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From: Borislav Petkov <bp@alien8.de>
To: "Luck, Tony" <tony.luck@intel.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Andy Lutomirski" <luto@kernel.org>,
	"Aili Yao" <yaoaili@kingsoft.com>,
	"HORIGUCHI NAOYA( 堀口 直也)" <naoya.horiguchi@nec.com>
Subject: Re: [PATCH 3/4] mce/copyin: fix to not SIGBUS when copying from user hits poison
Date: Thu, 8 Apr 2021 10:49:58 +0200	[thread overview]
Message-ID: <20210408084958.GC10192@zn.tnic> (raw)
In-Reply-To: <20210407214310.GA479383@agluck-desk2.amr.corp.intel.com>

On Wed, Apr 07, 2021 at 02:43:10PM -0700, Luck, Tony wrote:
> On Wed, Apr 07, 2021 at 11:18:16PM +0200, Borislav Petkov wrote:
> > On Thu, Mar 25, 2021 at 05:02:34PM -0700, Tony Luck wrote:
> > > Andy Lutomirski pointed out that sending SIGBUS to tasks that
> > > hit poison in the kernel copying syscall parameters from user
> > > address space is not the right semantic.
> > 
> > What does that mean exactly?
> 
> Andy said that a task could check a memory range for poison by
> doing:
> 
> 	ret = write(fd, buf, size);
> 	if (ret == size) {
> 		memory range is all good
> 	}
> 
> That doesn't work if the kernel sends a SIGBUS.
> 
> It doesn't seem a likely scenario ... but Andy is correct that
> the above ought to work.

We need to document properly what this is aiming to fix. He said
something yesterday along the lines of kthread_use_mm() hitting a SIGBUS
when a kthread "attaches" to an address space. I'm still unclear as to
how exactly that happens - there are only a handful of kthread_use_mm()
users in the tree...

> Yes. This is for kernel reading memory belongng to "current" task.

Provided "current" is really the task to which the poison page belongs.
That kthread_use_mm() thing sounded like the wrong task gets killed. But that
needs more details.

> Same in that the page gets unmapped. Different in that there
> is no SIGBUS if the kernel did the access for the user.

What is even the actual use case with sending tasks SIGBUS on poison
consumption? KVM? Others?

Are we documenting somewhere: "if your process gets a SIGBUS and this
and that, which means your page got offlined, you should do this and
that to recover"?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette


  reply	other threads:[~2021-04-08  8:50 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-26  0:02 [RFC 0/4] Fix machine check recovery for copy_from_user Tony Luck
2021-03-26  0:02 ` [PATCH 1/4] x86/mce: Fix copyin code to return -EFAULT on machine check Tony Luck
2021-04-06 19:24   ` Borislav Petkov
2021-03-26  0:02 ` [PATCH 2/4] mce/iter: Check for copyin failure & return error up stack Tony Luck
2021-03-26  0:02 ` [PATCH 3/4] mce/copyin: fix to not SIGBUS when copying from user hits poison Tony Luck
2021-04-07 21:18   ` Borislav Petkov
2021-04-07 21:43     ` Luck, Tony
2021-04-08  8:49       ` Borislav Petkov [this message]
2021-04-08 17:08         ` Luck, Tony
2021-04-13 10:07           ` Borislav Petkov
2021-04-13 16:13             ` Luck, Tony
2021-04-14 13:05               ` Borislav Petkov
2021-03-26  0:02 ` [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery Tony Luck
2021-04-08 13:36   ` Borislav Petkov
2021-04-08 16:06     ` Luck, Tony
2021-04-08  2:13 ` [RFC 0/4] Fix machine check recovery for copy_from_user Aili Yao
2021-04-08 14:39   ` Luck, Tony
2021-04-09  6:49     ` Aili Yao
2021-04-14  5:47 [PATCH 3/4] mce/copyin: fix to not SIGBUS when copying from user hits poison Jue Wang
2021-04-14 13:10 ` Borislav Petkov
2021-04-14 14:46   ` Jue Wang
2021-04-14 15:35     ` Borislav Petkov
2021-04-19 20:32 Jue Wang

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