relay: handle alloc_percpu returning NULL in relay_open

hulk inclusion
category: bugfix
bugzilla: 13690
CVE: CVE-2019-19462

-------------------------------------------------

alloc_percpu() may return NULL, which means chan->buf may be set to
NULL. In that case, when we do *per_cpu_ptr(chan->buf, ...), we
dereference an invalid pointer:

BUG: Unable to handle kernel data access at 0x7dae0000
Faulting instruction address: 0xc0000000003f3fec
...
NIP [c0000000003f3fec] relay_open+0x29c/0x600
LR [c0000000003f3fc0] relay_open+0x270/0x600
Call Trace:
[c000000054353a70] [c0000000003f3fb4] relay_open+0x264/0x600 (unreliable)
[c000000054353b00] [c000000000451764] __blk_trace_setup+0x254/0x600
[c000000054353bb0] [c000000000451b78] blk_trace_setup+0x68/0xa0
[c000000054353c10] [c0000000010da77c] sg_ioctl+0x7bc/0x2e80
[c000000054353cd0] [c000000000758cbc] do_vfs_ioctl+0x13c/0x1300
[c000000054353d90] [c000000000759f14] ksys_ioctl+0x94/0x130
[c000000054353de0] [c000000000759ff8] sys_ioctl+0x48/0xb0
[c000000054353e20] [c00000000000bcd0] system_call+0x5c/0x68

Check if alloc_percpu returns NULL.

This was found by syzkaller both on x86 and powerpc, and the reproducer
it found on powerpc is capable of hitting the issue as an unprivileged
user.

https://lore.kernel.org/lkml/20191219121256.26480-1-dja%40axtens.net/


Fixes: 017c59c0 ("relay: Use per CPU constructs for the relay channel buffer pointers")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>