On Thu, 1 Aug 2019 13:16:22 +0100
Will Deacon [off-list ref] wrote:

On Thu, Jul 25, 2019 at 05:15:54PM +0900, Masami Hiramatsu wrote:

quoted

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 <redacted>
Fixes: commit 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>
---
  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

Oops, daifflags.h is included via kvm_host.h...
OK, kprobes.c must include daifflags.h directly in this case.

I'll update this patch too.

Thank you,

Will

-- 
Masami Hiramatsu [off-list ref]

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