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Subject[PATCH AUTOSEL 5.2 52/59] arm64: kprobes: Recover pstate.D in single-step exception handler
Date
From: Masami Hiramatsu <mhiramat@kernel.org>

[ Upstream commit b3980e48528c4d2a9e70b145a5bba328b73a0f93 ]

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: 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>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 ae7e605085d71..9c0e0178ea291 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 88ce502c8e6f1..624f2501f3f87 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>
@@ -165,33 +166,6 @@ 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.
@@ -203,17 +177,17 @@ 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
@@ -250,8 +224,6 @@ 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);
--
2.20.1
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