lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Thu, 30 Apr 2020 17:39:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

On Thu, Apr 30, 2020 at 5:23 PM Andy Lutomirski <luto@...capital.net> wrote:
>
> > But anyway, I don't hate something like "copy_to_user_fallible()"
> > conceptually. The naming needs to be fixed, in that "user" can always
> > take a fault, so it's the _source_ that can fault, not the "user"
> > part.
>
> I don’t like this.  “user” already implied that basically anything can be wrong with the memory

Maybe I didn't explain.

"user" already implies faulting. We agree.

And since we by definition cannot know what the user has mapped into
user space, *every* normal copy_to_user() has to be able to handle
whatever faults that throws at us.

The reason I dislike "copy_to_user_fallible()" is that the user side
already has that 'fallible".

If it's the _source_ being "fallible" (it really needs a better name -
I will not call it just "f") then it should be "copy_f_to_user()".

That would be ok.

So "copy_f_to_user()" makes sense. But "copy_to_user_f()" does not.
That puts the "f" on the "user", which we already know can fault.

See what I want in the name? I want the name to say which side can
cause problems!

If you are copying memory from some f source, it must not be
"copy_safe()". That doesn't say if the _source_ is f, or the
destination is f.

So "copy_to_f()" makes sense (we don't say "copy_kernel_to_user()" -
the "normal" case is silent), and "copy_from_f()" makes sense.
"copy_in_f()" makes sense too.

But not this "randomly copy some randomly f memory area that I don't
know if it's the source or the destination".

Sometimes you may then use a common implementation for the different
directions - if that works on the architecture.

For example, "copy_to_user()" and "copy_from_user()" on x86 both end
up internally using a shared "copy_user_generic()" implementation. I
wish that wasn't the case (when I asked for what was to become
STAC/CLAC, I asked for one that could determine which direction of a
"rep movs" could touch user space), but it so happens that the
implementations end up being symmetric on x86.

But that's a pure implementation issue, and it very much is not going
to be true in general, and it shouldn't be exposed to users as such
(and we obviously don't).

                Linus

Powered by blists - more mailing lists