Messages in this thread | | | Date | Thu, 30 Aug 2018 12:40:53 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area |
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On Tue, 7 Aug 2018, Dave Hansen wrote: > On 08/07/2018 10:29 AM, Sean Christopherson wrote: > > if (unlikely(fault_in_kernel_space(address))) { > > + /* > > + * We should never encounter a protection keys fault on a > > + * kernel address as kernel address are always mapped with > > + * _PAGE_USER=0, i.e. PKRU isn't enforced. > > + */ > > + if (WARN_ON_ONCE(error_code & X86_PF_PK)) > > + goto bad_kernel_address; > > I just realized one more thing: the vsyscall page can bite us here. > It's at a fault_in_kernel_space() address and we *can* trigger a pkey > fault on it if we jump to an instruction that reads from a > pkey-protected area. > > We can make a gadget out of unaligned vsyscall instructions that does > that. See: > > 0xffffffffff600002: shlb $0x0,0x0(%rax) > > Then, we turn off access to all pkeys, including pkey-0, then jump to > the unaligned vsyscall instruction, which reads %rax, which is a kernel > address: > > asm("movl $0xffffffff, %eax;\ > movl $0x00000000, %ecx;\ > movl $0x00000000, %edx;\ > wrpkru;\ > movq $0xffffffffff600000, %rax;\ > movq $0xffffffffff600002, %rbx;\ > jmpq *%rbx;"); > > So, my bad. It was not a good suggestion to do a WARN_ON(). But, the > other funny thing is I would have expected spurious_fault() to get us > into a fault loop, which it doesn't. It's definitely getting *called* > with my little test program (I see it in ftrace) but it's not quite > doing what I expect. > > I need to dig a bit more.
Given the time span you should be close to ground water with your digging by now.
Thanks,
tglx
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