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Subject[PATCH v2 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler
Date
On arm64, if a nested kprobes hit, it can crash the kernel with below
error message.

[ 152.118921] Unexpected kernel single-step exception at EL1

This is because commit 7419333fa15e ("arm64: kprobe: Always clear
pstate.D in breakpoint exception handler") unmask pstate.D for
doing single step but does not recover it after single step in
the nested kprobes. That is correct *unless* any nested kprobes
(single-stepping) runs inside other kprobes user handler.

When the 1st kprobe hits, do_debug_exception() will be called. At this
point, debug exception (= pstate.D) must be masked (=1). When the 2nd
(nested) kprobe is hit before single-step of the first kprobe, it
unmask 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.

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")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
Changes in v2:
- Save and restore all DAIF flags.
- Operate pstate directly and remove spsr_set_debug_flag().
---
arch/arm64/kernel/probes/kprobes.c | 41 ++++++------------------------------
1 file changed, 7 insertions(+), 34 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bd5dfffca272..348e02b799a2 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -29,6 +29,8 @@

#include "decode-insn.h"

+#define PSR_DAIF_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
+
DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

@@ -167,33 +169,6 @@ 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 +180,17 @@ 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 & PSR_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 &= ~PSR_DAIF_MASK;
+ regs->pstate |= kcb->saved_irqflag;
}

static void __kprobes
@@ -252,8 +227,6 @@ 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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