linux-kernel - Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

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Date:	Mon, 3 Mar 2008 16:11:55 +0100
From:	"Klaus S. Madsen" <ksm@...rnemadsen.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Suspend-devel list <suspend-devel@...ts.sourceforge.net>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

On Mon, Mar 03, 2008 at 13:17:35 +0100, Pavel Machek wrote:
> Hi!
> 
> > > > Klaus S. Madsen wrote:
> > > >> open("/dev/mem", O_RDWR)                = 5
> > > >> mmap2(NULL, 1282, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_SHARED|MAP_FIXED, 5, 0) = 0
> > > >> mmap2(0xa0000, 393216, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, 5, 0xa0) = 0xa0000
> > > >                          ^^^^^^^^^^^^^^^^^^^^
> > > >> close(5)                                = 0
> > > >> ioperm(0, 0x400, 0x1)                   = 0
> > > >> iopl(0x3)                               = 0
> > > >> access("/sys/bus/pci", R_OK)            = 0
> > > >> write(1, "Calling get_mode\n", 17)      = 17
> > > >> vm86(0x1, 0xb7f14ccc, 0xb7f14830, 0xc000, 0x18b6 <unfinished ...>
> > > >> --- SIGSEGV (Segmentation fault) @ 0 (0) ---
> > > >> +++ killed by SIGSEGV (core dumped) +++
> > > >
> > > > This is the VGA BIOS being mapped, it's mapped
> > > > PROT_READ|PROT_WRITE, but  no PROT_EXEC; if the kernel is
> > > > NX-capable it *should* segfault trying to execute out of this
> > > > area, which is exactly what will happen when vm86 executes INT
> > > > 10h.
> > > >
> > > > If we can find that mmap() in the s2ram source code and add
> > > > PROT_EXEC to it, it would be interesting.
> > > 
> > > Klaus, could you send your .config as well? Lets make sure that NX is 
> > > even relevant in this context.
> > Allright. The mmap in question is in the x86-common.c file in libx86,
> > and adding PROT_EXEC to it solves the problem.
> 
> Ok, sw should probably fix that in s2ram... can you mail a patch to
> me, and suspend-devel?
As stated above, the problem is actually in libx86, and not in s2ram. I
have included Matthew Garrett in the CC list, as he's hosting the libx86
source code I have used as a base for my patch.

The following patch solves the segfault, by changing the mmap flags of
the video memory area, to allow execution. The patch is against
libx86-0.99 available from http://www.codon.org.uk/~mjg59/libx86/

--- libx86-0.99/x86-common.c	2006-09-08 00:44:27.000000000 +0200
+++ libx86-0.99.new/x86-common.c	2008-03-01 10:08:25.000000000 +0100
@@ -232,7 +232,7 @@
 	}
 
 	m = mmap((void *)0xa0000, 0x100000 - 0xa0000,
-	 PROT_READ | PROT_WRITE,
+	 PROT_READ | PROT_WRITE | PROT_EXEC,
 	 MAP_FIXED | MAP_SHARED, fd_mem, 0xa0000);
 
 	if (m == (void *)-1) {

> > The only thing I don't understand is why this is suddenly a problem with
> > 2.6.25, and not with 2.6.24? Is there a bug in 2.6.24 and previously
> > that allows real-mode execution of non-executable pages?
> 
> It is strange indeed... Should it be traced as an regression?
I honestly don't know. From hpa's comments, it seems as is the segfault
would be the expected behaviour, but as 2.6.24 and previously doesn't
segfault, I guess it's a matter of determining if the original
non-segfaulting behaviour is by design, or a bug.

-- 
Kind regards,
	Klaus S. Madsen
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