Created attachment 281103 [details] NULL POINTER DEFERENCE DURING __skb_unlink() In the function call, __skb_try_recv_from_queue() (net/core/datagram.c), sbk_queue_walk() walks through the queue without checking if the next member in the queue has valid next pointer/address. When a socket buffer has to unlink, __skb_unlink() is called. Inside __skb_unlink() function, it doesn't verify if skb->next has a valid address. skb->next is assigned and used, without verifying the value inside it. What could be probable solution, in this scenario? Should we check if skb->next is not NULL, before calling __skb_unlink()? -------------------------------------------------------------------------- -------------------------------------------------------------------------- net/core/datagram.c struct sk_buff *__skb_try_recv_from_queue(struct sock *sk, struct sk_buff_head *queue, unsigned int flags,void (*destructor)(struct sock *sk, struct sk_buff *skb),int *peeked, int *off, int *err, struct sk_buff **last) { ... ... ... skb_queue_walk(queue, skb) { ... ... ... } else { ==> __skb_unlink(skb, queue); if(destructor){ .... ... ... }
You don't give a back trace or program to reproduce this.
Hi Stephen, I have already attached the back trace in a file. Please find the attachment. However, I am pasting it here again: Backtrace: [ 3.589241] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 [ 3.598006] IP: __skb_try_recv_from_queue+0x4e/0x1b0 [ 3.606376] Oops: 0002 [#1] PREEMPT SMP NOPTI [ 3.720520] RIP: 0010:__skb_try_recv_from_queue+0x4e/0x1b0 [ 3.726645] RSP: 0018:ffffb03400e53ac8 EFLAGS: 00010046 [ 3.732470] RAX: ffff9040a78988c8 RBX: ffff904096bbef00 RCX: 000000000000000 [ 3.740441] RDX: 0000000000000000 RSI: ffff9040a78988c8 RDI: fff9040a7898800 [ 3.748411] RBP: ffffb03400e53ae8 R08: ffffb03400e53bf8 R09: fffb03400e53bfc [ 3.756382] R10: ffff904096989d00 R11: 0000000000000000 R12: fff9040a78988dc [ 3.764345] R13: ffff9040a78988c8 R14: 0000000000000202 R15: fffb03400e53ba8 [ 3.772305] FS: 00007feb1ef2f740(0000) GS:ffff9040bfd00000(0000) knlGS:0000000000000000 [ 3.781342] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 3.787757] CR2: 0000000000000008 CR3: 00000001e87fa000 CR4: 00000000003406a0 [ 3.795725] Call Trace: [ 3.798456] ? preempt_count_add+0x22/0x30 [ 3.803027] __skb_try_recv_datagram+0xe2/0x160 [ 3.808086] __skb_recv_datagram+0x8a/0xc0 [ 3.812657] skb_recv_datagram+0x3a/0x50 [ 3.817035] netlink_recvmsg+0x4e/0x3c0 [ 3.821315] ? copy_msghdr_from_user+0xcf/0x150 [ 3.826372] sock_recvmsg+0x3b/0x50 [ 3.830264] ___sys_recvmsg+0xd4/0x180 [ 3.834441] ? poll_select_copy_remaining+0x140/0x140 [ 3.840080] ? poll_select_copy_remaining+0x140/0x140 [ 3.845721] ? __switch_to_asm+0x34/0x70 [ 3.850098] ? __switch_to_asm+0x40/0x70 [ 3.854473] ? __switch_to_asm+0x34/0x70 [ 3.858849] ? __switch_to_asm+0x40/0x70 [ 3.863228] ? __switch_to_asm+0x34/0x70 [ 3.867604] ? __fget+0x71/0xa0 [ 3.871108] __sys_recvmsg+0x4c/0x90 [ 3.875097] ? __sys_recvmsg+0x4c/0x90 [ 3.879280] SyS_recvmsg+0x9/0x10 [ 3.882979] do_syscall_64+0x7e/0x350 [ 3.887064] ? trace_hardirqs_off_thunk+0x1a/0x1c [ 3.892315] entry_SYSCALL_64_after_hwframe+0x42/0xb7
It's caused by double kfree(). (Memory leak) Trying to access a already freed pointer. In the code: As it's a doubly linked list, the skb->next_pointer should point to the list. No problem with the code.