linux-kernel - Re: [RFC][PATCH 01/22] x86 user stack frame reads: switch to explicit __get

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Message-ID: <20200329092602.GB93574@gmail.com>
Date:   Sun, 29 Mar 2020 11:26:02 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC][PATCH 01/22] x86 user stack frame reads: switch to
 explicit __get_user()


* Al Viro <viro@...iv.linux.org.uk> wrote:

> > but the __get_user() API doesn't carry the 'unsafe' tag yet.
> > 
> > Should we add an __unsafe_get_user() alias to it perhaps, and use it 
> > in all code that adds it, like the chunk above? Or rename it to 
> > __unsafe_get_user() outright? No change to the logic, but it would be 
> > more obvious what code has inherited old __get_user() uses and which 
> > code uses __unsafe_get_user() intentionally.
> > 
> > Even after your series there's 700 uses of __get_user(), so it would 
> > make sense to make a distinction in name at least and tag all unsafe 
> > APIs with an 'unsafe_' prefix.
> 
> "unsafe" != "lacks access_ok", it's "done under user_access_begin".

Well, I thought the principle was that we'd mark generic APIs that had 
*either* a missing access_ok() check or a missing 
user_access_begin()/end() wrapping marked unsafe_*(), right?

__get_user() has __uaccess_begin()/end() on the inside, but doesn't have 
the access_ok() check, so those calls are 'unsafe' with regard to not 
being safe to untrusted (ptr,size) ranges.

I agree that all of these topics need equal attention:

 - leaking of cleared SMAP state (CLAC), which results in a silent 
   failure.

 - running user accesses without STAC, which results in a crash.

 - not doing an access_ok() check on untrusted (pointer,size) ranges, 
   which results in a silent failure as well.

I just think that any API that doesn't guarantee all of these are handled 
right probably needs to be unsafe_*() tagged.

> FWIW, with the currently linearized part I see 26 users in arch/x86 and 
> 108 - outside of arch/*.  With 43 of the latter supplied by the sodding 
> comedi_compat32.c, which needs to be rewritten anyway (or git rm'ed, 
> for that matter)...
> 
> We'll get there; the tricky part is the ones that come in pair with 
> something other than access_ok() in the first place (many of those are 
> KVM-related, but not all such are).
> 
> This part had been more about untangling uaccess_try stuff,,,

It's much appreciated! In my previous mail I just wanted to inquire about 
the long term plan, whether we are going to get rid of all uses of 
__get_user() - to which the answer appears to be "yes". :-)

Thanks,

	Ingo