Messages in this thread | | | Subject | Re: [PATCH] dma-mapping: Add BUG_ON for uninitialized dma_ops | From | James Bottomley <> | Date | Fri, 14 Jun 2013 09:14:43 -0700 |
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On Fri, 2013-06-14 at 16:36 +0200, Arnd Bergmann wrote: > On Thursday 13 June 2013, James Bottomley wrote: > > On Wed, 2013-06-12 at 17:06 +0200, Arnd Bergmann wrote: > > > On Tuesday 11 June 2013, James Bottomley wrote: > > > > Really, no, it's not a good idea at all. It invites tons of patches > > > > littering the code with BUG_ONs where we might possibly get a NULL > > > > dereference. All it does is add extra instructions to a code path for > > > > no actual benefit. > > > > > > > > If you can answer the question: what more information does the BUG_ON > > > > give you than the NULL deref Oops would not? then it might be > > > > reasonable. > > > > > > The question is if a user can trigger the NULL dereference intentionally, > > > in which case they might get the kernel to jump into a user-provided > > > buffer. > > > > Can you elaborate on how they could do this? If you're thinking they > > could alter the pointer and trigger the jump, then yes, but a BUG_ON > > won't prevent that because the altered pointer won't be NULL. > > The attack that has been demonstrated a couple of times uses an anomymous > mmap to virtual address 0. You fill that page with pointers to a > function in your program. If there is a NULL pointer to some operations > structure and kernel code calls an operation without checking the > ops pointer first, it gets read from the NULL page and the kernel > jumps into user space.
This is the MMAP_PAGE_ZERO exploit. The original exploit relied on a leaky personality capability clearing mask and was fixed in 2.6.31 by
commit f9fabcb58a6d26d6efde842d1703ac7cfa9427b6 Author: Julien Tinnes <jt@cr0.org> Date: Fri Jun 26 20:27:40 2009 +0200
personality: fix PER_CLEAR_ON_SETID
So it's not really relevant to 3.x kernels, is it?
James
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