On Sat, Jul 21, 2018 at 12:49 PM, M. Vefa Bicakci m.v.b@runbox.com 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?