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Subject[PATCH AUTOSEL 4.19 050/167] ARC: mm: do_page_fault fixes #1: relinquish mmap_sem if signal arrives while handle_mm_fault
Date
From: Vineet Gupta <vgupta@synopsys.com>

[ Upstream commit 4d447455e73b47c43dd35fcc38ed823d3182a474 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arc/mm/fault.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index db6913094be3c..f28db0b112a30 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -143,12 +143,17 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
*/
fault = handle_mm_fault(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;
+ }
}

perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
--
2.20.1
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