On Thu, Feb 25, 2016 at 2:39 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Thu, Feb 25, 2016 at 11:31 AM, Brian Gerst <brge...@gmail.com> wrote: >> On Thu, Feb 25, 2016 at 1:30 PM, Linus Torvalds >> <torva...@linux-foundation.org> wrote: >>> On Thu, Feb 25, 2016 at 10:20 AM, Andy Lutomirski <l...@amacapital.net> >>> wrote: >>>> >>>> Ideally we'd fix this up and restore flags on sysexit. At least >>>> failing to restore arithmetic flags isn't an info leak because the >>>> exit code clobbers them with entirely predictable data. I doubt >>>> anyone cares all that much if we clobber AC. >>> >>> As long as the "clobber AC" is purely about clearing it, it's probably fine. >>> >>> Although there may be programs that set AC in order to actually get >>> notified about alignment issues (perhaps for portability reasons, >>> perhaps for small performance reasons). Clearing it will make those >>> programs still work, but they lose the checking. >>> >>>> I wrote a test for NT and the test fails for a different reason: our >>>> TF handling appears broken as well. (Our sysenter TF handling is >>>> *crap*, but it seems to work on 64-bit kernels at least.) >>> >>> TF should be entirely immaterial for system calls. Why would we care? >>> We need it for correct handling of real traps, but not for the system >>> call case afaik. Returning with TF clear is the right thing, since >>> we're not returning *to* the system call instruction, but the >>> instruction after. >>> >>>> My personal preference would be to add the missing popf. >>> >>> I don't mind adding the popf, but it won't help for iopl. Only iret >>> restores iopl, if I recall correctly (but maybe I don't, and I'm too >>> lazy to take the 30 seconds to look it up). >>> >>> Linus >> >> According to the SDM, popf will change IOPL only at CPL0, which is why >> Xen (which runs at CPL1 on 32-bit) has a paravirt hook for it. > > But maybe we can ditch that paravirt hook and just modify regs->flags > in sys_iopl. Xen never uses sysexit at all: > > /* XEN PV guests always use IRET path */ > ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \ > "jmp .Lsyscall_32_done", X86_FEATURE_XENPV > > and if we add the missing popf, we should be good to go.
IRET won't change IOPL either at CPL != 0, so Xen still needs that hook. -- Brian Gerst
