[Devel] [PATCH rh7] x86, mm, perf: Allow recursive faults from interrupts

[Andrey Ryabinin]

From: Peter Zijlstra <peterz at infradead.org>

Waiman managed to trigger a PMI while in a emulate_vsyscall() fault,
the PMI in turn managed to trigger a fault while obtaining a stack
trace. This triggered the sig_on_uaccess_error recursive fault logic
and killed the process dead.

Fix this by explicitly excluding interrupts from the recursive fault
logic.

Reported-and-Tested-by: Waiman Long <waiman.long at hp.com>
Fixes: e00b12e64be9 ("perf/x86: Further optimize copy_from_user_nmi()")
Cc: Aswin Chandramouleeswaran <aswin at hp.com>
Cc: Scott J Norton <scott.norton at hp.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Andy Lutomirski <luto at amacapital.net>
Cc: Arnaldo Carvalho de Melo <acme at ghostprotocols.net>
Cc: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
Link: http://lkml.kernel.org/r/20140110200603.GJ7572@laptop.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo at kernel.org>
(cherry picked from commit c026b3591e4f2a4993df773183704bb31634e0bd)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 arch/x86/mm/fault.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 0e41582126a9..5a8f2cf83164 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -687,6 +687,20 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 
 	/* Are we prepared to handle this kernel fault? */
 	if (fixup_exception(regs)) {
+		/*
+		 * Any interrupt that takes a fault gets the fixup. This makes
+		 * the below recursive fault logic only apply to a faults from
+		 * task context.
+		 */
+		if (in_interrupt())
+			return;
+
+		/*
+		 * Per the above we're !in_interrupt(), aka. task context.
+		 *
+		 * In this case we need to make sure we're not recursively
+		 * faulting through the emulate_vsyscall() logic.
+		 */
 		if (current_thread_info()->sig_on_uaccess_error && signal) {
 			tsk->thread.trap_nr = X86_TRAP_PF;
 			tsk->thread.error_code = error_code | X86_PF_USER;
@@ -696,6 +710,10 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 			force_sig_info_fault(signal, si_code, address,
 					     tsk, vma, 0);
 		}
+
+		/*
+		 * Barring that, we can do the fixup and be happy.
+		 */
 		return;
 	}
 
-- 
2.19.2