[PATCH RFC] mm: arc: fix potential double realease of mmap_sem

[Peter Xu <peterx@redhat.com>]

In do_page_fault() of ARC we have:

        ...
	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);     <---------------- [1]
                if (user_mode(regs))
                        return;
        }
        ...
	if (likely(!(fault & VM_FAULT_ERROR))) {
                ...
		return;
	}

	if (fault & VM_FAULT_OOM)
		goto out_of_memory;                <----------------- [2]
	else if (fault & VM_FAULT_SIGSEGV)
		goto bad_area;                     <----------------- [3]
	else if (fault & VM_FAULT_SIGBUS)
		goto do_sigbus;                    <----------------- [4]

Logically it's possible that we might try to release the mmap_sem twice
by having a scenario like:

        - task received SIGKILL,
        - task handled kernel mode page fault,
        - handle_mm_fault() returned with one of VM_FAULT_ERROR,

Then we'll go into path [1] to release the mmap_sem, however we won't
return immediately since user_mode(regs) check will fail (a kernel page
fault).  Then we might go into either [2]-[4] and either of them will
try to release the mmap_sem again.

To fix this, we only release the mmap_sem at [1] when we're sure we'll
quit immediately (after we checked with user_mode(regs)).

CC: Vineet Gupta <vgupta@synopsys.com>
CC: "Eric W. Biederman" <ebiederm@xmission.com>
CC: Peter Xu <peterx@redhat.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Souptick Joarder <jrdr.linux@gmail.com>
CC: Andrea Arcangeli <aarcange@redhat.com>
CC: linux-snps-arc@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Peter Xu <peterx@redhat.com>
---

I noticed this only by reading the code.  Neither have I verified the
issue, nor have I tested the patch since I even don't know how to (I'm
totally unfamiliar with the arc architecture).  However I'm posting this
out first to see whether there's any quick feedback, and in case it's a
valid issue that we've ignored.
---
 arch/arc/mm/fault.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index c9da6102eb4f..2d28c3dad5c1 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -142,11 +142,10 @@ 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))
+	if (unlikely(fatal_signal_pending(current) && user_mode(regs))) {
+		if (!(fault & VM_FAULT_RETRY))
 			up_read(&mm->mmap_sem);
-		if (user_mode(regs))
-			return;
+		return;
 	}
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
-- 
2.17.1