From: Jan Beulich <jbeulich@xxxxxxxx> commit 922eea2ce5c799228d9ff1be9890e6873ce8fff6 upstream. This can be had with two instead of six insns, by just checking the high CS.RPL bit. Also adjust the comment - there would be no #GP in the mentioned cases, as there's no segment limit violation or alike. Instead there'd be #PF, but that one reports the target EIP of said branch, not the address of the branch insn itself. Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Reviewed-by: Juergen Gross <jgross@xxxxxxxx> Link: https://lkml.kernel.org/r/a5986837-01eb-7bf8-bf42-4d3084d6a1f5@xxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/xen/xen-asm_32.S | 15 ++++----------- 1 file changed, 4 insertions(+), 11 deletions(-) --- a/arch/x86/xen/xen-asm_32.S +++ b/arch/x86/xen/xen-asm_32.S @@ -153,22 +153,15 @@ hyper_iret: * it's still on stack), we need to restore its value here. */ ENTRY(xen_iret_crit_fixup) - pushl %ecx /* * Paranoia: Make sure we're really coming from kernel space. * One could imagine a case where userspace jumps into the * critical range address, but just before the CPU delivers a - * GP, it decides to deliver an interrupt instead. Unlikely? - * Definitely. Easy to avoid? Yes. The Intel documents - * explicitly say that the reported EIP for a bad jump is the - * jump instruction itself, not the destination, but some - * virtual environments get this wrong. + * PF, it decides to deliver an interrupt instead. Unlikely? + * Definitely. Easy to avoid? Yes. */ - movl 3*4(%esp), %ecx /* nested CS */ - andl $SEGMENT_RPL_MASK, %ecx - cmpl $USER_RPL, %ecx - popl %ecx - je 2f + testb $2, 2*4(%esp) /* nested CS */ + jnz 2f /* * If eip is before iret_restore_end then stack
