CVE-2022-50569: xfrm: Update ipcomp_scratches with NULL when freed

In the Linux kernel, the following vulnerability has been resolved:

xfrm: Update ipcompscratches with NULL when freed

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

static void percpu ipcompallocscratches(void) { ... scratches = allocpercpu(void ); if (!scratches) return NULL; ipcompscratches does not know about this allocation failure. Therefore holding the old obsolete address. ... }

So when we free,

static void ipcompfreescratches(void) { ... scratches = ipcompscratches; Assigning obsolete address from ipcompscratches

if (!scratches) return;

foreachpossiblecpu(i) vfree(percpuptr(scratches, i)); Trying to free non existent page, causing warning: trying to vfree existent vm area. ... }

Fix this breakage by updating ipcompscrtches with NULL when scratches is freed