Re: Really good idea to allow mmap(0, FIXED)?

[Arjan van de Ven <arjan@infradead.org>]

On Thu, 2006-10-05 at 23:55 +0000, David Wagner wrote:
> Michael Buesch  wrote:
> >Is is really a good idea to allow processes to remap something
> >to address 0?
> >I say no, because this can potentially be used to turn rather harmless
> >kernel bugs into a security vulnerability.
> 
> Let me see if I understand.  If the kernel does this somewhere:
> 
>     struct s *foo;
>     foo->x->y = 0;
> 
> and if there is some way that userland code can cause this to be
> executed with 'foo' set to a NULL pointer, then user-land code can
> do this:
> 
>     mmap(0, 4096, PROT_READ|PROT_EXEC|PROT_WRITE,
>         MAP_FIXED|MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
>     struct s *bar = 0;

the question isn't if it's a good idea to allow mmap(0) but to allow
mmap PROT_WRITE | PROT_EXEC !


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