net: Whitelist the skbuff_head_cache "cb" field

Web:        https://git.kernel.org/torvalds/c/79a8a642bf05cd0dced20621f6fef9d884124abd
Commit:     79a8a642bf05cd0dced20621f6fef9d884124abd
Parent:     e728789c52afccc1275cba1dd812f03abe16ea3c
Refname:    refs/heads/master
Author:     Kees Cook <[email protected]>
AuthorDate: Wed Feb 7 17:44:38 2018 -0800
Committer:  David S. Miller <[email protected]>
CommitDate: Thu Feb 8 15:15:48 2018 -0500

    net: Whitelist the skbuff_head_cache "cb" field
    
    Most callers of put_cmsg() use a "sizeof(foo)" for the length argument.
    Within put_cmsg(), a copy_to_user() call is made with a dynamic size, as a
    result of the cmsg header calculations. This means that hardened usercopy
    will examine the copy, even though it was technically a fixed size and
    should be implicitly whitelisted. All the put_cmsg() calls being built
    from values in skbuff_head_cache are coming out of the protocol-defined
    "cb" field, so whitelist this field entirely instead of creating per-use
    bounce buffers, for which there are concerns about performance.
    
    Original report was:
    
    Bad or missing usercopy whitelist? Kernel memory exposure attempt detected from SLAB object 'skbuff_head_cache' (offset 64, size 16)!
    WARNING: CPU: 0 PID: 3663 at mm/usercopy.c:81 usercopy_warn+0xdb/0x100 mm/usercopy.c:76
    ...
     __check_heap_object+0x89/0xc0 mm/slab.c:4426
     check_heap_object mm/usercopy.c:236 [inline]
     __check_object_size+0x272/0x530 mm/usercopy.c:259
     check_object_size include/linux/thread_info.h:112 [inline]
     check_copy_size include/linux/thread_info.h:143 [inline]
     copy_to_user include/linux/uaccess.h:154 [inline]
     put_cmsg+0x233/0x3f0 net/core/scm.c:242
     sock_recv_errqueue+0x200/0x3e0 net/core/sock.c:2913
     packet_recvmsg+0xb2e/0x17a0 net/packet/af_packet.c:3296
     sock_recvmsg_nosec net/socket.c:803 [inline]
     sock_recvmsg+0xc9/0x110 net/socket.c:810
     ___sys_recvmsg+0x2a4/0x640 net/socket.c:2179
     __sys_recvmmsg+0x2a9/0xaf0 net/socket.c:2287
     SYSC_recvmmsg net/socket.c:2368 [inline]
     SyS_recvmmsg+0xc4/0x160 net/socket.c:2352
     entry_SYSCALL_64_fastpath+0x29/0xa0
    
    Reported-by: [email protected]
    Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
    Signed-off-by: Kees Cook <[email protected]>
    Signed-off-by: David S. Miller <[email protected]>
---
 net/core/skbuff.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 8c61c27c1b28..09bd89c90a71 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3894,10 +3894,12 @@ EXPORT_SYMBOL_GPL(skb_gro_receive);
 
 void __init skb_init(void)
 {
-	skbuff_head_cache = kmem_cache_create("skbuff_head_cache",
+	skbuff_head_cache = kmem_cache_create_usercopy("skbuff_head_cache",
 					      sizeof(struct sk_buff),
 					      0,
 					      SLAB_HWCACHE_ALIGN|SLAB_PANIC,
+					      offsetof(struct sk_buff, cb),
+					      sizeof_field(struct sk_buff, cb),
 					      NULL);
 	skbuff_fclone_cache = kmem_cache_create("skbuff_fclone_cache",
 						sizeof(struct sk_buff_fclones),
--
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