Re: + sys_getppid-oopses-on-debug-kernel.patch added to -mm tree

[Björn Steinbrink]

On 2006.08.08 17:39:37 +0300, Muli Ben-Yehuda wrote:
> On Tue, Aug 08, 2006 at 07:32:51AM -0700, akpm@xxxxxxxx wrote:
> > 
> > The patch titled
> > 
> >      sys_getppid() oopses on debug kernel
> > 
> > has been added to the -mm tree.  Its filename is
> > 
> >      sys_getppid-oopses-on-debug-kernel.patch
> > 
> > See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
> > out what to do about this
> > 
> > ------------------------------------------------------
> > Subject: sys_getppid() oopses on debug kernel
> > From: Kirill Korotaev <dev@xxxxx>
> > 
> > sys_getppid() optimization can access a freed memory.  On kernels with
> > DEBUG_SLAB turned ON, this results in Oops.
> > 
> > Signed-off-by: Kirill Korotaev <dev@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
> 
> I'm probably missing something, but is it really valid to access freed
> kernel memory even if CONFIG_DEBUG_SLAB is off - as this patch does?

There's a note right above the function that explains it:
 * NOTE! This depends on the fact that even if we _do_
 * get an old value of "parent", we can happily dereference
 * the pointer (it was and remains a dereferencable kernel pointer
 * no matter what): we just can't necessarily trust the result
 * until we know that the parent pointer is valid.

HTH
Björn
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