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

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Date:   Wed, 16 Jan 2019 07:34:30 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Josh Snyder <joshs@...flix.com>,
        Dave Chinner <david@...morbit.com>,
        Jiri Kosina <jikos@...nel.org>,
        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

Linus Torvalds wrote on Wed, Jan 16, 2019:
> 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..)

That seems to work on an x86_64 vm.

I've tested with the attached patch:
 - root can lookup pages on any file I tried;
 - user can lookup page on file it owns, assuming it can write to it
(e.g. it won't work on a 0400 file you own)
 - user cannot lookup pages on e.g. /lib64/libc-2.28.so

There is a difference with your previous patch though, that used to list
no page in core when it didn't know; this patch lists pages as in core
when it refuses to tell. I don't think that's very important, though.

If anything, the 0400 user-owner file might be a problem in some edge
case (e.g. if you're preloading git directories, many objects are 0444);
should we *also* check ownership?...

-- 
Dominique

View attachment "mincore.diff" of type "text/x-diff" (1198 bytes)