• Mark Rutland's avatar
    kcov: ensure irq code sees a valid area · a45f5ee6
    Mark Rutland authored
    [ Upstream commit c9484b986ef03492357fddd50afbdd02929cfa72 ]
    
    Patch series "kcov: fix unexpected faults".
    
    These patches fix a few issues where KCOV code could trigger recursive
    faults, discovered while debugging a patch enabling KCOV for arch/arm:
    
    * On CONFIG_PREEMPT kernels, there's a small race window where
      __sanitizer_cov_trace_pc() can see a bogus kcov_area.
    
    * Lazy faulting of the vmalloc area can cause mutual recursion between
      fault handling code and __sanitizer_cov_trace_pc().
    
    * During the context switch, switching the mm can cause the kcov_area to
      be transiently unmapped.
    
    These are prerequisites for enabling KCOV on arm, but the issues
    themsevles are generic -- we just happen to avoid them by chance rather
    than design on x86-64 and arm64.
    
    This patch (of 3):
    
    For kernels built with CONFIG_PREEMPT, some C code may execute before or
    after the interrupt handler, while the hardirq count is zero.  In these
    cases, in_task() can return true.
    
    A task can be interrupted in the middle of a KCOV_DISABLE ioctl while it
    resets the task's kcov data via kcov_task_init().  Instrumented code
    executed during this period will call __sanitizer_cov_trace_pc(), and as
    in_task() returns true, will inspect t->kcov_mode before trying to write
    to t->kcov_area.
    
    In kcov_init_task() we update t->kcov_{mode,area,size} with plain stores,
    which may be re-ordered, torn, etc.  Thus __sanitizer_cov_trace_pc() may
    see bogus values for any of these fields, and may attempt to write to
    memory which is not mapped.
    
    Let's avoid this by using WRITE_ONCE() to set t->kcov_mode, with a
    barrier() to ensure this is ordered before we clear t->kov_{area,size}.
    This ensures that any code execute while kcov_init_task() is preempted
    will either see valid values for t->kcov_{area,size}, or will see that
    t->kcov_mode is KCOV_MODE_DISABLED, and bail out without touching
    t->kcov_area.
    
    Link: http://lkml.kernel.org/r/20180504135535.53744-2-mark.rutland@arm.comSigned-off-by: 's avatarMark Rutland <mark.rutland@arm.com>
    Acked-by: 's avatarAndrey Ryabinin <aryabinin@virtuozzo.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: 's avatarSasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    a45f5ee6