Re: [PATCH] x86/mtrr: don't copy out-of-bounds data in mtrr_write

[Ingo Molnar <mingo@kernel.org>]

* Jann Horn <jannh@google.com> wrote:

>  - A malicious user can pass an arbitrary file to a setuid binary as
> stdin/stdout/stderr. When the setuid binary (expecting stdin/stdout to
> be something normal, like a proper file or a pipe) then calls read(0,
> <buf>, <len>), if the kernel disregards the length argument and writes
> beyond the end of the buffer, it can corrupt adjacent userspace data,
> potentially allowing a user to escalate their privileges; a write
> handler is somewhat less interesting because it can probably (as in
> this case) only leak out-of-bounds data from the caller, not corrupt
> it, but it's still a concern in theory.

BTW., a naive question: would it make sense to simply disallow 'special'
fds to be passed to setuid binaries, and fix any user-space that breaks?
(i.e. only allow regular files and pipes/sockets.)

Also, don't allow splice() on special files either, except if the driver 
explicitly opts in to it.

Sounds a lot more robust in the long run than playing whack-a-mole with the 
*inevitable* hole in special read() and write() handlers in our 3,000+ device 
drivers...

Thanks,

	Ingo