From: Masami Hiramatsu mhiramat@kernel.org
mainline inclusion from mainline-5.3-rc3 commit b3980e48528c category: bugfix bugzilla: 20080 CVE: NA
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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: Wei Li liwei391@huawei.com Reviewed-by: Hanjun Guo guohanjun@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- arch/arm64/include/asm/daifflags.h | 1 + arch/arm64/kernel/probes/kprobes.c | 40 ++++++-------------------------------- 2 files changed, 7 insertions(+), 34 deletions(-)
diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h index 3441ca0..1230923 100644 --- a/arch/arm64/include/asm/daifflags.h +++ b/arch/arm64/include/asm/daifflags.h @@ -24,6 +24,7 @@
#define DAIF_PROCCTX 0 #define DAIF_PROCCTX_NOIRQ PSR_I_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 2d63df1..fe9d207 100644 --- a/arch/arm64/kernel/probes/kprobes.c +++ b/arch/arm64/kernel/probes/kprobes.c @@ -30,6 +30,7 @@ #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> @@ -180,33 +181,6 @@ static void __kprobes set_current_kprobe(struct 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. * Without disabling interrupt on local CPU, there is a chance of @@ -217,17 +191,17 @@ static void __kprobes set_current_kprobe(struct kprobe *p) 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 @@ -264,8 +238,6 @@ 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);
-
Yang Yingliang