Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check

[Vineet Gupta]

On 1/14/20 1:22 PM, Linus Torvalds wrote:
> On Tue, Jan 14, 2020 at 12:09 PM Vineet Gupta
> <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
>>
>> This came up when switching ARC to word-at-a-time interface and using
>> generic/optimized strncpy_from_user
>>
>> It seems the existing code checks for user buffer/string range multiple
>> times and one of tem cn be avoided.
> 
> NO!
> 
> DO NOT DO THIS.
> 
> This is seriously buggy.
> 
>>  long strncpy_from_user(char *dst, const char __user *src, long count)
>>  {
>> -       unsigned long max_addr, src_addr;
>> -
>>         if (unlikely(count <= 0))
>>                 return 0;
>>
>> -       max_addr = user_addr_max();
>> -       src_addr = (unsigned long)untagged_addr(src);
>> -       if (likely(src_addr < max_addr)) {
>> -               unsigned long max = max_addr - src_addr;
>> +       kasan_check_write(dst, count);
>> +       check_object_size(dst, count, false);
>> +       if (user_access_begin(src, count)) {
> 
> You can't do that "user_access_begin(src, count)", because "count" is
> the maximum _possible_ length, but it is *NOT* necessarily the actual
> length of the string we really get from user space!
> 
> Think of this situation:
> 
>  - user has a 5-byte string at the end of the address space
> 
>  - kernel does a
> 
>      n = strncpy_from_user(uaddr, page, PAGE_SIZE)
> 
> now your "user_access_begin(src, count)" will _fail_, because "uaddr"
> is close to the end of the user address space, and there's not room
> for PAGE_SIZE bytes any more.

Oops indeed that was the case I didn't comprehend. In my initial tests with
debugger, every single hit on strncpy_from_user() had user addresses well into the
address space such that @max was ridiculously large (0xFFFF_FFFF - ptr) compared
to @count.

> But "count" isn't actually how many bytes we will access from user
> space, it's only the maximum limit on the *target*. IOW, it's about a
> kernel buffer size, not about the user access size.

Right I understood all that, but missed the case when user buffer is towards end
of address space and access_ok() will erroneously flag it.

> Because we'll only access that 5-byte string, which fits just fine in
> the user space, and doing that "user_access_begin(src, count)" gives
> the wrong answer.
> 
> The fact is, copying a string from user space is *very* different from
> copying a fixed number of bytes, and that whole dance with
> 
>         max_addr = user_addr_max();
> 
> is absolutely required and necessary.
> 
> You completely broke string copying.

I'm sorry and I wasn't sure to begin with hence the disclaimer in 0/4

> It is very possible that string copying was horribly broken on ARC
> before too - almost nobody ever gets this right, but the generic
> routine does.

No it is not. It is just dog slow since it does byte copy and uses the Zero delay
loops which I'm trying to get rid of. That's when I recalled the word-at-a-time
API which I'd meaning to go back to for last 7 years :-)

-Vineet