[PATCH v2 00/12] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups

From: Andy Lutomirski
Date: Wed Mar 09 2016 - 22:00:51 EST

hpa asked me to get rid of the ASM_CLAC at the beginning of the SYSENTER
path. Little did he know...

This series makes the observed behavior of SYSENTER wrt flags the same
for all sane flags and kernel bitnesses. That is, SYSENTER preserves
flags now unless you do a syscall that explicitly changes flags, and
the HW flags that the syscall executes with are sanitized. This
includes NT, TF, AC and all arithmetic flags. Prior to this series,
32-bit kernels clobbered TF and the arithmetic flags and behaved
highly erratically if NT was set. (If IF is cleared by evil userspace
when SYSENTER starts, IF will be set again on return. There's nothing
the kernel can do about this -- SYSENTER inherently forgets the state
of IF.)

This series speeds up SYSENTER on all kernels by a surprisingly large
amount on Skylake because it eliminates an unconditional CLAC.

While SYSENTER used to handle TF correctly as far as I can tell on
64-bit kernels, the means by which it did so was heavily tangled up in
the ptrace single-step logic. It now works just like all the other
kernel entries except insofar as do_debug has a simple special case
for it. Relatedly, the bizarre and poorly explained old fixup in
do_debug is now hidden behind a WARN_ON_ONCE in preparation for
deleting it at some point.

The code that fixed up NMI and #DB early in SYSENTER in 32-bit kernels
used to be both terrifying and incorrect. (It doesn't appear to have
been exploitably bad, but the reason for that is subtle, and the code
was certainy more fragile than it deserved to me.) We still need a
special fixup, but it's much simpler now.

While I was doing all this, I also noticed that DR6 and BTF handling
in do_debug was a bit off. Two of the patches in here try to fix it

Have fun!

tl;dr: Cleanups and sanity fixes here, but no security fixes, and I
don't think anything needs to be backported or put in x86/urgent.

This series applies to the result of merging tip:x86/asm and
tip:x86/urgent. I've been testing on a somewhat bastardized base,
because tip currently doesn't work on my laptop in 32-bit mode. (That
bug is fixed in Linus' tree.)

Changes from v1:
- s/Sysenter/SYSENTER in two places (Borislav)
- int main() -> int main(void) (Borislav)
- Fix some "SYENTER" typoed
- Add the last two patches (11 was a missed cleanup and 12 is a respin)
- Patch 12 differs from the old version in that Ingo's comments are hopefully addressed.

Andy Lutomirski (12):
selftests/x86: In syscall_nt, test NT|TF as well
x86/entry/compat: In SYSENTER, sink AC clearing below the existing
FLAGS test
x86/entry/32: Filter NT and speed up AC filtering in SYSENTER
x86/entry/32: Restore FLAGS on SYSEXIT
x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions
x86/traps: Clear DR6 early in do_debug and improve the comment
x86/entry: Vastly simplify SYSENTER TF handling
x86/entry: Only allocate space for SYSENTER_stack if needed
x86/entry/32: Simplify and fix up the SYSENTER stack #DB/NMI fixup
x86/entry/32: Add and check a stack canary for the SYSENTER stack
x86/entry: Remove TIF_SINGLESTEP entry work
x86/entry: Improve system call entry comments

arch/x86/entry/common.c | 10 --
arch/x86/entry/entry_32.S | 243 +++++++++++++++++++++----------
arch/x86/entry/entry_64.S | 10 ++
arch/x86/entry/entry_64_compat.S | 100 +++++++++----
arch/x86/include/asm/processor.h | 5 +-
arch/x86/include/asm/proto.h | 15 +-
arch/x86/include/asm/thread_info.h | 2 +-
arch/x86/kernel/asm-offsets_32.c | 5 +
arch/x86/kernel/process.c | 3 +
arch/x86/kernel/traps.c | 87 ++++++++---
tools/testing/selftests/x86/syscall_nt.c | 57 +++++++-
11 files changed, 392 insertions(+), 145 deletions(-)