• Steven Rostedt's avatar
    x86: Add counter when debug stack is used with interrupts enabled · 42181186
    Steven Rostedt authored
    Mathieu Desnoyers pointed out a case that can cause issues with
    NMIs running on the debug stack:
    
      int3 -> interrupt -> NMI -> int3
    
    Because the interrupt changes the stack, the NMI will not see that
    it preempted the debug stack. Looking deeper at this case,
    interrupts only happen when the int3 is from userspace or in
    an a location in the exception table (fixup).
    
      userspace -> int3 -> interurpt -> NMI -> int3
    
    All other int3s that happen in the kernel should be processed
    without ever enabling interrupts, as the do_trap() call will
    panic the kernel if it is called to process any other location
    within the kernel.
    
    Adding a counter around the sections that enable interrupts while
    using the debug stack allows the NMI to also check that case.
    If the NMI sees that it either interrupted a task using the debug
    stack or the debug counter is non-zero, then it will have to
    change the IDT table to make the int3 not change stacks (which will
    corrupt the stack if it does).
    
    Note, I had to move the debug_usage functions out of processor.h
    and into debugreg.h because of the static inlined functions to
    inc and dec the debug_usage counter. __get_cpu_var() requires
    smp.h which includes processor.h, and would fail to build.
    
    Link: http://lkml.kernel.org/r/1323976535.23971.112.camel@gandalf.stny.rr.comReported-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: H. Peter Anvin <hpa@linux.intel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Paul Turner <pjt@google.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    42181186
debugreg.h 4.34 KB