Re: xt_hashlimig build error (was Re: [RFC 01/17] x86/asm/64: Remove the restore_c_regs_and_iret label)

[Vishwanath Pai]

On 09/07/2017 04:45 PM, Linus Torvalds wrote:
> On Thu, Sep 7, 2017 at 1:16 PM, Vishwanath Pai <vpai@xxxxxxxxxx> wrote:
>>
>> Writing U32INT_MAX as 0xFFFFFFFFULL was a mistake on my part. I could
>> have avoided all of this by using built-in constants instead of trying
>> to define them myself. I will rewrite the function as below and send out
>> another patch:
>>
>> static u64 user2rate_bytes(u64 user)
>> {
>>         u64 r;
>>
>>         r = user ? U32_MAX / (u32) user : U32_MAX;
>>         r = (r - 1) << XT_HASHLIMIT_BYTE_SHIFT;
>>         return r;
>> }
> 
> No, that is *still* wrong.
> 
> In particular, the test for "user" being zero is done in 64 bits, but
> then when you do the divide, the cast to (u32) will take the low 32
> bits - which may be zero, because only upper bits were set.
> 
> So now you get a divide-by-zero.
> 
> What seems to be going on is that a value larger than UINT32_MAX is
> basically "invalid", since the reverse function cannot possibly
> generate that.
> 
> So one possible fix is to just make that an error case in the caller,
> and then make user2rate_bytes() not take (or return) "u64" at all, but
> simply use u32.
> 
> Please be more careful here.
> 
>               Linus
> 

Yes, that is true. Thanks for pointing it out. I will change the user
param to 'u32', and also change the return type to u32 as well. I will
add a check in hashlimit_mt_check() to make sure the userspace never
sends anything > U32_MAX and error out if they do.

Thanks,
Vishwanath