On Thu, Jul 25, 2019 at 05:15:54PM +0900, Masami Hiramatsu wrote:
> 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 <[email protected]>
> Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in
> breakpoint exception handler")
> Reviewed-by: James Morse <[email protected]>
> Tested-by: James Morse <[email protected]>
> Signed-off-by: Masami Hiramatsu <[email protected]>
> ---
> 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 | 39
> +++++-------------------------------
> 2 files changed, 7 insertions(+), 34 deletions(-)
I'm seeing an allmodconfig build failure with this:
arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_save_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:181:38: error: ‘DAIF_MASK’ undeclared (first
use in this function); did you mean ‘BIT_MASK’?
kcb->saved_irqflag = regs->pstate & DAIF_MASK;
^~~~~~~~~
BIT_MASK
arch/arm64/kernel/probes/kprobes.c:181:38: note: each undeclared identifier is
reported only once for each function it appears in
arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_restore_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:190:19: error: ‘DAIF_MASK’ undeclared (first
use in this function); did you mean ‘BIT_MASK’?
regs->pstate &= ~DAIF_MASK;
^~~~~~~~~
BIT_MASK
make[2]: *** [scripts/Makefile.build:274: arch/arm64/kernel/probes/kprobes.o]
Error 1
Will
