Re: EFAULT reading /dev/mem... - broken x86info

[Manfred Spraul]

Petr Vandrovec wrote:

> On Mon, Nov 10, 2003 at 06:17:37PM +0100, Manfred Spraul wrote:
>  
>
> > DEBUG_PAGEALLOC unmaps pages on kmem_cache_free and __free_pages(). The 
> > pages are mapped again during get_free_pages and kmem_cache_alloc.
> >
> > 0x86000 looks like a normal page - what guarantees that it's not used by 
> > the kernel?
> >    
>
> With DEBUG_PAGEALLOC there is no problem with page if it is USED by the kernel.
> Problem is if page is NOT USED - in this case kernel does not have it in its
> mapping, and bad thing happen.
>  
If the page is used by AGP, then it won't have a mapping either.

> x86info (and other simillar tools for dumping different BIOS structures) just
> scans physical memory for some well known signatures - hoping that kernel did
> not smash these structures.
>  
Scanning physical memory is very dangerous: it's undefined what happens 
if a page is mapped multiple times with different cache settings. Athlon 
cpus prefetch whatever they see, and they speculatively set the dirty 
bit in cachelines for WB cacheable pages.

> Up to now it was possible to read whole physical memory from userspace - some 
> pages by reading /dev/mem, some pages by mmaping /dev/mem. Now it is not possible 
> anymore - which I think is bad, as /dev/mem has semantic of it.
I think /dev/mem should lie and return 0x00 for pages that are not in 
the linear mapping.

Or /dev/mem users should expect random holes.

--
   Manfred

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/