[PATCH v3] xfrm: Update ipcomp_scratches with NULL if not allocated

Khalid Masum
2022-09-01T04:03:25Z

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;
Assigning 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: trying to vfree
existent vm area.
...
}

Fix this breakage by:
(1) Update ipcomp_scratches with NULL if the above mentioned
allocation fails.
(2) Update ipcomp_scrtches with NULL when scratches is freed

Reported-by: syzbot+5ec9bb042ddfe9644773@syzkaller.appspotmail.com
Tested-by: syzbot+5ec9bb042ddfe9644773@syzkaller.appspotmail.com
Signed-off-by: Khalid Masum <khalid.masum.92@gmail.com>
---
Changes since v2:
- Set ipcomp_scratches to NULL when scratches is freed.
- Update commit message.
- v2 Link: https://lore.kernel.org/lkml/20220831142938.5882-1-khalid.masum.92@gmail.com/

Changes since v1:
- Instead of altering usercount, update ipcomp_scratches to NULL
- Update commit message.
- v1 Link: https://lore.kernel.org/lkml/20220831014126.6708-1-khalid.masum.92@gmail.com/

diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
index cb40ff0ff28d..3774d07c5819 100644
--- a/net/xfrm/xfrm_ipcomp.c
+++ b/net/xfrm/xfrm_ipcomp.c
@@ -203,6 +203,7 @@ static void ipcomp_free_scratches(void)
vfree(*per_cpu_ptr(scratches, i));

free_percpu(scratches);
+ ipcomp_scratches = NULL;
}

static void * __percpu *ipcomp_alloc_scratches(void)
@@ -215,7 +216,7 @@ static void * __percpu *ipcomp_alloc_scratches(void)

scratches = alloc_percpu(void *);
if (!scratches)
- return NULL;
+ return ipcomp_scratches = NULL;

ipcomp_scratches = scratches;