Re: [PATCH -tip 1/2] x86/alternative: Sync bp_patching update for avoiding NULL pointer exception

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 27, 2019 at 02:56:52PM +0900, Masami Hiramatsu wrote:
> ftracetest multiple_kprobes.tc testcase hit a following NULL pointer
> exception.
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 800000007bf60067 P4D 800000007bf60067 PUD 7bf5f067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 6 PID: 0 Comm: swapper/6 Not tainted 5.4.0-rc8+ #23
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
> RIP: 0010:poke_int3_handler+0x39/0x100
> Code: 5b 5d c3 f6 87 88 00 00 00 03 75 f2 48 8b 87 80 00 00 00 48 89 fb 48 8d 68 ff 48 8b 05 80 98 72 01 83 fa 01 0f 8f 93 00 00 00 <48> 63 10 48 81 c2 00 00 00 81 48 39 d5 75 c5 0f b6 50 08 8d 4a 34
> RSP: 0018:ffffc900001a8eb8 EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffffc900001a8ee8 RCX: ffffffff81a00b57
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffffc900001a8ee8
> RBP: ffffffff81027635 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88807d980000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 000000007a970000 CR4: 00000000000006a0
> Call Trace:
>  <IRQ>
>  do_int3+0xd/0xf0
>  int3+0x42/0x50
> RIP: 0010:sched_clock+0x6/0x10
> 
> eu-addr2line told that poke_int3_handler+0x39 was alternatives:958.
> 
> static inline void *text_poke_addr(struct text_poke_loc *tp)
> {
>         return _stext + tp->rel_addr; <------ Here is line #958
> }
> 
> This seems like caused by the tp (bp_patching.vec) was NULL but
> bp_patching.nr_entries != 0. There is a small chance to do
> this, because we have no sync after zeroing bp_patching.nr_entries
> before clearing bp_patching.vec.
> 
> Steve suggested we could fix this by adding sync_core, because int3
> is done with interrupts disabled, and the on_each_cpu() requires
> all CPUs to have had their interrupts enabled.
> 
> Fixes: c0213b0ac03c ("x86/alternative: Batch of patch operations")
> Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  arch/x86/kernel/alternative.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 4552795a8df4..9505096e2cd1 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1134,8 +1134,14 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
>  	 * sync_core() implies an smp_mb() and orders this store against
>  	 * the writing of the new instruction.
>  	 */
> -	bp_patching.vec = NULL;
>  	bp_patching.nr_entries = 0;
> +	/*
> +	 * This sync_core () ensures that all int3 handlers in progress
> +	 * have finished. This allows poke_int3_handler () after this to
> +	 * avoid touching bp_paching.vec by checking nr_entries == 0.
> +	 */
> +	text_poke_sync();
> +	bp_patching.vec = NULL;
>  }

Hurm.. is there no way we can merge that with the 'last'
text_poke_sync() ? It seems a little daft to do 2 back-to-back IPI
things like that.