On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski <[email protected]> wrote: > Anish Bhatt noticed that user programs can set RFLAGS.NT before > syscall or sysenter, and the kernel entry code doesn't filter out > NT. This causes kernel C code and, depending on thread flags, the > exit slow path to run with NT set. >
Quick ping: now that the merge window is (sort of) open, what's happening with these patches? Thanks, Andy > The former is a little bit scary (imagine calling into EFI with NT > set), and the latter will fail with #GP and send a spurious SIGSEGV. > > One answer would be "don't do that". But the kernel can do better > here. > > These patches filter NT on all kernel entries. For syscall (both > bitnesses), this is free. For sysenter, it seems to cost very > little (less than my ability to measure, although I didn't try that > hard). Patch 2, which isn't tagged for -stable, speeds up context > switches by avoiding saving and restoring flags, so this series > should be a decent overall performance win. > > See: https://bugs.winehq.org/show_bug.cgi?id=33275 > > Note to bikeshedders: I have no desire to go crazy micro-optimizing > the sysenter path. :) This version seems to be good enough (and > should be a performance *increase* for most workloads). > > Changes from v3: > - Added a better description of the impact in patch 1 > > Changes from v2: > - Move the flag fixup out of line > - Fix a CFI buglet > > Changes from v1: > - Spell [email protected] correctly > - Tidy up changelog text > - Actually commit an asm constraint fix in patch 2 (egads!) > - Replace the unconditional popfq with a branch > > Andy Lutomirski (2): > x86_64,entry: Filter RFLAGS.NT on entry from userspace > x86_64: Don't save flags on context switch > > arch/x86/ia32/ia32entry.S | 18 +++++++++++++++++- > arch/x86/include/asm/switch_to.h | 12 ++++++++---- > arch/x86/kernel/cpu/common.c | 2 +- > 3 files changed, 26 insertions(+), 6 deletions(-) > > -- > 1.9.3 > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
