The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From 81065b35e2486c024c7aa86caed452e1f01a59d4 Mon Sep 17 00:00:00 2001
From: Tony Luck <tony.luck@xxxxxxxxx>
Date: Mon, 13 Sep 2021 14:52:39 -0700
Subject: [PATCH] x86/mce: Avoid infinite loop for copy from user recovery
There are two cases for machine check recovery:
1) The machine check was triggered by ring3 (application) code.
This is the simpler case. The machine check handler simply queues
work to be executed on return to user. That code unmaps the page
from all users and arranges to send a SIGBUS to the task that
triggered the poison.
2) The machine check was triggered in kernel code that is covered by
an exception table entry. In this case the machine check handler
still queues a work entry to unmap the page, etc. but this will
not be called right away because the #MC handler returns to the
fix up code address in the exception table entry.
Problems occur if the kernel triggers another machine check before the
return to user processes the first queued work item.
Specifically, the work is queued using the ->mce_kill_me callback
structure in the task struct for the current thread. Attempting to queue
a second work item using this same callback results in a loop in the
linked list of work functions to call. So when the kernel does return to
user, it enters an infinite loop processing the same entry for ever.
There are some legitimate scenarios where the kernel may take a second
machine check before returning to the user.
1) Some code (e.g. futex) first tries a get_user() with page faults
disabled. If this fails, the code retries with page faults enabled
expecting that this will resolve the page fault.
2) Copy from user code retries a copy in byte-at-time mode to check
whether any additional bytes can be copied.
On the other side of the fence are some bad drivers that do not check
the return value from individual get_user() calls and may access
multiple user addresses without noticing that some/all calls have
failed.
Fix by adding 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 four machine checks before moving on (e.g. one
user access with page faults disabled, then a repeat to the same address
with page faults enabled ... repeat in copy tail bytes). Just in case
there is some code that loops forever enforce a limit of 10.
[ bp: Massage commit message, drop noinstr, fix typo, extend panic
messages. ]
Fixes: 5567d11c21a1 ("x86/mce: Send #MC singal from task work")
Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/YT/IJ9ziLqmtqEPu@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 8cb7816d03b4..193204aee880 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -1253,6 +1253,9 @@ 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);
}
@@ -1262,6 +1265,7 @@ static void kill_me_maybe(struct callback_head *cb)
int flags = MF_ACTION_REQUIRED;
int ret;
+ p->mce_count = 0;
pr_err("Uncorrected hardware memory error in user-access at %llx", p->mce_addr);
if (!p->mce_ripv)
@@ -1290,17 +1294,34 @@ 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)
{
- 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);
+ int count = ++current->mce_count;
- if (kill_current_task)
- current->mce_kill_me.func = kill_me_now;
- else
- current->mce_kill_me.func = kill_me_maybe;
+ /* 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);
+
+ if (kill_current_task)
+ current->mce_kill_me.func = kill_me_now;
+ else
+ current->mce_kill_me.func = kill_me_maybe;
+ }
+
+ /* Ten is likely overkill. Don't expect more than two faults before task_work() */
+ if (count > 10)
+ mce_panic("Too many consecutive 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("Consecutive 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, ¤t->mce_kill_me, TWA_RESUME);
}
@@ -1438,7 +1459,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
/* If this triggers there is no way to recover. Die hard. */
BUG_ON(!on_thread_stack() || !user_mode(regs));
- queue_task_work(&m, kill_current_task);
+ queue_task_work(&m, msg, kill_current_task);
} else {
/*
@@ -1456,7 +1477,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
}
if (m.kflags & MCE_IN_KERNEL_COPYIN)
- queue_task_work(&m, kill_current_task);
+ queue_task_work(&m, msg, kill_current_task);
}
out:
mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1780260f237b..361c7bc72cbb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1468,6 +1468,7 @@ struct task_struct {
mce_whole_page : 1,
__mce_reserved : 62;
struct callback_head mce_kill_me;
+ int mce_count;
#endif
#ifdef CONFIG_KRETPROBES
