Patch "m68k: Fix spinlock race in kernel thread creation" has been added to the 4.19-stable tree

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This is a note to let you know that I've just added the patch titled

    m68k: Fix spinlock race in kernel thread creation

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     m68k-fix-spinlock-race-in-kernel-thread-creation.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 21d265ce13ca673129d4682c5e078ad9cbf25bfe
Author: Michael Schmitz <schmitzmic@xxxxxxxxx>
Date:   Thu Apr 11 15:36:31 2024 +1200

    m68k: Fix spinlock race in kernel thread creation
    
    [ Upstream commit da89ce46f02470ef08f0f580755d14d547da59ed ]
    
    Context switching does take care to retain the correct lock owner across
    the switch from 'prev' to 'next' tasks.  This does rely on interrupts
    remaining disabled for the entire duration of the switch.
    
    This condition is guaranteed for normal process creation and context
    switching between already running processes, because both 'prev' and
    'next' already have interrupts disabled in their saved copies of the
    status register.
    
    The situation is different for newly created kernel threads.  The status
    register is set to PS_S in copy_thread(), which does leave the IPL at 0.
    Upon restoring the 'next' thread's status register in switch_to() aka
    resume(), interrupts then become enabled prematurely.  resume() then
    returns via ret_from_kernel_thread() and schedule_tail() where run queue
    lock is released (see finish_task_switch() and finish_lock_switch()).
    
    A timer interrupt calling scheduler_tick() before the lock is released
    in finish_task_switch() will find the lock already taken, with the
    current task as lock owner.  This causes a spinlock recursion warning as
    reported by Guenter Roeck.
    
    As far as I can ascertain, this race has been opened in commit
    533e6903bea0 ("m68k: split ret_from_fork(), simplify kernel_thread()")
    but I haven't done a detailed study of kernel history so it may well
    predate that commit.
    
    Interrupts cannot be disabled in the saved status register copy for
    kernel threads (init will complain about interrupts disabled when
    finally starting user space).  Disable interrupts temporarily when
    switching the tasks' register sets in resume().
    
    Note that a simple oriw 0x700,%sr after restoring sr is not enough here
    - this leaves enough of a race for the 'spinlock recursion' warning to
    still be observed.
    
    Tested on ARAnyM and qemu (Quadra 800 emulation).
    
    Fixes: 533e6903bea0 ("m68k: split ret_from_fork(), simplify kernel_thread()")
    Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
    Closes: https://lore.kernel.org/all/07811b26-677c-4d05-aeb4-996cd880b789@xxxxxxxxxxxx
    Signed-off-by: Michael Schmitz <schmitzmic@xxxxxxxxx>
    Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
    Reviewed-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/20240411033631.16335-1-schmitzmic@xxxxxxxxx
    Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/m68k/kernel/entry.S b/arch/m68k/kernel/entry.S
index 9a66657773beb..417d8f0e89627 100644
--- a/arch/m68k/kernel/entry.S
+++ b/arch/m68k/kernel/entry.S
@@ -425,7 +425,9 @@ resume:
 	movec	%a0,%dfc
 
 	/* restore status register */
-	movew	%a1@(TASK_THREAD+THREAD_SR),%sr
+	movew	%a1@(TASK_THREAD+THREAD_SR),%d0
+	oriw	#0x0700,%d0
+	movew	%d0,%sr
 
 	rts
 




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