The patch below does not apply to the 4.20-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4d447455e73b47c43dd35fcc38ed823d3182a474 Mon Sep 17 00:00:00 2001

From: Vineet Gupta vgupta@synopsys.com Date: Mon, 10 Dec 2018 16:56:45 -0800 Subject: [PATCH] ARC: mm: do_page_fault fixes #1: relinquish mmap_sem if signal arrives while handle_mm_fault

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 a1d723197084..8df1638259f3 100644 --- a/arch/arc/mm/fault.c +++ b/arch/arc/mm/fault.c @@ -141,12 +141,17 @@ 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 (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);