linux-kernel - Re: vm86 segfaults with NX bit

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Message-ID: <s5hwsewawon.wl%tiwai@suse.de>
Date:	Sat, 22 Nov 2008 11:20:56 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Egbert Eich <eich@...e.de>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Luc Verhaegen <lverhaegen@...e.de>
Subject: Re: vm86 segfaults with NX bit

At Fri, 21 Nov 2008 18:42:04 +0100,
Egbert Eich wrote:
> 
> Takashi Iwai writes:
>  > At Thu, 20 Nov 2008 08:58:44 -0800,
>  > Arjan van de Ven wrote:
>  > > 
>  > > On Thu, 20 Nov 2008 16:39:06 +0100
>  > > Takashi Iwai <tiwai@...e.de> wrote:
>  > > 
>  > > > Hi,
>  > > > 
>  > > > X guys reported that VESA driver segfaults in vm86 mode with a kernel
>  > > > with CONFIG_X86_PAE, and it turned to be the NX bit.  See Novell
>  > > > bugzilla #443440:
>  > > > 	https://bugzilla.novell.com/show_bug.cgi?id=443440
>  > > > 
>  > > > I can confirm that the latest 2.6.28-rc still causes the same problem,
>  > > > and it disappears when booted with noexec=off option.
>  > > > 
>  > > > Also, when NX bit is reset for the first 1MB (or smaller) in
>  > > > do_sys_vm86(), it also works as expected.  But I have little clue
>  > > > where to restore the bit again.
>  > > > 
>  > > > Any good suggestions / solutions?
>  > > > 
>  > > 
>  > > did the code that mapped this memory setup use PROT_EXEC ?
>  > 
>  > A good question...
>  > Luc, Egbert, how does VESA driver handle it?
> 
> 
> Yes, we set the PROT_EXEC for the VBIOS. But the segfault doesn't happen
> in VBIOS as far as I can tell. It happens on the 'exit instruction', a
> hlt (0xf6) which is trapped by vm86 so it returns to the caller.
> This is done by putting 0xf6 into address 0x600 and putting this address
> on the stack.
> The memory that's mapped at 0x600 in the Xserver's address space is
> allocated thru shmget() and attached to the right place in the processes
> memory space for vm86.
> We should call mprotect() on these ranges to set this memory executable.
> Hey, this code was orginally written almost 10 years ago!
> 
> The patch below fixes the issue.

Good to know that a die-hard bug is fixed now.
Thanks!


Takashi
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