Problem with set_memory_rw

Problem with set_memory_rw

[Oleg Kutkov]

Hello, this is my first post in a mailing list, so excuse me if 
something wrong...
I'm new in kernel and try to do some manipulation with memory pages.
For manipulating memory page attributes i used set_memory_rw(),
but function returning 0 and i can't continue writing to memory,
because page, that i want to change, is read only by default.
This is part of my code:


long unsigned addr;

addr = 0x0509940;              //this is addres in memory, that i  want 
to overwrite (read only, for default)

set_memory_rw(addr, 1);    //try to set rw permission on this addres for 
one memory page.


So, function return zero, this is error, as i can understand. If i try 
to write by this address - kernel write in log error messages, such as 
"Unable to handle kernel request at address 0x0509940".
This is kernel bug or something wrong in my code?

Thank for any help.


Best regards,
Oleg.

Re: Problem with set_memory_rw

[KAMEZAWA Hiroyuki]

On Thu, 04 Feb 2010 07:26:25 +0200
Oleg Kutkov <elenbert@gmail.com> wrote:

> Hello, this is my first post in a mailing list, so excuse me if 
> something wrong...
> I'm new in kernel and try to do some manipulation with memory pages.
> For manipulating memory page attributes i used set_memory_rw(),
> but function returning 0 and i can't continue writing to memory,
> because page, that i want to change, is read only by default.
> This is part of my code:
> 
> 
> long unsigned addr;
> 
> addr = 0x0509940;              //this is addres in memory, that i  want 
> to overwrite (read only, for default)
> 
> set_memory_rw(addr, 1);    //try to set rw permission on this addres for 
> one memory page.
> 
> 
> So, function return zero, this is error, as i can understand. If i try 
> to write by this address - kernel write in log error messages, such as 
> "Unable to handle kernel request at address 0x0509940".
> This is kernel bug or something wrong in my code?
> 
> Thank for any help.
> 

set_memory_rw()'s 1st argument requires virtual address.
So, you need virtual address of the page you want.
 
  - phys_to_virt() ... convert physical address to virtual address.
  - virt_to_phys() ... convert virtual address to physical.

Anyway, RW is vitual address mapping's attribute and not for physical.

Thanks,
-Kame

Fwd: Problem with set_memory_rw

[Oleg Kutkov]

set_memory_rw()'s 1st argument requires virtual address.
So, you need virtual address of the page you want.

 - phys_to_virt() ... convert physical address to virtual address.
 - virt_to_phys() ... convert virtual address to physical.

Anyway, RW is vitual address mapping's attribute and not for physical.

Thanks,
-Kame



Thank for answer!
But this is a very strange, because 0x0509940 - it a virtual memory
address (i got it from System.map, this is a system call table, on my
machine). set_memory_rw return zero, anyway. Maybe, system call table
is much write protected, so i can't change attribute of memory page?
One more interesting thing:



struct page *pg;
pg = virt_to_page(addr);
unsigned long page_addr;
page_addr = (unsigned long) page_address(pg);


addr - this is my virtual address (provided by System.map)
But page_addr got another value!
What wrong?
Sorry, if my questions is to stupid.

P.S. I know, that overwriting system call table is very bad, i just
experimenting with my own network drivers and i need to replace some
network system calls..


Best regard,
Oleg.

Re: Fwd: Problem with set_memory_rw

[KAMEZAWA Hiroyuki]

On Thu, 4 Feb 2010 10:09:57 +0200
Oleg Kutkov <elenbert@gmail.com> wrote:
> Thank for answer!
> But this is a very strange, because 0x0509940 - it a virtual memory
> address (i got it from System.map, this is a system call table, on my
> machine). set_memory_rw return zero, anyway. Maybe, system call table
> is much write protected, so i can't change attribute of memory page?
> One more interesting thing:
> 
> 
> 
> struct page *pg;
> pg = virt_to_page(addr);
> unsigned long page_addr;
> page_addr = (unsigned long) page_address(pg);
> 
> 
> addr - this is my virtual address (provided by System.map)
> But page_addr got another value!
> What wrong?
> Sorry, if my questions is to stupid.
> 

Below is only about virt_to_page() etc... (not about set_memory_rw())
Maybe my answer for set_memory_rw() was pointless.

I think system call table is on .rodata section and set_memory_rw() doesn't
allow change attributes on .rodata sections(not .text)
..I'm sorry if I'm wrong.

==
At first, Linux maps physical memory in linear mapping. virt_to_page(), 
page_address()..etc works for this linear mapping area.

AFAIK, in some arch?, kernel's text area is on outside of linear mapping area.

For example, ia64 has
  0xa00xxxxxxxxxxxxxxxxxx  for kernel text.
  0xe000xxxxxxxxxxxxxxxxx  for linear mapping.

Another example, x86 has
  0xffffffff80000000       for kernel text.
  0xffff880000000000       for linear mapping's base address.

I know base address of linear mapping area is represented as __PAGE_OFFSET
in many archs. please check.

Anyway, "what 0x0509940 is" depends on your arch. I think.
Maybe it's good to investigate how __pa() and __va() is implemented on your arch.


Thanks,
-Kame

Re: Fwd: Problem with set_memory_rw

[Nick Piggin]

On Thu, Feb 04, 2010 at 05:51:33PM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 4 Feb 2010 10:09:57 +0200
> Oleg Kutkov <elenbert@gmail.com> wrote:
> > Thank for answer!
> > But this is a very strange, because 0x0509940 - it a virtual memory
> > address (i got it from System.map, this is a system call table, on my
> > machine). set_memory_rw return zero, anyway. Maybe, system call table
> > is much write protected, so i can't change attribute of memory page?
> > One more interesting thing:
> > 
> > 
> > 
> > struct page *pg;
> > pg = virt_to_page(addr);
> > unsigned long page_addr;
> > page_addr = (unsigned long) page_address(pg);
> > 
> > 
> > addr - this is my virtual address (provided by System.map)
> > But page_addr got another value!
> > What wrong?
> > Sorry, if my questions is to stupid.
> > 
> 
> Below is only about virt_to_page() etc... (not about set_memory_rw())
> Maybe my answer for set_memory_rw() was pointless.
> 
> I think system call table is on .rodata section and set_memory_rw() doesn't
> allow change attributes on .rodata sections(not .text)
> ..I'm sorry if I'm wrong.

You are right. static_protections() forbids rodata from being marked RW.