• Andy Lutomirski's avatar
    x86_64, entry: Filter RFLAGS.NT on entry from userspace · 8c7aa698
    Andy Lutomirski authored
    The NT flag doesn't do anything in long mode other than causing IRET
    to #GP.  Oddly, CPL3 code can still set NT using popf.
    
    Entry via hardware or software interrupt clears NT automatically, so
    the only relevant entries are fast syscalls.
    
    If user code causes kernel code to run with NT set, then there's at
    least some (small) chance that it could cause trouble.  For example,
    user code could cause a call to EFI code with NT set, and who knows
    what would happen?  Apparently some games on Wine sometimes do
    this (!), and, if an IRET return happens, they will segfault.  That
    segfault cannot be handled, because signal delivery fails, too.
    
    This patch programs the CPU to clear NT on entry via SYSCALL (both
    32-bit and 64-bit, by my reading of the AMD APM), and it clears NT
    in software on entry via SYSENTER.
    
    To save a few cycles, this borrows a trick from Jan Beulich in Xen:
    it checks whether NT is set before trying to clear it.  As a result,
    it seems to have very little effect on SYSENTER performance on my
    machine.
    
    There's another minor bug fix in here: it looks like the CFI
    annotations were wrong if CONFIG_AUDITSYSCALL=n.
    
    Testers beware: on Xen, SYSENTER with NT set turns into a GPF.
    
    I haven't touched anything on 32-bit kernels.
    
    The syscall mask change comes from a variant of this patch by Anish
    Bhatt.
    
    Note to stable maintainers: there is no known security issue here.
    A misguided program can set NT and cause the kernel to try and fail
    to deliver SIGSEGV, crashing the program.  This patch fixes Far Cry
    on Wine: https://bugs.winehq.org/show_bug.cgi?id=33275
    
    Cc: <stable@vger.kernel.org>
    Reported-by: default avatarAnish Bhatt <anish@chelsio.com>
    Signed-off-by: default avatarAndy Lutomirski <luto@amacapital.net>
    Link: http://lkml.kernel.org/r/395749a5d39a29bd3e4b35899cf3a3c1340e5595.1412189265.git.luto@amacapital.netSigned-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
    8c7aa698
ia32entry.S 13.5 KB