[PATCH v3 3/4] pipe: add proc_dopipe_max_size() to safely assign pipe_max_size

From: Joe Lawrence
Date: Tue Oct 10 2017 - 14:09:55 EST

pipe_max_size is assigned directly via procfs sysctl:

static struct ctl_table fs_table[] = {
...
{
.procname = "pipe-max-size",
.data = &pipe_max_size,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = &pipe_proc_fn,
.extra1 = &pipe_min_size,
},
...

int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
size_t *lenp, loff_t *ppos)
{
...
ret = proc_dointvec_minmax(table, write, buf, lenp, ppos)
...

and then later rounded in-place a few statements later:

...
pipe_max_size = round_pipe_size(pipe_max_size);
...

This leaves a window of time between initial assignment and rounding
that may be visible to other threads. (For example, one thread sets a
non-rounded value to pipe_max_size while another reads its value.)

Similar reads of pipe_max_size are potentially racey:

pipe.c :: alloc_pipe_info()
pipe.c :: pipe_set_size()

Add a new proc_dopipe_max_size() function that consolidates reading the
new value from the user buffer, verifying bounds, and calling
round_pipe_size() with a single assignment to pipe_max_size.

Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
---
fs/pipe.c | 18 +++--------------
include/linux/pipe_fs_i.h | 1 +
include/linux/sysctl.h | 3 +++
kernel/sysctl.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 56 insertions(+), 15 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 8cbc97d97753..e4eea5a51916 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -1019,7 +1019,7 @@ static int fifo_open(struct inode *inode, struct file *filp)
* Currently we rely on the pipe array holding a power-of-2 number
* of pages. Returns 0 on error.
*/
-static inline unsigned int round_pipe_size(unsigned int size)
+unsigned int round_pipe_size(unsigned int size)
{
unsigned long nr_pages;

@@ -1124,25 +1124,13 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
}

/*
- * This should work even if CONFIG_PROC_FS isn't set, as proc_dointvec_minmax
+ * This should work even if CONFIG_PROC_FS isn't set, as proc_dopipe_max_size
* will return an error.
*/
int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
size_t *lenp, loff_t *ppos)
{
- unsigned int rounded_pipe_max_size;
- int ret;
-
- ret = proc_douintvec_minmax(table, write, buf, lenp, ppos);
- if (ret < 0 || !write)
- return ret;
-
- rounded_pipe_max_size = round_pipe_size(pipe_max_size);
- if (rounded_pipe_max_size == 0)
- return -EINVAL;
-
- pipe_max_size = rounded_pipe_max_size;
- return ret;
+ return proc_dopipe_max_size(table, write, buf, lenp, ppos);
}

/*
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index e7497c9dde7f..485cf7a7aa8f 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -190,5 +190,6 @@ static inline int pipe_buf_steal(struct pipe_inode_info *pipe,
struct pipe_inode_info *get_pipe_info(struct file *file);

int create_pipe_files(struct file **, int);
+unsigned int round_pipe_size(unsigned int size);

#endif
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 1d4dba490fb6..ba24ca72800c 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -50,6 +50,9 @@ extern int proc_dointvec_minmax(struct ctl_table *, int,
extern int proc_douintvec_minmax(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp,
loff_t *ppos);
+extern int proc_dopipe_max_size(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp,
+ loff_t *ppos);
extern int proc_dointvec_jiffies(struct ctl_table *, int,
void __user *, size_t *, loff_t *);
extern int proc_dointvec_userhz_jiffies(struct ctl_table *, int,
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c976719bf37a..30b01b22014d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -67,6 +67,7 @@
#include <linux/kexec.h>
#include <linux/bpf.h>
#include <linux/mount.h>
+#include <linux/pipe_fs_i.h>

#include <linux/uaccess.h>
#include <asm/processor.h>
@@ -2631,6 +2632,47 @@ int proc_douintvec_minmax(struct ctl_table *table, int write,
do_proc_douintvec_minmax_conv, &param);
}

+struct do_proc_dopipe_max_size_conv_param {
+ unsigned int *min;
+};
+
+static int do_proc_dopipe_max_size_conv(unsigned long *lvalp,
+ unsigned int *valp,
+ int write, void *data)
+{
+ struct do_proc_dopipe_max_size_conv_param *param = data;
+
+ if (write) {
+ unsigned int val = round_pipe_size(*lvalp);
+
+ if (val == 0)
+ return -EINVAL;
+
+ if (param->min && *param->min > val)
+ return -ERANGE;
+
+ if (*lvalp > UINT_MAX)
+ return -EINVAL;
+
+ *valp = val;
+ } else {
+ unsigned int val = *valp;
+ *lvalp = (unsigned long) val;
+ }
+
+ return 0;
+}
+
+int proc_dopipe_max_size(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ struct do_proc_dopipe_max_size_conv_param param = {
+ .min = (unsigned int *) table->extra1,
+ };
+ return do_proc_douintvec(table, write, buffer, lenp, ppos,
+ do_proc_dopipe_max_size_conv, &param);
+}
+
static void validate_coredump_safety(void)
{
#ifdef CONFIG_COREDUMP
@@ -3136,6 +3178,12 @@ int proc_douintvec_minmax(struct ctl_table *table, int write,
return -ENOSYS;
}

+int proc_dopipe_max_size(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ return -ENOSYS;
+}
+
int proc_dointvec_jiffies(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos)
{
@@ -3179,6 +3227,7 @@ int proc_doulongvec_ms_jiffies_minmax(struct ctl_table *table, int write,
EXPORT_SYMBOL(proc_dointvec_jiffies);
EXPORT_SYMBOL(proc_dointvec_minmax);
EXPORT_SYMBOL_GPL(proc_douintvec_minmax);
+EXPORT_SYMBOL_GPL(proc_dopipe_max_size);
EXPORT_SYMBOL(proc_dointvec_userhz_jiffies);
EXPORT_SYMBOL(proc_dointvec_ms_jiffies);
EXPORT_SYMBOL(proc_dostring);
--
1.8.3.1