netdev - [PATCH v2] xfrm: ipcomp: Update ipcomp_scratches with NULL if alloc fails

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Message-Id: <20220831142938.5882-1-khalid.masum.92@gmail.com>
Date:   Wed, 31 Aug 2022 20:29:38 +0600
From:   Khalid Masum <khalid.masum.92@...il.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        syzbot+5ec9bb042ddfe9644773@...kaller.appspotmail.com,
        Khalid Masum <khalid.masum.92@...il.com>
Subject: [PATCH v2] xfrm: ipcomp: Update ipcomp_scratches with NULL if alloc fails

Currently if ipcomp_alloc_scratches() fails to allocate memory
ipcomp_scratches holds obsolete address. So when we try to free the
percpu scratches using ipcomp_free_scratches() it tries to vfree non
existent vm area. Described below:

static void * __percpu *ipcomp_alloc_scratches(void)
{
	...
	scratches = alloc_percpu(void *);
        if (!scratches)
                return NULL;
ipcomp_scratches does not know about this allocation failure.
Therefore holding the old obsolete address.
        ...
}

So when we free,

static void ipcomp_free_scratches(void)
{
	...

        scratches = ipcomp_scratches;
Receiving obsolete addresses from ipcomp_scratches
        
	if (!scratches)
                return;

        for_each_possible_cpu(i)
               vfree(*per_cpu_ptr(scratches, i));
Trying to free non existent page, causing warning.

        ...
}

Fix this breakage by updating ipcomp_scratches with NULL if
the above mentioned allocation fails.

Reported-and-tested-by: syzbot+5ec9bb042ddfe9644773@...kaller.appspotmail.com
Signed-off-by: Khalid Masum <khalid.masum.92@...il.com>

---
diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
index cb40ff0ff28d..17815cde8a7f 100644
--- a/net/xfrm/xfrm_ipcomp.c
+++ b/net/xfrm/xfrm_ipcomp.c
@@ -215,7 +215,7 @@ static void * __percpu *ipcomp_alloc_scratches(void)
 
 	scratches = alloc_percpu(void *);
 	if (!scratches)
-		return NULL;
+		return ipcomp_scratches = NULL;
 
 	ipcomp_scratches = scratches;