[PATCH v4] arm64: kprobes: Recover pstate.D in single-step exception handler

From: Masami Hiramatsu <mhiramat@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>
Cc: Dan Rue <dan.rue@linaro.org>,
	Daniel Diaz <daniel.diaz@linaro.org>,
	Anders Roxell <anders.roxell@linaro.org>,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
	Matt Hart <matthew.hart@linaro.org>,
	linux-arm-kernel@lists.infradead.org, mhiramat@kernel.org
Subject: [PATCH v4] arm64: kprobes: Recover pstate.D in single-step exception handler
Date: Thu,  1 Aug 2019 23:25:49 +0900	[thread overview]
Message-ID: <156466954920.8995.14798868044005509434.stgit@devnote2> (raw)
In-Reply-To: <20190801230818.72fadeea8a58d1f657179b30@kernel.org>

kprobes manipulates the interrupted PSTATE for single step, and
doesn't restore it. Thus, if we put a kprobe where the pstate.D
(debug) masked, the mask will be cleared after the kprobe hits.

Moreover, in the most complicated case, this can lead a kernel
crash with below message when a nested kprobe hits.

[  152.118921] Unexpected kernel single-step exception at EL1

When the 1st kprobe hits, do_debug_exception() will be called.
At this point, debug exception (= pstate.D) must be masked (=1).
But if another kprobes hits before single-step of the first kprobe
(e.g. inside user pre_handler), it unmask the debug exception
(pstate.D = 0) and return.
Then, when the 1st kprobe setting up single-step, it saves current
DAIF, mask DAIF, enable single-step, and restore DAIF.
However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
single-step exception happens soon after restoring DAIF.

This has been introduced by commit 7419333fa15e ("arm64: kprobe:
Always clear pstate.D in breakpoint exception handler")

To solve this issue, this stores all DAIF bits and restore it
after single stepping.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
  Changes in v4:
   - Include daifflags.h directly so that kprobe can use DAIF_MASK always.
  Changes in v3:
   - Update patch description
   - move PSR_DAIF_MASK in daifflags.h
  Changes in v2:
   - Save and restore all DAIF flags.
   - Operate pstate directly and remove spsr_set_debug_flag().
---
 arch/arm64/include/asm/daifflags.h |    2 ++
 arch/arm64/kernel/probes/kprobes.c |   40 +++++-------------------------------
 2 files changed, 8 insertions(+), 34 deletions(-)

diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h
index 987926ed535e..063c964af705 100644
--- a/arch/arm64/include/asm/daifflags.h
+++ b/arch/arm64/include/asm/daifflags.h
@@ -13,6 +13,8 @@
 #define DAIF_PROCCTX		0
 #define DAIF_PROCCTX_NOIRQ	PSR_I_BIT
 #define DAIF_ERRCTX		(PSR_I_BIT | PSR_A_BIT)
+#define DAIF_MASK		(PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
+
 
 /* mask/save/unmask/restore all exceptions, including interrupts. */
 static inline void local_daif_mask(void)
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bd5dfffca272..c4452827419b 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -21,6 +21,7 @@
 #include <asm/ptrace.h>
 #include <asm/cacheflush.h>
 #include <asm/debug-monitors.h>
+#include <asm/daifflags.h>
 #include <asm/system_misc.h>
 #include <asm/insn.h>
 #include <linux/uaccess.h>
@@ -167,33 +168,6 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
 	__this_cpu_write(current_kprobe, p);
 }
 
-/*
- * When PSTATE.D is set (masked), then software step exceptions can not be
- * generated.
- * SPSR's D bit shows the value of PSTATE.D immediately before the
- * exception was taken. PSTATE.D is set while entering into any exception
- * mode, however software clears it for any normal (none-debug-exception)
- * mode in the exception entry. Therefore, when we are entering into kprobe
- * breakpoint handler from any normal mode then SPSR.D bit is already
- * cleared, however it is set when we are entering from any debug exception
- * mode.
- * Since we always need to generate single step exception after a kprobe
- * breakpoint exception therefore we need to clear it unconditionally, when
- * we become sure that the current breakpoint exception is for kprobe.
- */
-static void __kprobes
-spsr_set_debug_flag(struct pt_regs *regs, int mask)
-{
-	unsigned long spsr = regs->pstate;
-
-	if (mask)
-		spsr |= PSR_D_BIT;
-	else
-		spsr &= ~PSR_D_BIT;
-
-	regs->pstate = spsr;
-}
-
 /*
  * Interrupts need to be disabled before single-step mode is set, and not
  * reenabled until after single-step mode ends.
@@ -205,17 +179,17 @@ spsr_set_debug_flag(struct pt_regs *regs, int mask)
 static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	kcb->saved_irqflag = regs->pstate;
+	kcb->saved_irqflag = regs->pstate & DAIF_MASK;
 	regs->pstate |= PSR_I_BIT;
+	/* Unmask PSTATE.D for enabling software step exceptions. */
+	regs->pstate &= ~PSR_D_BIT;
 }
 
 static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	if (kcb->saved_irqflag & PSR_I_BIT)
-		regs->pstate |= PSR_I_BIT;
-	else
-		regs->pstate &= ~PSR_I_BIT;
+	regs->pstate &= ~DAIF_MASK;
+	regs->pstate |= kcb->saved_irqflag;
 }
 
 static void __kprobes
@@ -252,8 +226,6 @@ static void __kprobes setup_singlestep(struct kprobe *p,
 
 		set_ss_context(kcb, slot);	/* mark pending ss */
 
-		spsr_set_debug_flag(regs, 0);
-
 		/* IRQs and single stepping do not mix well. */
 		kprobes_save_local_irqflag(kcb, regs);
 		kernel_enable_single_step(regs);


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