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SubjectRe: general protection fault in sockfs_setattr
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Pastebin says that it was 4.17.0-rc4+ rather than 4.17-rc7.
I suggest reporting to Al Viro and linux-fsdevel ML after
confirming that this bug still happens with linux.git , in
case this is a dentry related bug (e.g. someone is by error
calling dput() without getting a refcount).

Also, please don't eliminate kernel messages prior to the
crash. Sometimes previous kernel messages (e.g. memory
allocation fault injection) as-is indicate the cause.

On 2018/06/06 11:19, shankarapailoor wrote:
> Hi Cong,
>
> I added that check and it seems to stop the crash. Like you said, I
> don't see where the reference count for the file is increased. The
> inode lock also seems to be held during this call.
>
> Regards,
> Shankara
>
>
>
> On Tue, Jun 5, 2018 at 12:14 PM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>> On Mon, Jun 4, 2018 at 9:53 PM, shankarapailoor
>> <shankarapailoor@gmail.com> wrote:
>>> Hi,
>>>
>>> I have been fuzzing Linux 4.17-rc7 with Syzkaller and found the
>>> following crash: https://pastebin.com/ixX3RB9j
>>>
>>> Syzkaller isolated the cause of the bug to the following program:
>>>
>>> socketpair$unix(0x1, 0x1, 0x0,
>>> &(0x7f0000000000)={<r0=>0xffffffffffffffff, <r1=>0xffffffffffffffff})
>>> getresuid(&(0x7f0000000080)=<r2=>0x0, &(0x7f00000000c0),
>>> &(0x7f0000000700))r3 = getegid()
>>> fchownat(r0, &(0x7f0000000040)='\x00', r2, r3, 0x1000)
>>> dup3(r1, r0, 0x80000)
>>>
>>>
>>> The problematic area appears to be here:
>>>
>>> static int sockfs_setattr(struct dentry *dentry, struct iattr *iattr)
>>> {
>>> int err = simple_setattr(dentry, iattr);
>>>
>>> if (!err && (iattr->ia_valid & ATTR_UID)) {
>>> struct socket *sock = SOCKET_I(d_inode(dentry));
>>>
>>> sock->sk->sk_uid = iattr->ia_uid; //KASAN GPF
>>> }
>>> return err;
>>> }
>>>
>>> If dup3 is called concurrently with fchownat then can sock->sk be NULL?
>>
>> Although dup3() implies a close(), fd is refcnt'ted, if dup3() runs
>> concurrently with fchownat() it should not be closed until whoever
>> the last closes it.
>>
>> Or maybe fchownat() doesn't even hold refcnt of fd, since it aims
>> to change the file backed.
>>
>>
>> Not sure if the following is sufficient, inode might need to be protected
>> with some lock...
>>
>> diff --git a/net/socket.c b/net/socket.c
>> index f10f1d947c78..6294b4b3132e 100644
>> --- a/net/socket.c
>> +++ b/net/socket.c
>> @@ -537,7 +537,10 @@ static int sockfs_setattr(struct dentry *dentry,
>> struct iattr *iattr)
>> if (!err && (iattr->ia_valid & ATTR_UID)) {
>> struct socket *sock = SOCKET_I(d_inode(dentry));
>>
>> - sock->sk->sk_uid = iattr->ia_uid;
>> + if (sock->sk)
>> + sock->sk->sk_uid = iattr->ia_uid;
>> + else
>> + err = -ENOENT;
>> }
>>
>> return err;
>
>
>

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