Date: Wed, 7 Sep 2011 13:43:35 +0400 From: Solar Designer <solar@...nwall.com> To: kernel-hardening@...ts.openwall.com Subject: Re: [RFC] x86, mm: start mmap allocation for libs from low addresses Vasiliy, On Wed, Sep 07, 2011 at 01:34:11PM +0400, Vasiliy Kulikov wrote: > On Wed, Sep 07, 2011 at 13:30 +0400, Solar Designer wrote: > > What if mmap_min_addr set really low, or is even 0? I think we want to > > skip low addresses even if processes are permitted to use those. > > (Permitted does not mean encouraged.) So how about ASCII_ARMOR_MIN_ADDR > > 0x19000 (100 KB) when !CONFIG_VM86? > > Are you talking about safety with NULL pointer dereferencing? Yes. Besides deliberate attacks, we want inadvertent NULL pointer dereferences to be detected by the kernel as such. This wouldn't happen if the kernel itself happens to map something at NULL in almost every process just because someone set mmap_min_addr to 0 e.g. to be able to occasionally run a certain userspace app that needs that. Then, there may be attack scenarios where the attacker is not able to invoke mmap() in the process via which the attack on the kernel is initiated - e.g., some daemon process over which the attacker has only partial control over a certain protocol. Thus, low mmap_min_addr wouldn't be directly exploitable by the attacker, and we shouldn't be making matters worse with this patch. Alexander
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