[PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Dan Carpenter]

The "payload" variable is type size_t, however the nlmsg_total_size()
function will a few bytes to it and then truncate the result to type
int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
function might allocate a buffer which is smaller than intended.

Cc: stable@vger.kernel.org
Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
---
 include/net/netlink.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/net/netlink.h b/include/net/netlink.h
index e015ffbed819..ca7a8152e6d4 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1015,6 +1015,8 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
  */
 static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
 {
+	if (payload > INT_MAX)
+		return NULL;
 	return alloc_skb(nlmsg_total_size(payload), flags);
 }
 
-- 
2.45.2

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Przemek Kitszel]

On 1/22/25 14:49, Dan Carpenter wrote:
> The "payload" variable is type size_t, however the nlmsg_total_size()
> function will a few bytes to it and then truncate the result to type
> int.  That means that if "payload" is more than UINT_MAX the alloc_skb()

In the code it's INT_MAX, would be best to have the same used in both
places (or explain it so it's obvious)

> function might allocate a buffer which is smaller than intended.
> 
> Cc: stable@vger.kernel.org
> Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
>   include/net/netlink.h | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index e015ffbed819..ca7a8152e6d4 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -1015,6 +1015,8 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
>    */
>   static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
>   {
> +	if (payload > INT_MAX)
> +		return NULL;
>   	return alloc_skb(nlmsg_total_size(payload), flags);
>   }
>   

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Dan Carpenter]

On Wed, Jan 22, 2025 at 02:52:39PM +0100, Przemek Kitszel wrote:
> On 1/22/25 14:49, Dan Carpenter wrote:
> > The "payload" variable is type size_t, however the nlmsg_total_size()
> > function will a few bytes to it and then truncate the result to type
> > int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> 
> In the code it's INT_MAX, would be best to have the same used in both
> places (or explain it so it's obvious)
> 

Yeah.  It's not probably not obvious.

I don't like using UINT_MAX as a limit because why push so close to the
edge?  Normal allocation functions are capped at INT_MAX to avoid
integer overflows.  You'd have to use vmalloc() to allocate more than
2GB of RAM.  So it's not like we gain anything by using a higher, riskier
number.

The nlmsg_total_size() function adds potentially 19 bytes to the
payload.

INT_MAX plus anything less than 2 million number can't overflow to zero.
It could overflow to negative but you can't allocate negative bytes so
that's fine.

The vfs_read/write() functions use MAX_RW_COUNT to avoid integer
overflows.  That's basically INT_MAX - PAGE_SIZE.  There are quite
a few places like this in the kernel which assume small numbers like
sizeof() are generally going to return less than PAGE_SIZE.  Would
that be better to do this.  Then it couldn't overflow to negative.

regards,
dan carpenter

diff --git a/include/net/netlink.h b/include/net/netlink.h
index e015ffbed819..ceeea04fae4a 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1015,6 +1015,9 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
  */
 static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
 {
+	/* Prevent integer overflow */
+	if (payload > INT_MAX - PAGE_SIZE)
+		return NULL;
 	return alloc_skb(nlmsg_total_size(payload), flags);
 }
 

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Jakub Kicinski]

On Wed, 22 Jan 2025 16:49:17 +0300 Dan Carpenter wrote:
> The "payload" variable is type size_t, however the nlmsg_total_size()
> function will a few bytes to it and then truncate the result to type
> int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> function might allocate a buffer which is smaller than intended.

Is there a bug, or is this theoretical?

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Dan Carpenter]

On Wed, Jan 22, 2025 at 06:24:27AM -0800, Jakub Kicinski wrote:
> On Wed, 22 Jan 2025 16:49:17 +0300 Dan Carpenter wrote:
> > The "payload" variable is type size_t, however the nlmsg_total_size()
> > function will a few bytes to it and then truncate the result to type
> > int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> > function might allocate a buffer which is smaller than intended.
> 
> Is there a bug, or is this theoretical?

The rule here is that if we pass something very close to UINT_MAX to
nlmsg_new() the it leads to an integer overflow.  I'm not a networking
expert.  The caller that concerned me was:

*** 1 ***

net/netfilter/ipset/ip_set_core.c
  1762                  /* Error in restore/batch mode: send back lineno */
  1763                  struct nlmsghdr *rep, *nlh = nlmsg_hdr(skb);
  1764                  struct sk_buff *skb2;
  1765                  struct nlmsgerr *errmsg;
  1766                  size_t payload = min(SIZE_MAX,
  1767                                       sizeof(*errmsg) + nlmsg_len(nlh));

I don't know the limits of limits of nlmsg_len() here.

The min(SIZE_MAX is what scared me.  That was added to silence a Smatch
warning.  :P  It should be fixed or removed.

  1768                  int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
  1769                  struct nlattr *cda[IPSET_ATTR_CMD_MAX + 1];
  1770                  struct nlattr *cmdattr;
  1771                  u32 *errline;
  1772  
  1773                  skb2 = nlmsg_new(payload, GFP_KERNEL);
  1774                  if (!skb2)
  1775                          return -ENOMEM;

*** 2 ***
There is similar code in netlink_ack() where the payload comes from
nlmsg_len(nlh).

*** 3 ***

There is a potential issue in queue_userspace_packet() when we call:

	len = upcall_msg_size(upcall_info, hlen - cutlen, ...
                                           ^^^^^^^^^^^^^
	user_skb = genlmsg_new(len, GFP_ATOMIC);

It's possible that hlen is less than cutlen.  (That's a separate bug,
I'll send a fix for it).

regards,
dan carpenter

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Jakub Kicinski]

On Fri, 24 Jan 2025 17:35:24 +0300 Dan Carpenter wrote:
> On Wed, Jan 22, 2025 at 06:24:27AM -0800, Jakub Kicinski wrote:
> > On Wed, 22 Jan 2025 16:49:17 +0300 Dan Carpenter wrote:  
> > > The "payload" variable is type size_t, however the nlmsg_total_size()
> > > function will a few bytes to it and then truncate the result to type
> > > int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> > > function might allocate a buffer which is smaller than intended.  
> > 
> > Is there a bug, or is this theoretical?  
> 
> The rule here is that if we pass something very close to UINT_MAX to
> nlmsg_new() the it leads to an integer overflow.  I'm not a networking
> expert.  The caller that concerned me was:
> 
> *** 1 ***
> 
> net/netfilter/ipset/ip_set_core.c
>   1762                  /* Error in restore/batch mode: send back lineno */
>   1763                  struct nlmsghdr *rep, *nlh = nlmsg_hdr(skb);
>   1764                  struct sk_buff *skb2;
>   1765                  struct nlmsgerr *errmsg;
>   1766                  size_t payload = min(SIZE_MAX,
>   1767                                       sizeof(*errmsg) + nlmsg_len(nlh));
> 
> I don't know the limits of limits of nlmsg_len() here.

Practically speaking the limits are fairly small. The nlh comes from
user's request / sendmsg() call. So the user must have prepared 
a message of at least that len, and kernel must had been able to
kvmalloc() a linear buffer large enough to copy that message in.

> The min(SIZE_MAX is what scared me.  That was added to silence a Smatch
> warning.  :P  It should be fixed or removed.

Yeah, that ip_set code looks buggy. Mostly because we use @payload
for the nlmsg_put() call, but then raw nlh->nlmsg_len for memcpy() :S

>   1768                  int min_len = nlmsg_total_size(sizeof(struct nfgenmsg));
>   1769                  struct nlattr *cda[IPSET_ATTR_CMD_MAX + 1];
>   1770                  struct nlattr *cmdattr;
>   1771                  u32 *errline;
>   1772  
>   1773                  skb2 = nlmsg_new(payload, GFP_KERNEL);
>   1774                  if (!skb2)
>   1775                          return -ENOMEM;
> 
> *** 2 ***
> There is similar code in netlink_ack() where the payload comes from
> nlmsg_len(nlh).

This one is correct. Each piece of the message is nlmsg_put()
individually, which does bounds checking. So if the allocation 
of the skb was faulty and the skb is shorter than we expected 
we'll just error out on the put.

> *** 3 ***
> 
> There is a potential issue in queue_userspace_packet() when we call:
> 
> 	len = upcall_msg_size(upcall_info, hlen - cutlen, ...
>                                            ^^^^^^^^^^^^^
> 	user_skb = genlmsg_new(len, GFP_ATOMIC);
> 
> It's possible that hlen is less than cutlen.  (That's a separate bug,
> I'll send a fix for it).

Ack.

In general IMVHO the check in nlmsg_new() won't be too effective.
The callers can overflow their local message size calculation.
Not to mention that the size calculation is often inexact.
So using nla_put() and checking error codes is the best way
to prevent security issues..

Re: [PATCH net] net: netlink: prevent potential integer overflow in nlmsg_new()

[Simon Horman]

On Wed, Jan 22, 2025 at 04:49:17PM +0300, Dan Carpenter wrote:
> The "payload" variable is type size_t, however the nlmsg_total_size()
> function will a few bytes to it and then truncate the result to type
> int.  That means that if "payload" is more than UINT_MAX the alloc_skb()
> function might allocate a buffer which is smaller than intended.
> 
> Cc: stable@vger.kernel.org
> Fixes: bfa83a9e03cf ("[NETLINK]: Type-safe netlink messages/attributes interface")
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
>  include/net/netlink.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index e015ffbed819..ca7a8152e6d4 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -1015,6 +1015,8 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
>   */
>  static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
>  {
> +	if (payload > INT_MAX)
> +		return NULL;
>  	return alloc_skb(nlmsg_total_size(payload), flags);

Hi Dan,

I wonder if this is sufficient.

If payload is INT_MAX then won't the call to nlmsg_msg_size() inside
nlmsg_total_size() overflow. And likewise, it feels that NLMSG_ALIGN
could overflow somehow.