Date: Tue, 08 Jul 2014 15:15:47 -0700 From: Andy Lutomirski <luto@...capital.net> To: oss-security@...ts.openwall.com Subject: Re: CVE-2014-4699: Linux ptrace bug On 07/05/2014 12:35 PM, Solar Designer wrote: > Andy, all - > > On Sat, Jul 05, 2014 at 10:25:47PM +0400, Solar Designer wrote: >> "x86_64,ptrace: Enforce RIP <= TASK_SIZE_MAX (CVE-2014-4699)" > [...] >> "CVE-2014-4699 Kernel: x86_64,ptrace: Enforce RIP <= TASK_SIZE_MAX" > > BTW, I'm not convinced it's such a good idea to allow setting RIP to > exactly TASK_SIZE_MAX just because user code could run to that address > (this was Andy's rationale). Imagine that TASK_SIZE_MAX is ever set > such that it's the very first non-canonical address. If user code > simply runs to that address, it gets a user mode fault. However, if the > kernel tries to set user RIP to that address via SYSRET, it'll get #GP > while still in kernel mode - exactly the problem we're trying to fix. > > So when fixing the problem in this way, or when including this as a > hardening measure along with forcing the IRET path as well, I'd prefer > to allow only "< TASK_SIZE_MAX", not "<= TASK_SIZE_MAX". In the event that anyone changes TASK_SIZE_MAX to equal the first non-canonical address, then this is the least of your worries: someone can put a syscall instruction at the very last canonical address, and game over. This bug affected a lot of operating systems a few years ago, but AFAIK Linux was never vulnerable. --Andy
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