Re: [PATCH] powerpc/mm: Fix missing mmap_sem release

[Davidlohr Bueso]

On Thu, 07 Sep 2017, Laurent Dufour wrote:

The commit b5c8f0fd595d ("powerpc/mm: Rework mm_fault_error()") reviewed
the way the error path is managed in __do_page_fault() but it was a bit too
agressive when handling a case by returning without releasing the mmap_sem.

By the way, replacing current->mm->mmap_sem by mm->mmap_sem as mm is set to
current->mm.

Fixes: b5c8f0fd595d ("powerpc/mm: Rework mm_fault_error()")
Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org>
Signed-off-by: Laurent Dufour <laurent....@free.fr>
---
arch/powerpc/mm/fault.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 4797d08581ce..f799ccf37d27 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c

But... here:

        /*
         * If we need to retry the mmap_sem has already been released,
         * and if there is a fatal signal pending there is no guarantee
         * that we made any progress. Handle this case first.
         */

@@ -521,10 +521,11 @@ static int __do_page_fault(struct pt_regs *regs, unsigned 
long address,
                 * User mode? Just return to handle the fatal exception 
otherwise
                 * return to bad_page_fault
                 */
+               up_read(&mm->mmap_sem);
                return is_user ? 0 : SIGBUS;
        }

Per the above comment, for that case handle_mm_fault()
has already released mmap_sem. The same occurs in x86,
for example.

Thanks,
Davidlohr