[net-next] net: switch to storing KCOV handle directly in sk_buff
diff mbox series

Message ID 20201125173436.1894624-1-elver@google.com
State New, archived
Headers show
Series
  • [net-next] net: switch to storing KCOV handle directly in sk_buff
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Commit Message

Marco Elver Nov. 25, 2020, 5:34 p.m. UTC
It turns out that usage of skb extensions can cause memory leaks. Ido
Schimmel reported: "[...] there are instances that blindly overwrite
'skb->extensions' by invoking skb_copy_header() after __alloc_skb()."

Therefore, give up on using skb extensions for KCOV handle, and instead
directly store kcov_handle in sk_buff.

Fixes: 6370cc3bbd8a ("net: add kcov handle to skb extensions")
Fixes: 85ce50d337d1 ("net: kcov: don't select SKB_EXTENSIONS when there is no NET")
Fixes: 97f53a08cba1 ("net: linux/skbuff.h: combine SKB_EXTENSIONS + KCOV handling")
Link: https://lore.kernel.org/linux-wireless/20201121160941.GA485907@shredder.lan/
Reported-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/skbuff.h | 37 +++++++++++++------------------------
 lib/Kconfig.debug      |  1 -
 net/core/skbuff.c      | 12 +-----------
 3 files changed, 14 insertions(+), 36 deletions(-)


base-commit: 470dfd808ac4135f313967f9d3e107b87fc6a0b3

Comments

Jakub Kicinski Nov. 25, 2020, 8:43 p.m. UTC | #1
On Wed, 25 Nov 2020 18:34:36 +0100 Marco Elver wrote:
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index ffe3dcc0ebea..070b1077d976 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -233,6 +233,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  	skb->end = skb->tail + size;
>  	skb->mac_header = (typeof(skb->mac_header))~0U;
>  	skb->transport_header = (typeof(skb->transport_header))~0U;
> +	skb_set_kcov_handle(skb, kcov_common_handle());
>  
>  	/* make sure we initialize shinfo sequentially */
>  	shinfo = skb_shinfo(skb);
> @@ -249,9 +250,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  
>  		fclones->skb2.fclone = SKB_FCLONE_CLONE;
>  	}
> -
> -	skb_set_kcov_handle(skb, kcov_common_handle());

Why the move?

>  out:
>  	return skb;
>  nodata:
> @@ -285,8 +283,6 @@ static struct sk_buff *__build_skb_around(struct sk_buff *skb,
>  	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
>  	atomic_set(&shinfo->dataref, 1);
>  
> -	skb_set_kcov_handle(skb, kcov_common_handle());
> -
>  	return skb;
>  }

And why are we dropping this?

If this was omitted in earlier versions it's just a independent bug, 
I don't think build_skb() will call __alloc_skb(), so we need a to
set the handle here.
Marco Elver Nov. 25, 2020, 10:42 p.m. UTC | #2
On Wed, 25 Nov 2020 at 21:43, Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Wed, 25 Nov 2020 18:34:36 +0100 Marco Elver wrote:
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index ffe3dcc0ebea..070b1077d976 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -233,6 +233,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> >       skb->end = skb->tail + size;
> >       skb->mac_header = (typeof(skb->mac_header))~0U;
> >       skb->transport_header = (typeof(skb->transport_header))~0U;
> > +     skb_set_kcov_handle(skb, kcov_common_handle());
> >
> >       /* make sure we initialize shinfo sequentially */
> >       shinfo = skb_shinfo(skb);
> > @@ -249,9 +250,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> >
> >               fclones->skb2.fclone = SKB_FCLONE_CLONE;
> >       }
> > -
> > -     skb_set_kcov_handle(skb, kcov_common_handle());
>
> Why the move?

v2 of the original series had it above. I frankly don't mind.

1. Group it with the other fields above?

2. Leave it at the end here?

> >  out:
> >       return skb;
> >  nodata:
> > @@ -285,8 +283,6 @@ static struct sk_buff *__build_skb_around(struct sk_buff *skb,
> >       memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> >       atomic_set(&shinfo->dataref, 1);
> >
> > -     skb_set_kcov_handle(skb, kcov_common_handle());
> > -
> >       return skb;
> >  }
>
> And why are we dropping this?

It wasn't here originally.

> If this was omitted in earlier versions it's just a independent bug,
> I don't think build_skb() will call __alloc_skb(), so we need a to
> set the handle here.

Correct, that was an original omission.

Will send v2.
Willem de Bruijn Nov. 26, 2020, 4:34 p.m. UTC | #3
On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@google.com> wrote:
>
> On Wed, 25 Nov 2020 at 21:43, Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Wed, 25 Nov 2020 18:34:36 +0100 Marco Elver wrote:
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index ffe3dcc0ebea..070b1077d976 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -233,6 +233,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> > >       skb->end = skb->tail + size;
> > >       skb->mac_header = (typeof(skb->mac_header))~0U;
> > >       skb->transport_header = (typeof(skb->transport_header))~0U;
> > > +     skb_set_kcov_handle(skb, kcov_common_handle());
> > >
> > >       /* make sure we initialize shinfo sequentially */
> > >       shinfo = skb_shinfo(skb);
> > > @@ -249,9 +250,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> > >
> > >               fclones->skb2.fclone = SKB_FCLONE_CLONE;
> > >       }
> > > -
> > > -     skb_set_kcov_handle(skb, kcov_common_handle());
> >
> > Why the move?
>
> v2 of the original series had it above. I frankly don't mind.
>
> 1. Group it with the other fields above?
>
> 2. Leave it at the end here?
>
> > >  out:
> > >       return skb;
> > >  nodata:
> > > @@ -285,8 +283,6 @@ static struct sk_buff *__build_skb_around(struct sk_buff *skb,
> > >       memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> > >       atomic_set(&shinfo->dataref, 1);
> > >
> > > -     skb_set_kcov_handle(skb, kcov_common_handle());
> > > -
> > >       return skb;
> > >  }
> >
> > And why are we dropping this?
>
> It wasn't here originally.
>
> > If this was omitted in earlier versions it's just a independent bug,
> > I don't think build_skb() will call __alloc_skb(), so we need a to
> > set the handle here.
>
> Correct, that was an original omission.
>
> Will send v2.

Does it make more sense to revert the patch that added the extensions
and the follow-on fixes and add a separate new patch instead?

If adding a new field to the skb, even if only in debug builds,
please check with pahole how it affects struct layout if you
haven't yet.

The skb_extensions idea was mine. Apologies for steering
this into an apparently unsuccessful direction. Adding new fields
to skb is very rare because possibly problematic wrt allocation.
Marco Elver Nov. 27, 2020, 12:26 p.m. UTC | #4
On Thu, 26 Nov 2020 at 17:35, Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
> On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@google.com> wrote:
[...]
> > Will send v2.
>
> Does it make more sense to revert the patch that added the extensions
> and the follow-on fixes and add a separate new patch instead?

That doesn't work, because then we'll end up with a build-broken
commit in between the reverts and the new version, because mac80211
uses skb_get_kcov_handle().

> If adding a new field to the skb, even if only in debug builds,
> please check with pahole how it affects struct layout if you
> haven't yet.

Without KCOV:

        /* size: 224, cachelines: 4, members: 72 */
        /* sum members: 217, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 32 bytes */

With KCOV:

        /* size: 232, cachelines: 4, members: 73 */
        /* sum members: 225, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 40 bytes */


> The skb_extensions idea was mine. Apologies for steering
> this into an apparently unsuccessful direction. Adding new fields
> to skb is very rare because possibly problematic wrt allocation.
Willem de Bruijn Nov. 27, 2020, 4:50 p.m. UTC | #5
On Fri, Nov 27, 2020 at 7:26 AM Marco Elver <elver@google.com> wrote:
>
> On Thu, 26 Nov 2020 at 17:35, Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> > On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@google.com> wrote:
> [...]
> > > Will send v2.
> >
> > Does it make more sense to revert the patch that added the extensions
> > and the follow-on fixes and add a separate new patch instead?
>
> That doesn't work, because then we'll end up with a build-broken
> commit in between the reverts and the new version, because mac80211
> uses skb_get_kcov_handle().
>
> > If adding a new field to the skb, even if only in debug builds,
> > please check with pahole how it affects struct layout if you
> > haven't yet.
>
> Without KCOV:
>
>         /* size: 224, cachelines: 4, members: 72 */
>         /* sum members: 217, holes: 1, sum holes: 2 */
>         /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
>         /* forced alignments: 2 */
>         /* last cacheline: 32 bytes */
>
> With KCOV:
>
>         /* size: 232, cachelines: 4, members: 73 */
>         /* sum members: 225, holes: 1, sum holes: 2 */
>         /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
>         /* forced alignments: 2 */
>         /* last cacheline: 40 bytes */

Thanks. defconfig leaves some symbols disabled, but manually enabling
them just fills a hole, so 232 is indeed the worst case allocation.

I recall a firm edict against growing skb, but I don't know of a
hard limit at exactly 224.

There is a limit at 2048 - sizeof(struct skb_shared_data) == 1728B
when using pages for two ETH_FRAME_LEN (1514) allocations.

This would leave 1728 - 1514 == 214B if also squeezing the skb itself
in with the same allocation.

But I have no idea if this is used anywhere. Certainly have no example
ready. And as you show, the previous default already is at 224.

If no one else knows of a hard limit at 224 or below, I suppose the
next technical limit is just 256 for kmem cache purposes.

My understanding was that skb_extensions was supposed to solve this
problem of extending the skb without growing the main structure. Not
for this patch, but I wonder if we can resolve the issues exposed here
and make usable in more conditions.

Patch
diff mbox series

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0a1239819fd2..333bcdc39635 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -701,6 +701,7 @@  typedef unsigned char *sk_buff_data_t;
  *	@transport_header: Transport layer header
  *	@network_header: Network layer header
  *	@mac_header: Link layer header
+ *	@kcov_handle: KCOV remote handle for remote coverage collection
  *	@tail: Tail pointer
  *	@end: End pointer
  *	@head: Head of buffer
@@ -904,6 +905,10 @@  struct sk_buff {
 	__u16			network_header;
 	__u16			mac_header;
 
+#ifdef CONFIG_KCOV
+	u64			kcov_handle;
+#endif
+
 	/* private: */
 	__u32			headers_end[0];
 	/* public: */
@@ -4150,9 +4155,6 @@  enum skb_ext_id {
 #endif
 #if IS_ENABLED(CONFIG_MPTCP)
 	SKB_EXT_MPTCP,
-#endif
-#if IS_ENABLED(CONFIG_KCOV)
-	SKB_EXT_KCOV_HANDLE,
 #endif
 	SKB_EXT_NUM, /* must be last */
 };
@@ -4608,35 +4610,22 @@  static inline void skb_reset_redirect(struct sk_buff *skb)
 #endif
 }
 
-#if IS_ENABLED(CONFIG_KCOV) && IS_ENABLED(CONFIG_SKB_EXTENSIONS)
 static inline void skb_set_kcov_handle(struct sk_buff *skb,
 				       const u64 kcov_handle)
 {
-	/* Do not allocate skb extensions only to set kcov_handle to zero
-	 * (as it is zero by default). However, if the extensions are
-	 * already allocated, update kcov_handle anyway since
-	 * skb_set_kcov_handle can be called to zero a previously set
-	 * value.
-	 */
-	if (skb_has_extensions(skb) || kcov_handle) {
-		u64 *kcov_handle_ptr = skb_ext_add(skb, SKB_EXT_KCOV_HANDLE);
-
-		if (kcov_handle_ptr)
-			*kcov_handle_ptr = kcov_handle;
-	}
+#ifdef CONFIG_KCOV
+	skb->kcov_handle = kcov_handle;
+#endif
 }
 
 static inline u64 skb_get_kcov_handle(struct sk_buff *skb)
 {
-	u64 *kcov_handle = skb_ext_find(skb, SKB_EXT_KCOV_HANDLE);
-
-	return kcov_handle ? *kcov_handle : 0;
-}
+#ifdef CONFIG_KCOV
+	return skb->kcov_handle;
 #else
-static inline void skb_set_kcov_handle(struct sk_buff *skb,
-				       const u64 kcov_handle) { }
-static inline u64 skb_get_kcov_handle(struct sk_buff *skb) { return 0; }
-#endif /* CONFIG_KCOV && CONFIG_SKB_EXTENSIONS */
+	return 0;
+#endif
+}
 
 #endif	/* __KERNEL__ */
 #endif	/* _LINUX_SKBUFF_H */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 826a205ffd1c..1d15cdaf1b89 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1879,7 +1879,6 @@  config KCOV
 	depends on CC_HAS_SANCOV_TRACE_PC || GCC_PLUGINS
 	select DEBUG_FS
 	select GCC_PLUGIN_SANCOV if !CC_HAS_SANCOV_TRACE_PC
-	select SKB_EXTENSIONS if NET
 	help
 	  KCOV exposes kernel code coverage information in a form suitable
 	  for coverage-guided fuzzing (randomized testing).
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index ffe3dcc0ebea..070b1077d976 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -233,6 +233,7 @@  struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 	skb->end = skb->tail + size;
 	skb->mac_header = (typeof(skb->mac_header))~0U;
 	skb->transport_header = (typeof(skb->transport_header))~0U;
+	skb_set_kcov_handle(skb, kcov_common_handle());
 
 	/* make sure we initialize shinfo sequentially */
 	shinfo = skb_shinfo(skb);
@@ -249,9 +250,6 @@  struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
 		fclones->skb2.fclone = SKB_FCLONE_CLONE;
 	}
-
-	skb_set_kcov_handle(skb, kcov_common_handle());
-
 out:
 	return skb;
 nodata:
@@ -285,8 +283,6 @@  static struct sk_buff *__build_skb_around(struct sk_buff *skb,
 	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
 	atomic_set(&shinfo->dataref, 1);
 
-	skb_set_kcov_handle(skb, kcov_common_handle());
-
 	return skb;
 }
 
@@ -4208,9 +4204,6 @@  static const u8 skb_ext_type_len[] = {
 #if IS_ENABLED(CONFIG_MPTCP)
 	[SKB_EXT_MPTCP] = SKB_EXT_CHUNKSIZEOF(struct mptcp_ext),
 #endif
-#if IS_ENABLED(CONFIG_KCOV)
-	[SKB_EXT_KCOV_HANDLE] = SKB_EXT_CHUNKSIZEOF(u64),
-#endif
 };
 
 static __always_inline unsigned int skb_ext_total_length(void)
@@ -4227,9 +4220,6 @@  static __always_inline unsigned int skb_ext_total_length(void)
 #endif
 #if IS_ENABLED(CONFIG_MPTCP)
 		skb_ext_type_len[SKB_EXT_MPTCP] +
-#endif
-#if IS_ENABLED(CONFIG_KCOV)
-		skb_ext_type_len[SKB_EXT_KCOV_HANDLE] +
 #endif
 		0;
 }