rxrpc: Use skb_unshare() rather than skb_cow_data()

The in-place decryption routines in AF_RXRPC's rxkad security module
currently call skb_cow_data() to make sure the data isn't shared and that
the skb can be written over.  This has a problem, however, as the softirq
handler may be still holding a ref or the Rx ring may be holding multiple
refs when skb_cow_data() is called in rxkad_verify_packet() - and so
skb_shared() returns true and __pskb_pull_tail() dislikes that.  If this
occurs, something like the following report will be generated.

	kernel BUG at net/core/skbuff.c:1463!
	...
	RIP: 0010:pskb_expand_head+0x253/0x2b0
	...
	Call Trace:
	 __pskb_pull_tail+0x49/0x460
	 skb_cow_data+0x6f/0x300
	 rxkad_verify_packet+0x18b/0xb10 [rxrpc]
	 rxrpc_recvmsg_data.isra.11+0x4a8/0xa10 [rxrpc]
	 rxrpc_kernel_recv_data+0x126/0x240 [rxrpc]
	 afs_extract_data+0x51/0x2d0 [kafs]
	 afs_deliver_fs_fetch_data+0x188/0x400 [kafs]
	 afs_deliver_to_call+0xac/0x430 [kafs]
	 afs_wait_for_call_to_complete+0x22f/0x3d0 [kafs]
	 afs_make_call+0x282/0x3f0 [kafs]
	 afs_fs_fetch_data+0x164/0x300 [kafs]
	 afs_fetch_data+0x54/0x130 [kafs]
	 afs_readpages+0x20d/0x340 [kafs]
	 read_pages+0x66/0x180
	 __do_page_cache_readahead+0x188/0x1a0
	 ondemand_readahead+0x17d/0x2e0
	 generic_file_read_iter+0x740/0xc10
	 __vfs_read+0x145/0x1a0
	 vfs_read+0x8c/0x140
	 ksys_read+0x4a/0xb0
	 do_syscall_64+0x43/0xf0
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fix this by using skb_unshare() instead in the input path for DATA packets
that have a security index != 0.  Non-DATA packets don't need in-place
encryption and neither do unencrypted DATA packets.

Fixes: 248f219cb8bc ("rxrpc: Rewrite the data and ack handling code")
Reported-by: Julian Wollrath <jwollrath@web.de>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h
index e2356c51883b..a13a62db3565 100644
--- a/include/trace/events/rxrpc.h
+++ b/include/trace/events/rxrpc.h
@@ -32,6 +32,8 @@ enum rxrpc_skb_trace {
 	rxrpc_skb_received,
 	rxrpc_skb_rotated,
 	rxrpc_skb_seen,
+	rxrpc_skb_unshared,
+	rxrpc_skb_unshared_nomem,
 };
 
 enum rxrpc_local_trace {
@@ -231,7 +233,9 @@ enum rxrpc_tx_point {
 	EM(rxrpc_skb_purged,			"PUR") \
 	EM(rxrpc_skb_received,			"RCV") \
 	EM(rxrpc_skb_rotated,			"ROT") \
-	E_(rxrpc_skb_seen,			"SEE")
+	EM(rxrpc_skb_seen,			"SEE") \
+	EM(rxrpc_skb_unshared,			"UNS") \
+	E_(rxrpc_skb_unshared_nomem,		"US0")
 
 #define rxrpc_local_traces \
 	EM(rxrpc_local_got,			"GOT") \
@@ -633,9 +637,9 @@ TRACE_EVENT(rxrpc_call,
 
 TRACE_EVENT(rxrpc_skb,
 	    TP_PROTO(struct sk_buff *skb, enum rxrpc_skb_trace op,
-		     int usage, int mod_count, const void *where),
+		     int usage, int mod_count, u8 flags,    const void *where),
 
-	    TP_ARGS(skb, op, usage, mod_count, where),
+	    TP_ARGS(skb, op, usage, mod_count, flags, where),
 
 	    TP_STRUCT__entry(
 		    __field(struct sk_buff *,		skb		)
@@ -648,7 +652,7 @@ TRACE_EVENT(rxrpc_skb,
 
 	    TP_fast_assign(
 		    __entry->skb = skb;
-		    __entry->flags = rxrpc_skb(skb)->rx_flags;
+		    __entry->flags = flags;
 		    __entry->op = op;
 		    __entry->usage = usage;
 		    __entry->mod_count = mod_count;