Re: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

Re: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

[Jue Wang]

On Thu, 25 Mar 2021 17:02:35 -0700, Tony Luck wrote:
...

> 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.

What about return EHWPOISON instead of EFAULT and update the callers
to handle EHWPOISON explicitly: i.e., not retry but give up on the page?

My main concern is that the strong assumptions that the kernel can't hit more
than a fixed number of poisoned cache lines before turning to user space
may simply not be true.

When DIMM goes bad, it can easily affect an entire bank or entire ram device
chip. Even with memory interleaving, it's possible that a kernel control path
touches lots of poisoned cache lines in the buffer it is working through.

RE: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

[Luck, Tony]

>> 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.
>
> What about return EHWPOISON instead of EFAULT and update the callers
> to handle EHWPOISON explicitly: i.e., not retry but give up on the page?

That seems like a good idea to me. But I got some pushback when I started
on this path earlier with some patches to the futex code.  But back then I
wasn't using error return of EHWPOISON ... possibly the code would look
less hacky with that explicitly called out.

The futex case was specifically for code using pagefault_disable(). Likely
all the other callers would need to be audited (but there are only a few dozen
places, so not too big of a deal).

> My main concern is that the strong assumptions that the kernel can't hit more
> than a fixed number of poisoned cache lines before turning to user space
> may simply not be true.

Agreed.

> When DIMM goes bad, it can easily affect an entire bank or entire ram device
> chip. Even with memory interleaving, it's possible that a kernel control path
> touches lots of poisoned cache lines in the buffer it is working through.

These larger failures have other problems ... dozens of unrelated pages
may be affected. In a perfect world Linux would be told on the first error
that this is just one of many errors ... and be given a list. But in the real
world that isn't likely to happen :-(

-Tony

RE: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

[Luck, Tony]

> What I'm still unclear on, does this new version address that
> "mysterious" hang or panic which the validation team triggered or you
> haven't checked yet?

No :-(

They are triggering some case where multiple threads in a process hit the same
poison, and somehow memory_failure() fails to complete offlining the page. At this
point any other threads that hit that page get the early return from memory_failure
(because the page flags say it is poisoned) ... and so we loop.

But the "recover from cases where multiple machine checks happen
simultaneously" case is orthogonal to the "do the right thing to recover
when the kernel touches poison at a user address". So I think we can
tackle them separately

-Tony

Re: [PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

[Borislav Petkov]

On Thu, Mar 25, 2021 at 05:02:35PM -0700, Tony Luck wrote:
...
> Expected worst case is two machine checks before moving on (e.g. one user
> access with page faults disabled, then a repeat to the same addrsss with
> page faults enabled). Just in case there is some code that loops forever
> enforce a limit of 10.
> 
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
>  arch/x86/kernel/cpu/mce/core.c | 40 ++++++++++++++++++++++++++--------
>  include/linux/sched.h          |  1 +
>  2 files changed, 32 insertions(+), 9 deletions(-)

What I'm still unclear on, does this new version address that
"mysterious" hang or panic which the validation team triggered or you
haven't checked yet?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[PATCH 4/4] x86/mce: Avoid infinite loop for copy from user recovery

[Tony Luck]

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 counter (current->mce_count) to keep track of repeated machine checks
before task_work() is called. First machine check saves the address information
and calls task_work_add(). Subsequent machine checks before that task_work
call back is executed check that the address is in the same page as the first
machine check (since the callback will offline exactly one page).

Expected worst case is two machine checks before moving on (e.g. one user
access with page faults disabled, then a repeat to the same addrsss with
page faults enabled). Just in case there is some code that loops forever
enforce a limit of 10.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
 arch/x86/kernel/cpu/mce/core.c | 40 ++++++++++++++++++++++++++--------
 include/linux/sched.h          |  1 +
 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 1570310cadab..999fd7f0330b 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1250,6 +1250,9 @@ static void __mc_scan_banks(struct mce *m, struct pt_regs *regs, struct mce *fin
 
 static void kill_me_now(struct callback_head *ch)
 {
+	struct task_struct *p = container_of(ch, struct task_struct, mce_kill_me);
+
+	p->mce_count = 0;
 	force_sig(SIGBUS);
 }
 
@@ -1258,6 +1261,7 @@ 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)
@@ -1277,18 +1281,36 @@ static void kill_me_never(struct callback_head *cb)
 {
 	struct task_struct *p = container_of(cb, struct task_struct, mce_kill_me);
 
+	p->mce_count = 0;
 	pr_err("Kernel accessed poison in user space at %llx\n", p->mce_addr);
 	if (!memory_failure(p->mce_addr >> PAGE_SHIFT, 0))
 		set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
 }
 
-static void queue_task_work(struct mce *m, void (*func)(struct callback_head *))
+static void queue_task_work(struct mce *m, char *msg, void (*func)(struct callback_head *))
 {
-	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);
-	current->mce_kill_me.func = func;
+	int count = ++current->mce_count;
+
+	/* First call, save all the details */
+	if (count == 1) {
+		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);
+		current->mce_kill_me.func = func;
+	}
+
+	/* Ten is likley overkill. Don't expect more than two faults before task_work() */
+	if (count > 10)
+		mce_panic("Too many machine checks while accessing user data", m, msg);
+
+	/* Second or later call, make sure page address matches the one from first call */
+	if (count > 1 && (current->mce_addr >> PAGE_SHIFT) != (m->addr >> PAGE_SHIFT))
+		mce_panic("Machine checks to different user pages", m, msg);
+
+	/* Do not call task_work_add() more than once */
+	if (count > 1)
+		return;
 
 	task_work_add(current, &current->mce_kill_me, TWA_RESUME);
 }
@@ -1427,9 +1449,9 @@ noinstr void do_machine_check(struct pt_regs *regs)
 		BUG_ON(!on_thread_stack() || !user_mode(regs));
 
 		if (kill_current_task)
-			queue_task_work(&m, kill_me_now);
+			queue_task_work(&m, msg, kill_me_now);
 		else
-			queue_task_work(&m, kill_me_maybe);
+			queue_task_work(&m, msg, kill_me_maybe);
 
 	} else {
 		/*
@@ -1447,7 +1469,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
 		}
 
 		if (m.kflags & MCE_IN_KERNEL_COPYIN)
-			queue_task_work(&m, kill_me_never);
+			queue_task_work(&m, msg, kill_me_never);
 	}
 out:
 	mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2d213b52730c..8f9dc91498cf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1364,6 +1364,7 @@ struct task_struct {
 					mce_whole_page : 1,
 					__mce_reserved : 62;
 	struct callback_head		mce_kill_me;
+	int				mce_count;
 #endif
 
 #ifdef CONFIG_KRETPROBES
-- 
2.29.2