Re: [PATCH v3] x86/mce: Avoid infinite loop for copy from user recovery
From: Borislav Petkov <bp@alien8.de>
Date: 2021-01-15 15:28:57
Also in:
linux-mm, lkml
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@intel.com> --- 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.
quoted hunk ↗ jump to hunk
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.
https://people.kernel.org/tglx/notes-about-netiquette