Re: [PATCH] relay: handle alloc_percpu returning NULL in relay_open

[Daniel Axtens <dja@axtens.net>]

Guenter Roeck <linux@roeck-us.net> writes:

> On Fri, Nov 29, 2019 at 12:37:45PM +1100, Daniel Axtens wrote:
>> 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. Because we can readily catch and
>> handle this situation, switch to alloc_cpu_gfp and pass in __GFP_NOWARN.
>> 
>> 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.
>> 
>> Fixes: 017c59c042d0 ("relay: Use per CPU constructs for the relay channel buffer pointers")
>> Reported-by: syzbot+1e925b4b836afe85a1c6@syzkaller-ppc64.appspotmail.com
>> Reported-by: syzbot+587b2421926808309d21@syzkaller-ppc64.appspotmail.com
>> Reported-by: syzbot+58320b7171734bf79d26@syzkaller.appspotmail.com
>> Reported-by: syzbot+d6074fb08bdb2e010520@syzkaller.appspotmail.com
>> Cc: Akash Goel <akash.goel@intel.com>
>> Cc: Andrew Donnellan <ajd@linux.ibm.com> # syzkaller-ppc64
>> Cc: stable@vger.kernel.org # v4.10+
>> Signed-off-by: Daniel Axtens <dja@axtens.net>
>> 
>
> So there is a CVE now, but it appears that the patch went nowhere.
> Are there any plans to actually apply it ?

I sent a v2 that addresses some review comments, I guess if anything is
applied it will be that.

Daniel

>
> Thanks,
> Guenter
>
>> --
>> 
>> There's a syz reproducer on the powerpc syzbot that eventually hits
>> the bug, but it can take up to an hour or so before it keels over on a
>> kernel with all the syzkaller debugging on, and even longer on a
>> production kernel. I have been able to reproduce it once on a stock
>> Ubuntu 5.0 ppc64le kernel.
>> 
>> I will ask MITRE for a CVE - while only the process doing the syscall
>> gets killed, it gets killed while holding the relay_channels_mutex,
>> so it blocks all future relay activity.
>> ---
>>  kernel/relay.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/relay.c b/kernel/relay.c
>> index ade14fb7ce2e..a376cc6b54ec 100644
>> --- a/kernel/relay.c
>> +++ b/kernel/relay.c
>> @@ -580,7 +580,13 @@ struct rchan *relay_open(const char *base_filename,
>>  	if (!chan)
>>  		return NULL;
>>  
>> -	chan->buf = alloc_percpu(struct rchan_buf *);
>> +	chan->buf = alloc_percpu_gfp(struct rchan_buf *,
>> +				     GFP_KERNEL | __GFP_NOWARN);
>> +	if (!chan->buf) {
>> +		kfree(chan);
>> +		return NULL;
>> +	}
>> +
>>  	chan->version = RELAYFS_CHANNEL_VERSION;
>>  	chan->n_subbufs = n_subbufs;
>>  	chan->subbuf_size = subbuf_size;
>> -- 
>> 2.20.1
>>