Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a mutex

From: Joe Lawrence
Date: Tue Sep 19 2017 - 17:47:42 EST

On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> On Tue, 5 Sep 2017, Joe Lawrence wrote:
>
>> 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()
>>
>> Protect them and the procfs sysctl assignment with a mutex.
>>
>> Reported-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>> Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
>> ---
>> fs/pipe.c | 28 ++++++++++++++++++++++++----
>> 1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/pipe.c b/fs/pipe.c
>> index fa28910b3c59..33bb11b0d78e 100644
>> --- a/fs/pipe.c
>> +++ b/fs/pipe.c
>> @@ -35,6 +35,11 @@
>> unsigned int pipe_max_size = 1048576;
>>
>> /*
>> + * Provide mutual exclusion around access to pipe_max_size
>> + */
>> +static DEFINE_MUTEX(pipe_max_mutex);
>> +
>> +/*
>> * Minimum pipe size, as required by POSIX
>> */
>> unsigned int pipe_min_size = PAGE_SIZE;
>> @@ -623,13 +628,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
>> unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
>> struct user_struct *user = get_current_user();
>> unsigned long user_bufs;
>> + unsigned int max_size;
>>
>> pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
>> if (pipe == NULL)
>> goto out_free_uid;
>>
>> - if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> - pipe_bufs = pipe_max_size >> PAGE_SHIFT;
>> + mutex_lock(&pipe_max_mutex);
>> + max_size = pipe_max_size;
>> + mutex_unlock(&pipe_max_mutex);
>> +
>> + if (pipe_bufs * PAGE_SIZE > max_size && !capable(CAP_SYS_RESOURCE))
>> + pipe_bufs = max_size >> PAGE_SHIFT;
>>
>> user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>>
>> @@ -1039,6 +1049,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>> struct pipe_buffer *bufs;
>> unsigned int size, nr_pages;
>> unsigned long user_bufs;
>> + unsigned int max_size;
>> long ret = 0;
>>
>> size = round_pipe_size(arg);
>> @@ -1056,8 +1067,11 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
>> * Decreasing the pipe capacity is always permitted, even
>> * if the user is currently over a limit.
>> */
>> + mutex_lock(&pipe_max_mutex);
>> + max_size = pipe_max_size;
>> + mutex_unlock(&pipe_max_mutex);
>> if (nr_pages > pipe->buffers &&
>> - size > pipe_max_size && !capable(CAP_SYS_RESOURCE))
>> + size > max_size && !capable(CAP_SYS_RESOURCE))
>> return -EPERM;
>>
>> user_bufs = account_pipe_buffers(pipe->user, pipe->buffers, nr_pages);
>> @@ -1131,18 +1145,24 @@ int pipe_proc_fn(struct ctl_table *table, int write, void __user *buf,
>> unsigned int rounded_pipe_max_size;
>> int ret;
>>
>> + mutex_lock(&pipe_max_mutex);
>> orig_pipe_max_size = pipe_max_size;
>> ret = proc_dointvec_minmax(table, write, buf, lenp, ppos);
>> - if (ret < 0 || !write)
>> + if (ret < 0 || !write) {
>> + mutex_unlock(&pipe_max_mutex);
>> return ret;
>> + }
>>
>> rounded_pipe_max_size = round_pipe_size(pipe_max_size);
>> if (rounded_pipe_max_size == 0) {
>> pipe_max_size = orig_pipe_max_size;
>> + mutex_unlock(&pipe_max_mutex);
>> return -EINVAL;
>> }
>>
>> pipe_max_size = rounded_pipe_max_size;
>> + mutex_unlock(&pipe_max_mutex);
>> +
>> return ret;
>> }
>>
>> --
>> 1.8.3.1
>>
> I think this mutex is too heavy - if multiple processes simultaneously
> create a pipe, the mutex would cause performance degradation.
>
> You can call do_proc_dointvec with a custom callback "conv" function
> that does the rounding of the pipe size value.

Okay, that would consolidate the fiddling with pipe_max_size into a
single place / assignment.

Implementation wise, are you suggesting something like the following
(work in progress). round_pipe_size() would still be used by
fs/pipe.c::pipe_set_size() so it would need to be visible to both source
files. The proc_do... function would be pipe-max-size specific as it's
basically:

- proc_douintvec_minmax, but
- without the "max" check
- with an added PAGE_SIZE floor
- PAGE_SIZE increment

and exported for pipe.c to call.

Plumbing in the opposite direction (export do_proc_douintvec() and stick
the new conv implementation in fs/pipe.c) might be a little easier, but
seems like the same ick-factor in the end.

Regards,

-- Joe

-->8-- -->8-- -->8-- -->8--

diff --git a/fs/pipe.c b/fs/pipe.c
index 8cbc97d97753..4db3cd2d139c 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;

@@ -1130,19 +1130,7 @@ static long pipe_set_size(struct pipe_inode_info
*pipe, unsigned long arg)
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..7a2913c5546e 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
@@ -3179,6 +3221,7 @@ 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