diff options
author | Andy Lutomirski <luto@kernel.org> | 2016-03-09 19:00:30 -0800 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2016-03-10 09:48:13 +0100 |
commit | f2b375756c839ff655a3e0b45e339f8fbf973093 (patch) | |
tree | b7e4af1375fe03f342aea24fb20da5a1accf0426 /arch/x86/kernel | |
parent | 8bb5643686d2898bec181c407651cf84e77d413f (diff) |
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>
Diffstat (limited to 'arch/x86/kernel')
-rw-r--r-- | arch/x86/kernel/traps.c | 52 |
1 files changed, 43 insertions, 9 deletions
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 7394be8b1a66..b0ddb81926f9 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; |