x86/entry: Vastly simplify SYSENTER TF (single-step) handling

Due to a blatant design error, SYSENTER doesn't clear TF (single-step).

As a result, if a user does SYSENTER with TF set, we will single-step
through the kernel until something clears TF.  There is absolutely
nothing we can do to prevent this short of turning off SYSENTER [1].

Simplify the handling considerably with two changes:

  1. We already sanitize EFLAGS in SYSENTER to clear NT and AC.  We can
     add TF to that list of flags to sanitize with no overhead whatsoever.

  2. Teach do_debug() to ignore single-step traps in the SYSENTER prologue.

That's all we need to do.

Don't get too excited -- our handling is still buggy on 32-bit
kernels.  There's nothing wrong with the SYSENTER code itself, but
the #DB prologue has a clever fixup for traps on the very first
instruction of entry_SYSENTER_32, and the fixup doesn't work quite
correctly.  The next two patches will fix that.

[1] We could probably prevent it by forcing BTF on at all times and
    making sure we clear TF before any branches in the SYSENTER
    code.  Needless to say, this is a bad idea.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/a30d2ea06fe4b621fe6a9ef911b02c0f38feb6f2.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
index a4a77286cb1d..9b9b30b19441 100644
--- a/arch/x86/include/asm/proto.h
+++ b/arch/x86/include/asm/proto.h
@@ -7,12 +7,23 @@
 
 void syscall_init(void);
 
+#ifdef CONFIG_X86_64
 void entry_SYSCALL_64(void);
-void entry_SYSCALL_compat(void);
+#endif
+
+#ifdef CONFIG_X86_32
 void entry_INT80_32(void);
-void entry_INT80_compat(void);
 void entry_SYSENTER_32(void);
+void __begin_SYSENTER_singlestep_region(void);
+void __end_SYSENTER_singlestep_region(void);
+#endif
+
+#ifdef CONFIG_IA32_EMULATION
 void entry_SYSENTER_compat(void);
+void __end_entry_SYSENTER_compat(void);
+void entry_SYSCALL_compat(void);
+void entry_INT80_compat(void);
+#endif
 
 void x86_configure_nx(void);
 void x86_report_nx(void);