxxxxxxx
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 50a3fad..6effcae 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -475,6 +475,14 @@ static int hw_breakpoint_handler(struct die_args *args)
 	for (i = 0; i < HBP_NUM; ++i) {
 		if (likely(!(dr6 & (DR_TRAP0 << i))))
 			continue;
+		/*
+		 * Set up resume flag to avoid breakpoint recursion when
+		 * returning back to origin in the event an int1
+		 * exception is triggered and no event handler
+		 * is present.
+		 */
+		if ((dr7 & (3 << ((i * 4) + 16))) == 0)
+			args->regs->flags |= X86_EFLAGS_RF;

 		/*
 		 * The counter may be concurrently released but that can only
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