ARC: mm: do_page_fault fixes #1: relinquish mmap_sem if signal arrives while handle_m...

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

Cc: stable@vger.kernel.org
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index a1d7231..8df1638 100644 (file)
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -141,12 +141,17 @@ good_area:
         */
        fault = handle_mm_fault(vma, address, flags);
 
-       /* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
        if (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;
+               }
        }
 
        perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);