Re: [netfilter-core] kernel panic: Out of memory and no killable processes... (2)

[Andrew Morton]

On Tue, 30 Jan 2018 15:01:04 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> > Well, this is not about syzkaller, it merely pointed out a potential
> > DoS... And that has to be addressed somehow.
> 
> So how about this?
> ---

argh ;)

> >From d48e950f1b04f234b57b9e34c363bdcfec10aeee Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxxx>
> Date: Tue, 30 Jan 2018 14:51:07 +0100
> Subject: [PATCH] net/netfilter/x_tables.c: make allocation less aggressive
> 
> syzbot has noticed that xt_alloc_table_info can allocate a lot of
> memory. This is an admin only interface but an admin in a namespace
> is sufficient as well. eacd86ca3b03 ("net/netfilter/x_tables.c: use
> kvmalloc() in xt_alloc_table_info()") has changed the opencoded
> kmalloc->vmalloc fallback into kvmalloc. It has dropped __GFP_NORETRY on
> the way because vmalloc has simply never fully supported __GFP_NORETRY
> semantic. This is still the case because e.g. page tables backing the
> vmalloc area are hardcoded GFP_KERNEL.
> 
> Revert back to __GFP_NORETRY as a poors man defence against excessively
> large allocation request here. We will not rule out the OOM killer
> completely but __GFP_NORETRY should at least stop the large request
> in most cases.
> 
> Fixes: eacd86ca3b03 ("net/netfilter/x_tables.c: use kvmalloc() in xt_alloc_table_info()")
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  net/netfilter/x_tables.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
> index d8571f414208..a5f5c29bcbdc 100644
> --- a/net/netfilter/x_tables.c
> +++ b/net/netfilter/x_tables.c
> @@ -1003,7 +1003,13 @@ struct xt_table_info *xt_alloc_table_info(unsigned int size)
>  	if ((SMP_ALIGN(size) >> PAGE_SHIFT) + 2 > totalram_pages)
>  		return NULL;

offtopic: preceding comment here is "prevent them from hitting BUG() in
vmalloc.c".  I suspect this is ancient code and vmalloc sure as heck
shouldn't go BUG with this input.  And it should be using `sz' ;)

So I suspect and hope that this code can be removed.  If not, let's fix
vmalloc!

> -	info = kvmalloc(sz, GFP_KERNEL);
> +	/*
> +	 * __GFP_NORETRY is not fully supported by kvmalloc but it should
> +	 * work reasonably well if sz is too large and bail out rather
> +	 * than shoot all processes down before realizing there is nothing
> +	 * more to reclaim.
> +	 */
> +	info = kvmalloc(sz, GFP_KERNEL | __GFP_NORETRY);
>  	if (!info)
>  		return NULL;

checkpatch sayeth

networking block comments don't use an empty /* line, use /* Comment...

So I'll do that and shall scoot the patch Davewards.