• Russell King's avatar
    ARM: vfp: fix a hole in VFP thread migration · f8f2a852
    Russell King authored
    Fix a hole in the VFP thread migration.  Lets define two threads.
    
    Thread 1, we'll call 'interesting_thread' which is a thread which is
    running on CPU0, using VFP (so vfp_current_hw_state[0] =
    &interesting_thread->vfpstate) and gets migrated off to CPU1, where
    it continues execution of VFP instructions.
    
    Thread 2, we'll call 'new_cpu0_thread' which is the thread which takes
    over on CPU0.  This has also been using VFP, and last used VFP on CPU0,
    but doesn't use it again.
    
    The following code will be executed twice:
    
    		cpu = thread->cpu;
    
    		/*
    		 * On SMP, if VFP is enabled, save the old state in
    		 * case the thread migrates to a different CPU. The
    		 * restoring is done lazily.
    		 */
    		if ((fpexc & FPEXC_EN) && vfp_current_hw_state[cpu]) {
    			vfp_save_state(vfp_current_hw_state[cpu], fpexc);
    			vfp_current_hw_state[cpu]->hard.cpu = cpu;
    		}
    		/*
    		 * Thread migration, just force the reloading of the
    		 * state on the new CPU in case the VFP registers
    		 * contain stale data.
    		 */
    		if (thread->vfpstate.hard.cpu != cpu)
    			vfp_current_hw_state[cpu] = NULL;
    
    The first execution will be on CPU0 to switch away from 'interesting_thread'.
    interesting_thread->cpu will be 0.
    
    So, vfp_current_hw_state[0] points at interesting_thread->vfpstate.
    The hardware state will be saved, along with the CPU number (0) that
    it was executing on.
    
    'thread' will be 'new_cpu0_thread' with new_cpu0_thread->cpu = 0.
    Also, because it was executing on CPU0, new_cpu0_thread->vfpstate.hard.cpu = 0,
    and so the thread migration check is not triggered.
    
    This means that vfp_current_hw_state[0] remains pointing at interesting_thread.
    
    The second execution will be on CPU1 to switch _to_ 'interesting_thread'.
    So, 'thread' will be 'interesting_thread' and interesting_thread->cpu now
    will be 1.  The previous thread executing on CPU1 is not relevant to this
    so we shall ignore that.
    
    We get to the thread migration check.  Here, we discover that
    interesting_thread->vfpstate.hard.cpu = 0, yet interesting_thread->cpu is
    now 1, indicating thread migration.  We set vfp_current_hw_state[1] to
    NULL.
    
    So, at this point vfp_current_hw_state[] contains the following:
    
    [0] = &interesting_thread->vfpstate
    [1] = NULL
    
    Our interesting thread now executes a VFP instruction, takes a fault
    which loads the state into the VFP hardware.  Now, through the assembly
    we now have:
    
    [0] = &interesting_thread->vfpstate
    [1] = &interesting_thread->vfpstate
    
    CPU1 stops due to ptrace (and so saves its VFP state) using the thread
    switch code above), and CPU0 calls vfp_sync_hwstate().
    
    	if (vfp_current_hw_state[cpu] == &thread->vfpstate) {
    		vfp_save_state(&thread->vfpstate, fpexc | FPEXC_EN);
    
    BANG, we corrupt interesting_thread's VFP state by overwriting the
    more up-to-date state saved by CPU1 with the old VFP state from CPU0.
    
    Fix this by ensuring that we have sane semantics for the various state
    describing variables:
    
    1. vfp_current_hw_state[] points to the current owner of the context
       information stored in each CPUs hardware, or NULL if that state
       information is invalid.
    2. thread->vfpstate.hard.cpu always contains the most recent CPU number
       which the state was loaded into or NR_CPUS if no CPU owns the state.
    
    So, for a particular CPU to be a valid owner of the VFP state for a
    particular thread t, two things must be true:
    
     vfp_current_hw_state[cpu] == &t->vfpstate && t->vfpstate.hard.cpu == cpu.
    
    and that is valid from the moment a CPU loads the saved VFP context
    into the hardware.  This gives clear and consistent semantics to
    interpreting these variables.
    
    This patch also fixes thread copying, ensuring that t->vfpstate.hard.cpu
    is invalidated, otherwise CPU0 may believe it was the last owner.  The
    hole can happen thus:
    
    - thread1 runs on CPU2 using VFP, migrates to CPU3, exits and thread_info
      freed.
    - New thread allocated from a previously running thread on CPU2, reusing
      memory for thread1 and copying vfp.hard.cpu.
    
    At this point, the following are true:
    
    	new_thread1->vfpstate.hard.cpu == 2
    	&new_thread1->vfpstate == vfp_current_hw_state[2]
    
    Lastly, this also addresses thread flushing in a similar way to thread
    copying.  Hole is:
    
    - thread runs on CPU0, using VFP, migrates to CPU1 but does not use VFP.
    - thread calls execve(), so thread flush happens, leaving
      vfp_current_hw_state[0] intact.  This vfpstate is memset to 0 causing
      thread->vfpstate.hard.cpu = 0.
    - thread migrates back to CPU0 before using VFP.
    
    At this point, the following are true:
    
    	thread->vfpstate.hard.cpu == 0
    	&thread->vfpstate == vfp_current_hw_state[0]
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    f8f2a852
vfphw.S 8.24 KB