Messages in this thread Patch in this message | | | From | Masami Hiramatsu <> | Subject | [PATCH v2 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler | Date | Mon, 22 Jul 2019 16:48:36 +0900 |
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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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