linux-kernel - Re: [PATCH] rbtree: fix the red root

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Date:   Fri, 11 Jan 2019 07:17:43 -1000
From:   Joey Pabalinas <joeypabalinas@...il.com>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, esploit@...tonmail.ch,
        jejb@...ux.ibm.com, dgilbert@...erlog.com,
        martin.petersen@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Joey Pabalinas <joeypabalinas@...il.com>
Subject: Re: [PATCH] rbtree: fix the red root

On Fri, Jan 11, 2019 at 11:51:45AM -0500, Qian Cai wrote:
> A GFP was reported,
> 
> kasan: CONFIG_KASAN_INLINE enabled
> kasan: GPF could be caused by NULL-ptr deref or user memory access
> general protection fault: 0000 [#1] SMP KASAN
>         kasan_die_handler.cold.22+0x11/0x31
>         notifier_call_chain+0x17b/0x390
>         atomic_notifier_call_chain+0xa7/0x1b0
>         notify_die+0x1be/0x2e0
>         do_general_protection+0x13e/0x330
>         general_protection+0x1e/0x30
>         rb_insert_color+0x189/0x1480
>         create_object+0x785/0xca0
>         kmemleak_alloc+0x2f/0x50
>         kmem_cache_alloc+0x1b9/0x3c0
>         getname_flags+0xdb/0x5d0
>         getname+0x1e/0x20
>         do_sys_open+0x3a1/0x7d0
>         __x64_sys_open+0x7e/0xc0
>         do_syscall_64+0x1b3/0x820
>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> It turned out,
> 
> gparent = rb_red_parent(parent);
> tmp = gparent->rb_right; <-- GFP triggered here.
> 
> Apparently, "gparent" is NULL which indicates "parent" is rbtree's root
> which is red. Otherwise, it will be treated properly a few lines above.

Good catch, acked. After thinking through the logic a bit your solution
seems like the simplest fix.

Now, I didn't do _extensive_ testing but a quick compile and bootup of
the patch with CONFIG_KASAN_INLINE enabled has yet to throw any GFPs,
so take that as you will.

Reviewed-by: Joey Pabalinas <joeypabalinas@...il.com>
Tested-by: Joey Pabalinas <joeypabalinas@...il.com>

> /*
>  * If there is a black parent, we are done.
>  * Otherwise, take some corrective action as,
>  * per 4), we don't want a red root or two
>  * consecutive red nodes.
>  */
> if(rb_is_black(parent))
> 	break;
> 
> Hence, it violates the rule #1 and need a fix up.
> 
> Reported-by: Esme <esploit@...tonmail.ch>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  lib/rbtree.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/lib/rbtree.c b/lib/rbtree.c
> index d3ff682fd4b8..acc969ad8de9 100644
> --- a/lib/rbtree.c
> +++ b/lib/rbtree.c
> @@ -127,6 +127,13 @@ __rb_insert(struct rb_node *node, struct rb_root *root,
>  			break;
>  
>  		gparent = rb_red_parent(parent);
> +		if (unlikely(!gparent)) {
> +			/*
> +			 * The root is red so correct it.
> +			 */
> +			rb_set_parent_color(parent, NULL, RB_BLACK);
> +			break;
> +		}
>  
>  		tmp = gparent->rb_right;
>  		if (parent != tmp) {	/* parent == gparent->rb_left */
> -- 
> 2.17.2 (Apple Git-113)
> 

-- 
Cheers,
Joey Pabalinas

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