Subject: Re: [PATCH] netfiler: ipset: fix unaligned atomic access

From: Russell King - ARM Linux admin <[email-masked]>
Date: Sun, 21 Jun 2020 20:45:14 +0100

Gentle ping...

This patch fixes a remotely triggerable kernel oops, and as such can
be viewed as a remote denial of service attack depending on the
netfilter rule setup.

On Wed, Jun 10, 2020 at 09:51:11PM +0100, Russell King wrote:
> When using ip_set with counters and comment, traffic causes the kernel
> to panic on 32-bit ARM:
> 
> Alignment trap: not handling instruction e1b82f9f at [<bf01b0dc>]
> Unhandled fault: alignment exception (0x221) at 0xea08133c
> PC is at ip_set_match_extensions+0xe0/0x224 [ip_set]
> 
> The problem occurs when we try to update the 64-bit counters - the
> faulting address above is not 64-bit aligned.  The problem occurs
> due to the way elements are allocated, for example:
> 
> 	set->dsize = ip_set_elem_len(set, tb, 0, 0);
> 	map = ip_set_alloc(sizeof(*map) + elements * set->dsize);
> 
> If the element has a requirement for a member to be 64-bit aligned,
> and set->dsize is not a multiple of 8, but is a multiple of four,
> then every odd numbered elements will be misaligned - and hitting
> an atomic64_add() on that element will cause the kernel to panic.
> 
> ip_set_elem_len() must return a size that is rounded to the maximum
> alignment of any extension field stored in the element.  This change
> ensures that is the case.
> 
> Signed-off-by: Russell King <[email-masked]>
> ---
> Patch against v5.6, where I tripped over this bug.  This bug has
> caused a kernel panic on my new router twice today.
> 
>  net/netfilter/ipset/ip_set_core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
> index 8dd17589217d..be9cd6a500fb 100644
> --- a/net/netfilter/ipset/ip_set_core.c
> +++ b/net/netfilter/ipset/ip_set_core.c
> @@ -459,6 +459,8 @@ ip_set_elem_len(struct ip_set *set, struct nlattr *tb[], size_t len,
>  	for (id = 0; id < IPSET_EXT_ID_MAX; id++) {
>  		if (!add_extension(id, cadt_flags, tb))
>  			continue;
> +		if (align < ip_set_extensions[id].align)
> +			align = ip_set_extensions[id].align;
>  		len = ALIGN(len, ip_set_extensions[id].align);
>  		set->offset[id] = len;
>  		set->extensions |= ip_set_extensions[id].type;
> -- 
> 2.20.1
> 
> 

-- 
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