Re: vm86 segfaults with NX bit

From: Takashi Iwai
Date: Sat Nov 22 2008 - 05:21:16 EST


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@xxxxxxx> 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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