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Message-Id: <8b9bd05f187231df008d48cf818a6a311cbd5c98.1597882384.git.luto@kernel.org>
Date: Wed, 19 Aug 2020 17:15:43 -0700
From: Andy Lutomirski <luto@...nel.org>
To: x86@...nel.org
Cc: Kyle Huey <me@...ehuey.com>,
Alexandre Chartre <alexandre.chartre@...cle.com>,
Robert O'Callahan <rocallahan@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Petr Mladek <pmladek@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Joel Fernandes <joel@...lfernandes.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Juergen Gross <jgross@...e.com>,
Brian Gerst <brgerst@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Will Deacon <will@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Josh Poimboeuf <jpoimboe@...hat.com>, stable@...r.kernel.org
Subject: [PATCH] x86/debug: Allow a single level of #DB recursion
Trying to clear DR7 around a #DB from usermode malfunctions if we
schedule when delivering SIGTRAP. Rather than trying to define a
special no-recursion region, just allow a single level of recursion.
We do the same thing for NMI, and it hasn't caused any problems yet.
Reported-by: Kyle Huey <me@...ehuey.com>
Debugged-by: Josh Poimboeuf <jpoimboe@...hat.com>
Fixes: 9f58fdde95c9 ("x86/db: Split out dr6/7 handling")
Cc: stable@...r.kernel.org
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
arch/x86/kernel/traps.c | 69 +++++++++++++++++++++++------------------
1 file changed, 38 insertions(+), 31 deletions(-)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 1f66d2d1e998..bf852b72f15c 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -729,20 +729,9 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
#endif
}
-static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
+static __always_inline unsigned long debug_read_clear_dr6(void)
{
- /*
- * Disable breakpoints during exception handling; recursive exceptions
- * are exceedingly 'fun'.
- *
- * Since this function is NOKPROBE, and that also applies to
- * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
- * HW_BREAKPOINT_W on our stack)
- *
- * Entry text is excluded for HW_BP_X and cpu_entry_area, which
- * includes the entry stack is excluded for everything.
- */
- *dr7 = local_db_save();
+ unsigned long dr6;
/*
* The Intel SDM says:
@@ -755,15 +744,21 @@ static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
*
* Keep it simple: clear DR6 immediately.
*/
- get_debugreg(*dr6, 6);
+ get_debugreg(dr6, 6);
set_debugreg(0, 6);
/* Filter out all the reserved bits which are preset to 1 */
- *dr6 &= ~DR6_RESERVED;
+ dr6 &= ~DR6_RESERVED;
+
+ return dr6;
+}
+
+static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
+{
+ *dr6 = debug_read_clear_dr6();
}
static __always_inline void debug_exit(unsigned long dr7)
{
- local_db_restore(dr7);
}
/*
@@ -863,6 +858,19 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
static __always_inline void exc_debug_kernel(struct pt_regs *regs,
unsigned long dr6)
{
+ /*
+ * Disable breakpoints during exception handling; recursive exceptions
+ * are exceedingly 'fun'.
+ *
+ * Since this function is NOKPROBE, and that also applies to
+ * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
+ * HW_BREAKPOINT_W on our stack)
+ *
+ * Entry text is excluded for HW_BP_X and cpu_entry_area, which
+ * includes the entry stack is excluded for everything.
+ */
+ unsigned long dr7 = local_db_save();
+
bool irq_state = idtentry_enter_nmi(regs);
instrumentation_begin();
@@ -883,6 +891,8 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
instrumentation_end();
idtentry_exit_nmi(regs, irq_state);
+
+ local_db_restore(dr7);
}
static __always_inline void exc_debug_user(struct pt_regs *regs,
@@ -894,6 +904,15 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
*/
WARN_ON_ONCE(!user_mode(regs));
+ /*
+ * NB: We can't easily clear DR7 here because
+ * idtentry_exit_to_usermode() can invoke ptrace, schedule, access
+ * user memory, etc. This means that a recursive #DB is possible. If
+ * this happens, that #DB will hit exc_debug_kernel() and clear DR7.
+ * Since we're not on the IST stack right now, everything will be
+ * fine.
+ */
+
irqentry_enter_from_user_mode(regs);
instrumentation_begin();
@@ -907,36 +926,24 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
/* IST stack entry */
DEFINE_IDTENTRY_DEBUG(exc_debug)
{
- unsigned long dr6, dr7;
-
- debug_enter(&dr6, &dr7);
- exc_debug_kernel(regs, dr6);
- debug_exit(dr7);
+ exc_debug_kernel(regs, debug_read_clear_dr6());
}
/* User entry, runs on regular task stack */
DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
{
- unsigned long dr6, dr7;
-
- debug_enter(&dr6, &dr7);
- exc_debug_user(regs, dr6);
- debug_exit(dr7);
+ exc_debug_user(regs, debug_read_clear_dr6());
}
#else
/* 32 bit does not have separate entry points. */
DEFINE_IDTENTRY_RAW(exc_debug)
{
- unsigned long dr6, dr7;
-
- debug_enter(&dr6, &dr7);
+ unsigned long dr6 = debug_read_clear_dr6();
if (user_mode(regs))
exc_debug_user(regs, dr6);
else
exc_debug_kernel(regs, dr6);
-
- debug_exit(dr7);
}
#endif
--
2.25.4
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