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Message-Id: <20230425225107.8525-5-beaub@linux.microsoft.com>
Date: Tue, 25 Apr 2023 15:51:07 -0700
From: Beau Belgrave <beaub@...ux.microsoft.com>
To: rostedt@...dmis.org, mhiramat@...nel.org,
mathieu.desnoyers@...icios.com, dcook@...ux.microsoft.com,
alanau@...ux.microsoft.com
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: [PATCH v2 4/4] tracing/user_events: Limit max fault-in attempts
When event enablement changes, user_events attempts to update a bit in
the user process. If a fault is hit, an attempt to fault-in the page and
the write is retried if the page made it in. While this normally requires
a couple attempts, it is possible a bad user process could attempt to
cause infinite loops.
Ensure fault-in attempts either sync or async are limited to a max of 10
attempts for each update. When the max is hit, return -EFAULT so another
attempt is not made in all cases.
Suggested-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>
---
kernel/trace/trace_events_user.c | 49 +++++++++++++++++++++++---------
1 file changed, 35 insertions(+), 14 deletions(-)
diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 4fc099fc7637..cab2c5891758 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -123,6 +123,7 @@ struct user_event_enabler_fault {
struct work_struct work;
struct user_event_mm *mm;
struct user_event_enabler *enabler;
+ int attempt;
};
static struct kmem_cache *fault_cache;
@@ -266,11 +267,19 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler)
kfree(enabler);
}
-static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr)
+static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr,
+ int attempt)
{
bool unlocked;
int ret;
+ /*
+ * Normally this is low, ensure that it cannot be taken advantage of by
+ * bad user processes to cause excessive looping.
+ */
+ if (attempt > 10)
+ return -EFAULT;
+
mmap_read_lock(mm->mm);
/* Ensure MM has tasks, cannot use after exit_mm() */
@@ -289,7 +298,7 @@ static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long uaddr)
static int user_event_enabler_write(struct user_event_mm *mm,
struct user_event_enabler *enabler,
- bool fixup_fault);
+ bool fixup_fault, int *attempt);
static void user_event_enabler_fault_fixup(struct work_struct *work)
{
@@ -298,9 +307,10 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
struct user_event_enabler *enabler = fault->enabler;
struct user_event_mm *mm = fault->mm;
unsigned long uaddr = enabler->addr;
+ int attempt = fault->attempt;
int ret;
- ret = user_event_mm_fault_in(mm, uaddr);
+ ret = user_event_mm_fault_in(mm, uaddr, attempt);
if (ret && ret != -ENOENT) {
struct user_event *user = enabler->event;
@@ -329,7 +339,7 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
if (!ret) {
mmap_read_lock(mm->mm);
- user_event_enabler_write(mm, enabler, true);
+ user_event_enabler_write(mm, enabler, true, &attempt);
mmap_read_unlock(mm->mm);
}
out:
@@ -341,7 +351,8 @@ static void user_event_enabler_fault_fixup(struct work_struct *work)
}
static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
- struct user_event_enabler *enabler)
+ struct user_event_enabler *enabler,
+ int attempt)
{
struct user_event_enabler_fault *fault;
@@ -353,6 +364,7 @@ static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
INIT_WORK(&fault->work, user_event_enabler_fault_fixup);
fault->mm = user_event_mm_get(mm);
fault->enabler = enabler;
+ fault->attempt = attempt;
/* Don't try to queue in again while we have a pending fault */
set_bit(ENABLE_VAL_FAULTING_BIT, ENABLE_BITOPS(enabler));
@@ -372,7 +384,7 @@ static bool user_event_enabler_queue_fault(struct user_event_mm *mm,
static int user_event_enabler_write(struct user_event_mm *mm,
struct user_event_enabler *enabler,
- bool fixup_fault)
+ bool fixup_fault, int *attempt)
{
unsigned long uaddr = enabler->addr;
unsigned long *ptr;
@@ -383,6 +395,8 @@ static int user_event_enabler_write(struct user_event_mm *mm,
lockdep_assert_held(&event_mutex);
mmap_assert_locked(mm->mm);
+ *attempt += 1;
+
/* Ensure MM has tasks, cannot use after exit_mm() */
if (refcount_read(&mm->tasks) == 0)
return -ENOENT;
@@ -398,7 +412,7 @@ static int user_event_enabler_write(struct user_event_mm *mm,
if (!fixup_fault)
return -EFAULT;
- if (!user_event_enabler_queue_fault(mm, enabler))
+ if (!user_event_enabler_queue_fault(mm, enabler, *attempt))
pr_warn("user_events: Unable to queue fault handler\n");
return -EFAULT;
@@ -439,15 +453,19 @@ static void user_event_enabler_update(struct user_event *user)
struct user_event_enabler *enabler;
struct user_event_mm *mm = user_event_mm_get_all(user);
struct user_event_mm *next;
+ int attempt;
while (mm) {
next = mm->next;
mmap_read_lock(mm->mm);
rcu_read_lock();
- list_for_each_entry_rcu(enabler, &mm->enablers, link)
- if (enabler->event == user)
- user_event_enabler_write(mm, enabler, true);
+ list_for_each_entry_rcu(enabler, &mm->enablers, link) {
+ if (enabler->event == user) {
+ attempt = 0;
+ user_event_enabler_write(mm, enabler, true, &attempt);
+ }
+ }
rcu_read_unlock();
mmap_read_unlock(mm->mm);
@@ -695,6 +713,7 @@ static struct user_event_enabler
struct user_event_enabler *enabler;
struct user_event_mm *user_mm;
unsigned long uaddr = (unsigned long)reg->enable_addr;
+ int attempt = 0;
user_mm = current_user_event_mm();
@@ -715,7 +734,8 @@ static struct user_event_enabler
/* Attempt to reflect the current state within the process */
mmap_read_lock(user_mm->mm);
- *write_result = user_event_enabler_write(user_mm, enabler, false);
+ *write_result = user_event_enabler_write(user_mm, enabler, false,
+ &attempt);
mmap_read_unlock(user_mm->mm);
/*
@@ -735,7 +755,7 @@ static struct user_event_enabler
if (*write_result) {
/* Attempt to fault-in and retry if it worked */
- if (!user_event_mm_fault_in(user_mm, uaddr))
+ if (!user_event_mm_fault_in(user_mm, uaddr, attempt))
goto retry;
kfree(enabler);
@@ -2192,6 +2212,7 @@ static int user_event_mm_clear_bit(struct user_event_mm *user_mm,
{
struct user_event_enabler enabler;
int result;
+ int attempt = 0;
memset(&enabler, 0, sizeof(enabler));
enabler.addr = uaddr;
@@ -2202,14 +2223,14 @@ static int user_event_mm_clear_bit(struct user_event_mm *user_mm,
/* Force the bit to be cleared, since no event is attached */
mmap_read_lock(user_mm->mm);
- result = user_event_enabler_write(user_mm, &enabler, false);
+ result = user_event_enabler_write(user_mm, &enabler, false, &attempt);
mmap_read_unlock(user_mm->mm);
mutex_unlock(&event_mutex);
if (result) {
/* Attempt to fault-in and retry if it worked */
- if (!user_event_mm_fault_in(user_mm, uaddr))
+ if (!user_event_mm_fault_in(user_mm, uaddr, attempt))
goto retry;
}
--
2.25.1
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