linux-kernel - Re: [locking/ww_mutex] c0afb0ffc0: BUG:kernel_NULL_pointer

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Date:   Tue, 24 Aug 2021 16:47:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        lkp@...ts.01.org, lkp@...el.com
Subject: Re: [locking/ww_mutex]  c0afb0ffc0:
 BUG:kernel_NULL_pointer_dereference,address

On Tue, Aug 24, 2021 at 10:00:44PM +0800, kernel test robot wrote:
> 
> 
> Greeting,
> 
> FYI, we noticed the following commit (built with gcc-9):
> 
> commit: c0afb0ffc06e6b4e492a3b711f1fb32074f9949c ("locking/ww_mutex: Gather mutex_waiter initialization")

Fixed by...

commit b857174e68e26f9c4f0796971e11eb63ad5a3eb6
Author: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Date:   Thu Aug 19 21:30:30 2021 +0200

    locking/ww_mutex: Initialize waiter.ww_ctx properly
    
    The consolidation of the debug code for mutex waiter intialization sets
    waiter::ww_ctx to a poison value unconditionally. For regular mutexes this
    is intended to catch the case where waiter_ww_ctx is dereferenced
    accidentally.
    
    For ww_mutex the poison value has to be overwritten either with a context
    pointer or NULL for ww_mutexes without context.
    
    The rework broke this as it made the store conditional on the context
    pointer instead of the argument which signals whether ww_mutex code should
    be compiled in or optiized out. As a result waiter::ww_ctx ends up with the
    poison pointer for contextless ww_mutexes which causes a later dereference of
    the poison pointer because it is != NULL.
    
    Use the build argument instead so for ww_mutex the poison value is always
    overwritten.
    
    Fixes: c0afb0ffc06e6 ("locking/ww_mutex: Gather mutex_waiter initialization")
    Reported-by: Guenter Roeck <linux@...ck-us.net>
    Suggested-by: Peter Zijlstra <peterz@...radead.org>
    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
    Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
    Link: https://lore.kernel.org/r/20210819193030.zpwrpvvrmy7xxxiy@linutronix.de

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 3a65bf4bacfd..d456579d0952 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -618,7 +618,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 	debug_mutex_lock_common(lock, &waiter);
 	waiter.task = current;
-	if (ww_ctx)
+	if (use_ww_ctx)
 		waiter.ww_ctx = ww_ctx;
 
 	lock_contended(&lock->dep_map, ip);