linux-kernel - Re: [PATCH] mm/mincore: allow for making sys

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Message-ID: <nycvar.YFH.7.76.1901161710470.6626@cbobk.fhfr.pm>
Date:   Wed, 16 Jan 2019 17:12:24 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Josh Snyder <joshs@...flix.com>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Wed, 16 Jan 2019, Linus Torvalds wrote:

> > "Being owner or has cap" (whichever cap) is probably OK. On the other 
> > hand, writeability check makes more sense in general - could we 
> > somehow check if the user has write access to the file instead of 
> > checking if it currently is opened read-write?
> 
> That's likely the best option. We could say "is it open for write, or
> _could_ we open it for writing?"
> 
> It's a slightly annoying special case, and I'd have preferred to avoid
> it, but it doesn't sound *compilcated*.
> 
> I'm on the road, but I did send out this:
> 
>     https://lore.kernel.org/lkml/CAHk-=wif_9nvNHJiyxHzJ80_WUb0P7CXNBvXkjZz-r1u0ozp7g@mail.gmail.com/
> 
> originally. The "let's try to only do the mmap residency" was the
> optimistic "maybe we can just get rid of this complexity entirely"
> version..
> 
> Anybody willing to test the above patch instead? And replace the
> 
>    || capable(CAP_SYS_ADMIN)
> 
> check with something like
> 
>    || inode_permission(inode, MAY_WRITE) == 0
> 
> instead?
> 
> (This is obviously after you've reverted the "only check mmap
> residency" patch..)

So that seems to deal with mincore() in a reasonable way indeed.

It doesn't unfortunately really solve the preadv2(RWF_NOWAIT), nor does it 
provide any good answer what to do about it, does it?

Thanks,

-- 
Jiri Kosina
SUSE Labs