[PATCH 3.16 051/202] ARC: mm: do_page_fault fixes #1: relinquish mmap_sem if signal arrives while handle_mm_fault

[Ben Hutchings]

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Vineet Gupta <vgupta@xxxxxxxxxxxx>

commit 4d447455e73b47c43dd35fcc38ed823d3182a474 upstream.

do_page_fault() forgot to relinquish mmap_sem if a signal came while
handling handle_mm_fault() - due to say a ctl+c or oom etc.
This would later cause a deadlock by acquiring it twice.

This came to light when running libc testsuite tst-tls3-malloc test but
is likely also the cause for prior seen LTP failures. Using lockdep
clearly showed what the issue was.

| # while true; do ./tst-tls3-malloc ; done
| Didn't expect signal from child: got `Segmentation fault'
| ^C
| ============================================
| WARNING: possible recursive locking detected
| 4.17.0+ #25 Not tainted
| --------------------------------------------
| tst-tls3-malloc/510 is trying to acquire lock:
| 606c7728 (&mm->mmap_sem){++++}, at: __might_fault+0x28/0x5c
|
|but task is already holding lock:
|606c7728 (&mm->mmap_sem){++++}, at: do_page_fault+0x9c/0x2a0
|
| other info that might help us debug this:
|  Possible unsafe locking scenario:
|
|       CPU0
|       ----
|  lock(&mm->mmap_sem);
|  lock(&mm->mmap_sem);
|
| *** DEADLOCK ***
|

------------------------------------------------------------
What the change does is not obvious (note to myself)

prior code was

| do_page_fault
|
|   down_read()		<-- lock taken
|   handle_mm_fault	<-- signal pending as this runs
|   if fatal_signal_pending
|       if VM_FAULT_ERROR
|           up_read
|       if user_mode
|          return	<-- lock still held, this was the BUG

New code

| do_page_fault
|
|   down_read()		<-- lock taken
|   handle_mm_fault	<-- signal pending as this runs
|   if fatal_signal_pending
|       if VM_FAULT_RETRY
|          return       <-- not same case as above, but still OK since
|                           core mm already relinq lock for FAULT_RETRY
|    ...
|
|   < Now falls through for bug case above >
|
|   up_read()		<-- lock relinquished

Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/arc/mm/fault.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -131,12 +131,17 @@ good_area:
 	 */
 	fault = handle_mm_fault(mm, vma, address, flags);
 
-	/* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
 	if (unlikely(fatal_signal_pending(current))) {
-		if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY))
-			up_read(&mm->mmap_sem);
-		if (user_mode(regs))
+
+		/*
+		 * if fault retry, mmap_sem already relinquished by core mm
+		 * so OK to return to user mode (with signal handled first)
+		 */
+		if (fault & VM_FAULT_RETRY) {
+			if (!user_mode(regs))
+				goto no_context;
 			return;
+		}
 	}
 
 	if (likely(!(fault & VM_FAULT_ERROR))) {