[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen
On Sat, Jul 21, 2018 at 12:49 PM, M. Vefa Bicakci <m.v.b@xxxxxxxxxx> wrote: > Commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for > exceptions/interrupts, to reduce speculation attack surface") unintendedly > broke Xen PV virtual machines by clearing the %rbx register at the end of > xen_failsafe_callback before the latter jumps to error_exit. > error_exit expects the %rbx register to be a flag indicating whether > there should be a return to kernel mode. > > This commit makes sure that the %rbx register is not cleared by > the PUSH_AND_CLEAR_REGS macro, when the macro in question is instantiated > by xen_failsafe_callback, to avoid the issue. Seems like a genuine problem, but: > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index c7449f377a77..96e8ff34129e 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -1129,7 +1129,7 @@ ENTRY(xen_failsafe_callback) > addq $0x30, %rsp > UNWIND_HINT_IRET_REGS > pushq $-1 /* orig_ax = -1 => not a system call */ > - PUSH_AND_CLEAR_REGS > + PUSH_AND_CLEAR_REGS clear_rbx=0 > ENCODE_FRAME_POINTER > jmp error_exit The old code first set RBX to zero then, if frame pointers are on, sets it to some special non-zero value, then crosses its fingers and hopes for the best. Your patched code just skips the zeroing part, so RBX either contains the ENCODE_FRAME_POINTER result or is uninitialized. How about actually initializing rbx to something sensible like, say, 1? _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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