[tip:x86/asm] x86/entry: Vastly simplify SYSENTER TF (single-step) handling

From: tip-bot for Andy Lutomirski
Date: Thu Mar 10 2016 - 06:04:36 EST

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Commit-ID: f2b375756c839ff655a3e0b45e339f8fbf973093
Gitweb: http://git.kernel.org/tip/f2b375756c839ff655a3e0b45e339f8fbf973093
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 9 Mar 2016 19:00:30 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 10 Mar 2016 09:48:13 +0100

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@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/a30d2ea06fe4b621fe6a9ef911b02c0f38feb6f2.1457578375.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/entry/entry_32.S | 42 ++++++++++++++++++++++----------
arch/x86/entry/entry_64_compat.S | 9 ++++++-
arch/x86/include/asm/proto.h | 15 ++++++++++--
arch/x86/kernel/traps.c | 52 +++++++++++++++++++++++++++++++++-------
4 files changed, 94 insertions(+), 24 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 7610906..b2e1d44 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -287,7 +287,26 @@ need_resched:
END(resume_kernel)
#endif

- # SYSENTER call handler stub
+GLOBAL(__begin_SYSENTER_singlestep_region)
+/*
+ * All code from here through __end_SYSENTER_singlestep_region is subject
+ * to being single-stepped if a user program sets TF and executes SYSENTER.
+ * There is absolutely nothing that we can do to prevent this from happening
+ * (thanks Intel!). To keep our handling of this situation as simple as
+ * possible, we handle TF just like AC and NT, except that our #DB handler
+ * will ignore all of the single-step traps generated in this range.
+ */
+
+#ifdef CONFIG_XEN
+/*
+ * Xen doesn't set %esp to be precisely what the normal SYSENTER
+ * entry point expects, so fix it up before using the normal path.
+ */
+ENTRY(xen_sysenter_target)
+ addl $5*4, %esp /* remove xen-provided frame */
+ jmp sysenter_past_esp
+#endif
+
ENTRY(entry_SYSENTER_32)
movl TSS_sysenter_sp0(%esp), %esp
sysenter_past_esp:
@@ -301,19 +320,25 @@ sysenter_past_esp:
SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest */

/*
- * SYSENTER doesn't filter flags, so we need to clear NT and AC
- * ourselves. To save a few cycles, we can check whether
+ * SYSENTER doesn't filter flags, so we need to clear NT, AC
+ * and TF ourselves. To save a few cycles, we can check whether
* either was set instead of doing an unconditional popfq.
* This needs to happen before enabling interrupts so that
* we don't get preempted with NT set.
*
+ * If TF is set, we will single-step all the way to here -- do_debug
+ * will ignore all the traps. (Yes, this is slow, but so is
+ * single-stepping in general. This allows us to avoid having
+ * a more complicated code to handle the case where a user program
+ * forces us to single-step through the SYSENTER entry code.)
+ *
* NB.: .Lsysenter_fix_flags is a label with the code under it moved
* out-of-line as an optimization: NT is unlikely to be set in the
* majority of the cases and instead of polluting the I$ unnecessarily,
* we're keeping that code behind a branch which will predict as
* not-taken and therefore its instructions won't be fetched.
*/
- testl $X86_EFLAGS_NT|X86_EFLAGS_AC, PT_EFLAGS(%esp)
+ testl $X86_EFLAGS_NT|X86_EFLAGS_AC|X86_EFLAGS_TF, PT_EFLAGS(%esp)
jnz .Lsysenter_fix_flags
.Lsysenter_flags_fixed:

@@ -369,6 +394,7 @@ sysenter_past_esp:
pushl $X86_EFLAGS_FIXED
popfl
jmp .Lsysenter_flags_fixed
+GLOBAL(__end_SYSENTER_singlestep_region)
ENDPROC(entry_SYSENTER_32)

# system call handler stub
@@ -651,14 +677,6 @@ ENTRY(spurious_interrupt_bug)
END(spurious_interrupt_bug)

#ifdef CONFIG_XEN
-/*
- * Xen doesn't set %esp to be precisely what the normal SYSENTER
- * entry point expects, so fix it up before using the normal path.
- */
-ENTRY(xen_sysenter_target)
- addl $5*4, %esp /* remove xen-provided frame */
- jmp sysenter_past_esp
-
ENTRY(xen_hypervisor_callback)
pushl $-1 /* orig_ax = -1 => not a system call */
SAVE_ALL
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index da5c194..cbb5e69 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -94,13 +94,19 @@ ENTRY(entry_SYSENTER_compat)
* This needs to happen before enabling interrupts so that
* we don't get preempted with NT set.
*
+ * If TF is set, we will single-step all the way to here -- do_debug
+ * will ignore all the traps. (Yes, this is slow, but so is
+ * single-stepping in general. This allows us to avoid having
+ * a more complicated code to handle the case where a user program
+ * forces us to single-step through the SYSENTER entry code.)
+ *
* NB.: .Lsysenter_fix_flags is a label with the code under it moved
* out-of-line as an optimization: NT is unlikely to be set in the
* majority of the cases and instead of polluting the I$ unnecessarily,
* we're keeping that code behind a branch which will predict as
* not-taken and therefore its instructions won't be fetched.
*/
- testl $X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp)
+ testl $X86_EFLAGS_NT|X86_EFLAGS_AC|X86_EFLAGS_TF, EFLAGS(%rsp)
jnz .Lsysenter_fix_flags
.Lsysenter_flags_fixed:

@@ -121,6 +127,7 @@ ENTRY(entry_SYSENTER_compat)
pushq $X86_EFLAGS_FIXED
popfq
jmp .Lsysenter_flags_fixed
+GLOBAL(__end_entry_SYSENTER_compat)
ENDPROC(entry_SYSENTER_compat)

/*
diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
index a4a7728..9b9b30b 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);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7394be8..b0ddb819 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -559,6 +559,29 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
NOKPROBE_SYMBOL(fixup_bad_iret);
#endif

+static bool is_sysenter_singlestep(struct pt_regs *regs)
+{
+ /*
+ * We don't try for precision here. If we're anywhere in the region of
+ * code that can be single-stepped in the SYSENTER entry path, then
+ * assume that this is a useless single-step trap due to SYSENTER
+ * being invoked with TF set. (We don't know in advance exactly
+ * which instructions will be hit because BTF could plausibly
+ * be set.)
+ */
+#ifdef CONFIG_X86_32
+ return (regs->ip - (unsigned long)__begin_SYSENTER_singlestep_region) <
+ (unsigned long)__end_SYSENTER_singlestep_region -
+ (unsigned long)__begin_SYSENTER_singlestep_region;
+#elif defined(CONFIG_IA32_EMULATION)
+ return (regs->ip - (unsigned long)entry_SYSENTER_compat) <
+ (unsigned long)__end_entry_SYSENTER_compat -
+ (unsigned long)entry_SYSENTER_compat;
+#else
+ return false;
+#endif
+}
+
/*
* Our handling of the processor debug registers is non-trivial.
* We do not clear them on entry and exit from the kernel. Therefore
@@ -616,6 +639,18 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
*/
clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);

+ if (unlikely(!user_mode(regs) && (dr6 & DR_STEP) &&
+ is_sysenter_singlestep(regs))) {
+ dr6 &= ~DR_STEP;
+ if (!dr6)
+ goto exit;
+ /*
+ * else we might have gotten a single-step trap and hit a
+ * watchpoint at the same time, in which case we should fall
+ * through and handle the watchpoint.
+ */
+ }
+
/*
* If dr6 has no reason to give us about the origin of this trap,
* then it's very likely the result of an icebp/int01 trap.
@@ -624,7 +659,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
if (!dr6 && user_mode(regs))
user_icebp = 1;

- /* Catch kmemcheck conditions first of all! */
+ /* Catch kmemcheck conditions! */
if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
goto exit;

@@ -659,14 +694,13 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
goto exit;
}

- /*
- * Single-stepping through system calls: ignore any exceptions in
- * kernel space, but re-enable TF when returning to user mode.
- *
- * We already checked v86 mode above, so we can check for kernel mode
- * by just checking the CPL of CS.
- */
- if ((dr6 & DR_STEP) && !user_mode(regs)) {
+ if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) {
+ /*
+ * Historical junk that used to handle SYSENTER single-stepping.
+ * This should be unreachable now. If we survive for a while
+ * without anyone hitting this warning, we'll turn this into
+ * an oops.
+ */
tsk->thread.debugreg6 &= ~DR_STEP;
set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
regs->flags &= ~X86_EFLAGS_TF;

Next message: tip-bot for Andy Lutomirski: "[tip:x86/asm] x86/entry: Only allocate space for tss_struct::SYSENTER_stack if needed"
Previous message: tip-bot for Andy Lutomirski: "[tip:x86/asm] x86/entry/traps: Clear DR6 early in do_debug() and improve the comment"
In reply to: Andy Lutomirski: "[PATCH v2 07/12] x86/entry: Vastly simplify SYSENTER TF handling"
Next in thread: Andy Lutomirski: "[PATCH v2 01/12] selftests/x86: In syscall_nt, test NT|TF as well"
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