Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user

[Andy Lutomirski]

> On Feb 16, 2019, at 3:47 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 
>> On Sat, Feb 16, 2019 at 02:50:15PM -0800, Andy Lutomirski wrote:
>> 
>> What is the actual problem?  Weâre not actually demand-faulting this data, are we?  Are we just overrunning the buffer because the from_user helpers are too clever?  Can we fix it for real by having the fancy helpers do *aligned* loads so that they donât overrun the buffer?  Heck, this might be faster, too.
> 
> Unaligned _stores_ are not any cheaper, and you'd get one hell of
> extra arithmetics from trying to avoid both.  Check something
> like e.g. memcpy() on alpha, where you really have to keep all
> accesses aligned, both on load and on store side.

I think we should avoid unaligned loads and do unaligned stores instead.

I would general expect that unaligned stores are a bit cheaper since they donât need to complete for the comparisons to happen.

But I maintain my claim that this code should not be overrunning its input buffer into the next page, since it could have observable side effects.

> 
> Can't we just pad the buffers a bit?  Making sure that name_buf
> and symlink_buf are _not_ followed by unmapped pages shouldn't
> be hard.  Both are allocated by kmalloc(), so...
> 
> What am I missing here?