Date:   Mon, 25 Sep 2017 06:44:49 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH RFC 2/3] pipe: protect pipe_max_size access with a
 mutex



On Tue, 19 Sep 2017, Joe Lawrence wrote:

> On 09/14/2017 07:09 PM, Mikulas Patocka wrote:
> >
> > 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

Yes. I think this is correct.

Reviewed-by: Mikulas Patocka <mpatocka@...hat.com>

> -->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
> 

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