Re: [PATCH] xfrm: Don't increase scratch users if allocation fails

[Khalid Masum]

On 8/31/22 15:13, Herbert Xu wrote:
> On Wed, Aug 31, 2022 at 07:41:26AM +0600, Khalid Masum wrote:
> > ipcomp_alloc_scratches() routine increases ipcomp_scratch_users count
> > even if it fails to allocate memory. Therefore, ipcomp_free_scratches()
> > routine, when triggered, tries to vfree() non existent percpu
> > ipcomp_scratches.
> >
> > To fix this breakage, do not increase scratch users count if
> > ipcomp_alloc_scratches() fails to allocate scratches.
> >
> > Reported-and-tested-by: syzbot+5ec9bb042ddfe9644773@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Khalid Masum <khalid.masum.92@xxxxxxxxx>
> > ---
> >   net/xfrm/xfrm_ipcomp.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
> > index cb40ff0ff28d..af9097983139 100644
> > --- a/net/xfrm/xfrm_ipcomp.c
> > +++ b/net/xfrm/xfrm_ipcomp.c
> > @@ -210,13 +210,15 @@ static void * __percpu *ipcomp_alloc_scratches(void)
> >   	void * __percpu *scratches;
> >   	int i;
> >   
> > -	if (ipcomp_scratch_users++)
> > +	if (ipcomp_scratch_users) {
> > +		ipcomp_scratch_users++;
> >   		return ipcomp_scratches;
> > -
> > +	}
> >   	scratches = alloc_percpu(void *);
> >   	if (!scratches)
> >   		return NULL;
> >   
> > +	ipcomp_scratch_users++;
> >   	ipcomp_scratches = scratches;
>
> This patch is broken because on error we will always call
> ipcomp_free_scratches which frees any partially allocated memory
> and restores ipcomp_scratch_users to zero.
>
> With this patch ipcomp_scratch_users will turn negative on error.
>
> Cheers,

Thanks for the review. I think it can be fixed by assigning NULL in 
ipcomp_scratches when the allocation fails as ipcomp_free_scratches
checks for it. I shall follow this email with a v2 shortly.

thanks,
  -- Khalid Masum