Merge branch 'tcp-take-care-of-another-syzbot-issue'

Eric Dumazet says:

====================
tcp: take care of another syzbot issue

This is a minor issue: It took months for syzbot to find a C repro,
and even with it, I had to spend a lot of time to understand KFENCE
was a prereq. With the default kfence 500ms interval, I had to be
very patient to trigger the kernel warning and perform my analysis.

This series targets net-next tree, because I added a new generic helper
in the first patch, then fixed the issue in the second one.
They can be backported once proven solid.
====================

Link: https://lore.kernel.org/r/20220222032113.4005821-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -1536,6 +1536,11 @@ static inline unsigned int skb_end_offset(const struct sk_buff *skb)
 {
 	return skb->end;
 }
+
+static inline void skb_set_end_offset(struct sk_buff *skb, unsigned int offset)
+{
+	skb->end = offset;
+}
 #else
 static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
 {
@@ -1546,6 +1551,11 @@ static inline unsigned int skb_end_offset(const struct sk_buff *skb)
 {
 	return skb->end - skb->head;
 }
+
+static inline void skb_set_end_offset(struct sk_buff *skb, unsigned int offset)
+{
+	skb->end = skb->head + offset;
+}
 #endif
 
 /* Internal */
@@ -1785,19 +1795,19 @@ static inline int skb_unclone(struct sk_buff *skb, gfp_t pri)
 	return 0;
 }
 
-/* This variant of skb_unclone() makes sure skb->truesize is not changed */
+/* This variant of skb_unclone() makes sure skb->truesize
+ * and skb_end_offset() are not changed, whenever a new skb->head is needed.
+ *
+ * Indeed there is no guarantee that ksize(kmalloc(X)) == ksize(kmalloc(X))
+ * when various debugging features are in place.
+ */
+int __skb_unclone_keeptruesize(struct sk_buff *skb, gfp_t pri);
 static inline int skb_unclone_keeptruesize(struct sk_buff *skb, gfp_t pri)
 {
 	might_sleep_if(gfpflags_allow_blocking(pri));
 
-	if (skb_cloned(skb)) {
-		unsigned int save = skb->truesize;
-		int res;
-
-		res = pskb_expand_head(skb, 0, 0, pri);
-		skb->truesize = save;
-		return res;
-	}
+	if (skb_cloned(skb))
+		return __skb_unclone_keeptruesize(skb, pri);
 	return 0;
 }
 

net/core/skbuff.c

@@ -201,7 +201,7 @@ static void __build_skb_around(struct sk_buff *skb, void *data,
 	skb->head = data;
 	skb->data = data;
 	skb_reset_tail_pointer(skb);
-	skb->end = skb->tail + size;
+	skb_set_end_offset(skb, size);
 	skb->mac_header = (typeof(skb->mac_header))~0U;
 	skb->transport_header = (typeof(skb->transport_header))~0U;
 
@@ -1736,11 +1736,10 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	skb->head     = data;
 	skb->head_frag = 0;
 	skb->data    += off;
+
+	skb_set_end_offset(skb, size);
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
-	skb->end      = size;
 	off           = nhead;
-#else
-	skb->end      = skb->head + size;
 #endif
 	skb->tail	      += off;
 	skb_headers_offset_update(skb, nhead);
@@ -1788,6 +1787,38 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
 }
 EXPORT_SYMBOL(skb_realloc_headroom);
 
+int __skb_unclone_keeptruesize(struct sk_buff *skb, gfp_t pri)
+{
+	unsigned int saved_end_offset, saved_truesize;
+	struct skb_shared_info *shinfo;
+	int res;
+
+	saved_end_offset = skb_end_offset(skb);
+	saved_truesize = skb->truesize;
+
+	res = pskb_expand_head(skb, 0, 0, pri);
+	if (res)
+		return res;
+
+	skb->truesize = saved_truesize;
+
+	if (likely(skb_end_offset(skb) == saved_end_offset))
+		return 0;
+
+	shinfo = skb_shinfo(skb);
+
+	/* We are about to change back skb->end,
+	 * we need to move skb_shinfo() to its new location.
+	 */
+	memmove(skb->head + saved_end_offset,
+		shinfo,
+		offsetof(struct skb_shared_info, frags[shinfo->nr_frags]));
+
+	skb_set_end_offset(skb, saved_end_offset);
+
+	return 0;
+}
+
 /**
  *	skb_expand_head - reallocate header of &sk_buff
  *	@skb: buffer to reallocate
@@ -6044,11 +6075,7 @@ static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
 	skb->head = data;
 	skb->data = data;
 	skb->head_frag = 0;
-#ifdef NET_SKBUFF_DATA_USES_OFFSET
-	skb->end = size;
-#else
-	skb->end = skb->head + size;
-#endif
+	skb_set_end_offset(skb, size);
 	skb_set_tail_pointer(skb, skb_headlen(skb));
 	skb_headers_offset_update(skb, 0);
 	skb->cloned = 0;
@@ -6186,11 +6213,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
 	skb->head = data;
 	skb->head_frag = 0;
 	skb->data = data;
-#ifdef NET_SKBUFF_DATA_USES_OFFSET
-	skb->end = size;
-#else
-	skb->end = skb->head + size;
-#endif
+	skb_set_end_offset(skb, size);
 	skb_reset_tail_pointer(skb);
 	skb_headers_offset_update(skb, 0);
 	skb->cloned   = 0;