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From: Christoph Hellwig <hch@lst.de>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-arch@vger.kernel.org, Nick Hu <nickhu@andestech.com>,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Greentime Hu <green.hu@gmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vincent Chen <deanbo422@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user_check
Date: Tue, 21 Jul 2020 07:20:22 +0200	[thread overview]
Message-ID: <20200721052022.GA10011@lst.de> (raw)
In-Reply-To: <eb470677-b569-a6f0-e63b-60149b54863a@roeck-us.net>

On Mon, Jul 20, 2020 at 10:15:37PM -0700, Guenter Roeck wrote:
> >> -       if (CHECK_DATA_CORRUPTION(uaccess_kernel(),
> >> +       if (CHECK_DATA_CORRUPTION(!uaccess_kernel(),
> >>
> >> How does this work anywhere ?
> > 
> > No, that is the wrong check - we want to make sure the address
> > space override doesn't leak to userspace.  The problem is that
> > armnommu (and m68knommu, but that doesn't call the offending
> > function) pretends to not have a kernel address space, which doesn't
> > really work.  Here is the fix I sent out yesterday, which I should
> > have Cc'ed you on, sorry:
> > 
> 
> The patch below makes sense, and it does work, but I still suspect
> that something with your original patch is wrong, or at least suspicious.
> Reason: My change above (Adding the "!") works for _all_ of my arm boot
> tests. Or, in other words, it doesn't make a difference if true
> or false is passed as first parameter of CHECK_DATA_CORRUPTION(), except
> for nommu systems. Also, unless I am really missing something, your
> original patch _does_ reverse the logic.

Well.  segment_eq is in current mainline used in two places:

 1) to implement uaccess_kernel
 2) in addr_limit_user_check to implement uaccess_kernel-like
    semantics using a strange reverse notation

I think the explanation for your observation is how addr_limit_user_check
is called on arm.  The addr_limit_check_failed wrapper for it is called
from assembly code, but only after already checking the addr_limit,
basically duplicating the segment_eq check.  So for mmu builds it won't
get called unless we leak the kernel address space override, which
is a pretty fatal error and won't show up in your boot tests.  The
only good way to test it is by explicit injecting it using the
lkdtm module.

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  reply	other threads:[~2020-07-21  5:20 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-14 10:54 clean up address limit helpers v2 Christoph Hellwig
2020-07-14 10:55 ` [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user_check Christoph Hellwig
2020-07-18  1:38   ` Guenter Roeck
2020-07-18  9:48     ` Christoph Hellwig
2020-07-18 14:54       ` Guenter Roeck
2020-07-20 22:10       ` Guenter Roeck
2020-07-21  4:58         ` Christoph Hellwig
2020-07-21  5:15           ` Guenter Roeck
2020-07-21  5:20             ` Christoph Hellwig [this message]
2020-07-21  5:30               ` Guenter Roeck
2020-07-21  5:35                 ` Christoph Hellwig
2020-07-14 10:55 ` [PATCH 2/6] nds32: use uaccess_kernel in show_regs Christoph Hellwig
2020-07-14 10:55 ` [PATCH 3/6] riscv: include <asm/pgtable.h> in <asm/uaccess.h> Christoph Hellwig
2020-07-14 10:55 ` [PATCH 4/6] uaccess: remove segment_eq Christoph Hellwig
2020-07-14 15:27   ` Linus Torvalds
2020-07-14 10:55 ` [PATCH 5/6] uaccess: add force_uaccess_{begin,end} helpers Christoph Hellwig
2020-07-14 15:29   ` Linus Torvalds
2020-07-14 10:55 ` [PATCH 6/6] exec: use force_uaccess_begin during exec and exit Christoph Hellwig
2020-07-15  3:33   ` Eric W. Biederman
2020-07-15  6:06     ` Christoph Hellwig
2020-07-16 23:49 ` clean up address limit helpers v2 Andrew Morton
2020-07-17  6:06   ` Christoph Hellwig
2020-07-20 15:54 ` [PATCH 0/6] arm: don't call addr_limit_user_check for nommu Christoph Hellwig
  -- strict thread matches above, loose matches on Subject: below --
2020-07-10 13:57 clean up address limit helpers Christoph Hellwig
2020-07-10 13:57 ` [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user_check Christoph Hellwig

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