linux/kernel/trace/trace_recursion_record.c
Steven Rostedt (VMware) 21ccc9cd72 tracing: Disable "other" permission bits in the tracefs files
When building the files in the tracefs file system, do not by default set
any permissions for OTH (other). This will make it easier for admins who
want to define a group for accessing tracefs and not having to first
disable all the permission bits for "other" in the file system.

As tracing can leak sensitive information, it should never by default
allowing all users access. An admin can still set the permission bits for
others to have access, which may be useful for creating a honeypot and
seeing who takes advantage of it and roots the machine.

Link: https://lkml.kernel.org/r/20210818153038.864149276@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
2021-10-08 18:08:43 -04:00

237 lines
6.1 KiB
C

// SPDX-License-Identifier: GPL-2.0
#include <linux/seq_file.h>
#include <linux/kallsyms.h>
#include <linux/module.h>
#include <linux/ftrace.h>
#include <linux/fs.h>
#include "trace_output.h"
struct recursed_functions {
unsigned long ip;
unsigned long parent_ip;
};
static struct recursed_functions recursed_functions[CONFIG_FTRACE_RECORD_RECURSION_SIZE];
static atomic_t nr_records;
/*
* Cache the last found function. Yes, updates to this is racey, but
* so is memory cache ;-)
*/
static unsigned long cached_function;
void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip)
{
int index = 0;
int i;
unsigned long old;
again:
/* First check the last one recorded */
if (ip == cached_function)
return;
i = atomic_read(&nr_records);
/* nr_records is -1 when clearing records */
smp_mb__after_atomic();
if (i < 0)
return;
/*
* If there's two writers and this writer comes in second,
* the cmpxchg() below to update the ip will fail. Then this
* writer will try again. It is possible that index will now
* be greater than nr_records. This is because the writer
* that succeeded has not updated the nr_records yet.
* This writer could keep trying again until the other writer
* updates nr_records. But if the other writer takes an
* interrupt, and that interrupt locks up that CPU, we do
* not want this CPU to lock up due to the recursion protection,
* and have a bug report showing this CPU as the cause of
* locking up the computer. To not lose this record, this
* writer will simply use the next position to update the
* recursed_functions, and it will update the nr_records
* accordingly.
*/
if (index < i)
index = i;
if (index >= CONFIG_FTRACE_RECORD_RECURSION_SIZE)
return;
for (i = index - 1; i >= 0; i--) {
if (recursed_functions[i].ip == ip) {
cached_function = ip;
return;
}
}
cached_function = ip;
/*
* We only want to add a function if it hasn't been added before.
* Add to the current location before incrementing the count.
* If it fails to add, then increment the index (save in i)
* and try again.
*/
old = cmpxchg(&recursed_functions[index].ip, 0, ip);
if (old != 0) {
/* Did something else already added this for us? */
if (old == ip)
return;
/* Try the next location (use i for the next index) */
index++;
goto again;
}
recursed_functions[index].parent_ip = parent_ip;
/*
* It's still possible that we could race with the clearing
* CPU0 CPU1
* ---- ----
* ip = func
* nr_records = -1;
* recursed_functions[0] = 0;
* i = -1
* if (i < 0)
* nr_records = 0;
* (new recursion detected)
* recursed_functions[0] = func
* cmpxchg(recursed_functions[0],
* func, 0)
*
* But the worse that could happen is that we get a zero in
* the recursed_functions array, and it's likely that "func" will
* be recorded again.
*/
i = atomic_read(&nr_records);
smp_mb__after_atomic();
if (i < 0)
cmpxchg(&recursed_functions[index].ip, ip, 0);
else if (i <= index)
atomic_cmpxchg(&nr_records, i, index + 1);
}
EXPORT_SYMBOL_GPL(ftrace_record_recursion);
static DEFINE_MUTEX(recursed_function_lock);
static struct trace_seq *tseq;
static void *recursed_function_seq_start(struct seq_file *m, loff_t *pos)
{
void *ret = NULL;
int index;
mutex_lock(&recursed_function_lock);
index = atomic_read(&nr_records);
if (*pos < index) {
ret = &recursed_functions[*pos];
}
tseq = kzalloc(sizeof(*tseq), GFP_KERNEL);
if (!tseq)
return ERR_PTR(-ENOMEM);
trace_seq_init(tseq);
return ret;
}
static void *recursed_function_seq_next(struct seq_file *m, void *v, loff_t *pos)
{
int index;
int p;
index = atomic_read(&nr_records);
p = ++(*pos);
return p < index ? &recursed_functions[p] : NULL;
}
static void recursed_function_seq_stop(struct seq_file *m, void *v)
{
kfree(tseq);
mutex_unlock(&recursed_function_lock);
}
static int recursed_function_seq_show(struct seq_file *m, void *v)
{
struct recursed_functions *record = v;
int ret = 0;
if (record) {
trace_seq_print_sym(tseq, record->parent_ip, true);
trace_seq_puts(tseq, ":\t");
trace_seq_print_sym(tseq, record->ip, true);
trace_seq_putc(tseq, '\n');
ret = trace_print_seq(m, tseq);
}
return ret;
}
static const struct seq_operations recursed_function_seq_ops = {
.start = recursed_function_seq_start,
.next = recursed_function_seq_next,
.stop = recursed_function_seq_stop,
.show = recursed_function_seq_show
};
static int recursed_function_open(struct inode *inode, struct file *file)
{
int ret = 0;
mutex_lock(&recursed_function_lock);
/* If this file was opened for write, then erase contents */
if ((file->f_mode & FMODE_WRITE) && (file->f_flags & O_TRUNC)) {
/* disable updating records */
atomic_set(&nr_records, -1);
smp_mb__after_atomic();
memset(recursed_functions, 0, sizeof(recursed_functions));
smp_wmb();
/* enable them again */
atomic_set(&nr_records, 0);
}
if (file->f_mode & FMODE_READ)
ret = seq_open(file, &recursed_function_seq_ops);
mutex_unlock(&recursed_function_lock);
return ret;
}
static ssize_t recursed_function_write(struct file *file,
const char __user *buffer,
size_t count, loff_t *ppos)
{
return count;
}
static int recursed_function_release(struct inode *inode, struct file *file)
{
if (file->f_mode & FMODE_READ)
seq_release(inode, file);
return 0;
}
static const struct file_operations recursed_functions_fops = {
.open = recursed_function_open,
.write = recursed_function_write,
.read = seq_read,
.llseek = seq_lseek,
.release = recursed_function_release,
};
__init static int create_recursed_functions(void)
{
struct dentry *dentry;
dentry = trace_create_file("recursed_functions", TRACE_MODE_WRITE,
NULL, NULL, &recursed_functions_fops);
if (!dentry)
pr_warn("WARNING: Failed to create recursed_functions\n");
return 0;
}
fs_initcall(create_recursed_functions);