Re: [PATCH 2/2] staging/lustre/llite: fix ll_getname user buffer copy

[Oleg Drokin <green@linuxhacker.ru>]

On Jun 10, 2015, at 3:52 AM, Dan Carpenter wrote:

> On Wed, Jun 10, 2015 at 12:41:23AM -0400, green@linuxhacker.ru wrote:
>> From: Oleg Drokin <green@linuxhacker.ru>
>> 
>> strncpy_from_user could return negative values on error,
>> so need to take those into account.
>> Since ll_getname is used to get a single component name from userspace
>> to transfer to server as-is, there's no need to allocate 4k buffer
>> as done by __getname. Allocate NAME_MAX+1 buffer instead to ensure
>> we have enough for a null terminated max valid length buffer.
>> 
>> This was discovered by Al Viro in https://lkml.org/lkml/2015/4/11/243
>> 
>> Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
>> ---
>> drivers/staging/lustre/lustre/llite/dir.c | 20 +++++++++++---------
>> 1 file changed, 11 insertions(+), 9 deletions(-)
>> 
>> diff --git a/drivers/staging/lustre/lustre/llite/dir.c b/drivers/staging/lustre/lustre/llite/dir.c
>> index 87a042c..e0b9043 100644
>> --- a/drivers/staging/lustre/lustre/llite/dir.c
>> +++ b/drivers/staging/lustre/lustre/llite/dir.c
>> @@ -1213,29 +1213,31 @@ out:
>> 	return rc;
>> }
>> 
>> -static char *
>> -ll_getname(const char __user *filename)
>> +/* This function tries to get a single name component,
>> + * to send to the server. No actual path traversal involved,
>> + * so we limit to NAME_MAX */
>> +static char *ll_getname(const char __user *filename)
>> {
>> 	int ret = 0, len;
>> -	char *tmp = __getname();
>> +	char *tmp = kzalloc(NAME_MAX + 1, GFP_KERNEL);
> 
> Doing allocations in the declaration block is rare in the kernel but it
> accounts for around a quarter of the missing NULL checks and many memory
> leaks in the kbuild zero day bot testing.  It's a bad idea and some
> subsystems ban the practice, but Greg is fine with it so I'm not going
> to complain.

Fair. I can redo this.

>> 
>> 	if (!tmp)
>> 		return ERR_PTR(-ENOMEM);
>> 
>> -	len = strncpy_from_user(tmp, filename, PATH_MAX);
>> -	if (len == 0)
>> +	len = strncpy_from_user(tmp, filename, NAME_MAX);
>> +	if (len < 0)
>> +		ret = len;
>> +	else if (len == 0)
>> 		ret = -ENOENT;
>> -	else if (len > PATH_MAX)
>> -		ret = -ENAMETOOLONG;
> 
> I don't like how this does silent truncation.  strncpy_from_user()
> return -EFAULT if we run into unmapped memory.  Otherwise if the user
> supplies a too long name it returns len == PATH_MAX.  (I think, the
> documentation for this function is hard to understand).
> 
> Of course, the check was never true in the original code…

Right.
It's no big deal to ask for NAME_MAX+1 and then restore the len > NAME_MAX
check and return -ENAMETOOLONG in that case. Then the silent truncate
is gone and logic wise we don't care all that much either way, I imagine.

Bye,
    Oleg