Re: [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery

[Borislav Petkov]

On Thu, Jan 14, 2021 at 04:38:17PM -0800, Tony Luck wrote:
> Recovery action when get_user() triggers a machine check uses the fixup
> path to make get_user() return -EFAULT.  Also queue_task_work() sets up
> so that kill_me_maybe() will be called on return to user mode to send a
> SIGBUS to the current process.
> 
> But there are places in the kernel where the code assumes that this
> EFAULT return was simply because of a page fault. The code takes some
> action to fix that, and then retries the access. This results in a second
> machine check.
> 
> While processing this second machine check queue_task_work() is called
> again. But since this uses the same callback_head structure that
> was used in the first call, the net result is an entry on the
> current->task_works list that points to itself. When task_work_run()
> is called it loops forever in this code:
> 
> 		do {
> 			next = work->next;
> 			work->func(work);
> 			work = next;
> 			cond_resched();
> 		} while (work);
> 
> Add a "mce_busy" counter so that task_work_add() is only called once
> per faulty page in this task.

Yeah, that sentence can be removed now too.

> Do not allow too many repeated machine checks, or machine checks to
> a different page from the first.
> 
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> 
> V3: Thanks to extensive commentary from Andy & Boris
> 
> Throws out the changes to get_user() and subsequent changes to core
> code. Everything is now handled in the machine check code. Downside is
> that we can (and do) take multiple machine checks from a single poisoned
> page before generic kernel code finally gets the message that a page is
> really and truly gone (but all the failed get_user() calls still return
> the legacy -EFAULT code, so none of that code will ever mistakenly use
> a value from a bad page). But even on an old machine that does broadcast
> interrupts for each machine check things survive multiple cycles of my
> test injection into a futex operation.

Nice.

> 
> I picked "10" as the magic upper limit for how many times the machine
> check code will allow a fault from the same page before deciding to
> panic.  We can bike shed that value if you like.
> 
>  arch/x86/kernel/cpu/mce/core.c | 27 ++++++++++++++++++++-------
>  include/linux/sched.h          |  1 +
>  2 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 13d3f1cbda17..25daf6517dc9 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -1246,6 +1246,7 @@ static void kill_me_maybe(struct callback_head *cb)
>  	struct task_struct *p = container_of(cb, struct task_struct, mce_kill_me);
>  	int flags = MF_ACTION_REQUIRED;
>  
> +	p->mce_count = 0;
>  	pr_err("Uncorrected hardware memory error in user-access at %llx", p->mce_addr);
>  
>  	if (!p->mce_ripv)
> @@ -1266,12 +1267,24 @@ static void kill_me_maybe(struct callback_head *cb)
>  	}
>  }
>  
> -static void queue_task_work(struct mce *m, int kill_current_task)
> +static void queue_task_work(struct mce *m, char *msg, int kill_current_task)

So this function gets called in the user mode MCE case too:

	if ((m.cs & 3) == 3) {

		queue_task_work(&m, msg, kill_current_task);
	}

Do we want to panic for multiple MCEs to different addresses in user
mode?

I don't think so - that should go down the memory failure page
offlining path...

> -	current->mce_addr = m->addr;
> -	current->mce_kflags = m->kflags;
> -	current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
> -	current->mce_whole_page = whole_page(m);
> +	if (current->mce_count++ == 0) {
> +		current->mce_addr = m->addr;
> +		current->mce_kflags = m->kflags;
> +		current->mce_ripv = !!(m->mcgstatus & MCG_STATUS_RIPV);
> +		current->mce_whole_page = whole_page(m);
> +	}
> +

	/* Magic number should be large enough */

> +	if (current->mce_count > 10)
> +		mce_panic("Too many machine checks while accessing user data", m, msg);
> +
> +	if (current->mce_count > 1 || (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT))
> +		mce_panic("Machine checks to different user pages", m, msg);

Will this second part of the test expression, after the "||" ever hit?

You do above in the first branch:

	if (current->mce_count++ == 0) {

		...

		current->mce_addr = m->addr;

and ->mce_count becomes 1.

In that case that

	(current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT)

gets tested but that won't ever be true because ->mce_addr = ->addr
above.

And then, for other values of mce_count, mce_count > 1 will hit.

In any case, what are you trying to catch with this? Two get_user() to
different pages both catching MCEs?

> +
> +	/* Do not call task_work_add() more than once */
> +	if (current->mce_count > 1)
> +		return;

That won't happen either, AFAICT. It'll panic above.

Regardless, I like how this is all confined to the MCE code and there's
no need to touch stuff outside...

Thx.

-- 
Regards/Gruss,
    Boris.

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