Re: [PATCH 2/3] exec: let do_coredump limit the number ofconcurrent dumps to pipes (v8)

[Neil Horman]

Reposting with Oleg Cc'd so he can ACK it

On Fri, Jul 03, 2009 at 06:51:29AM -0400, Neil Horman wrote:
> core_pattern: Introduce core pipe limiting sysctl
> 
> Since we can dump cores to pipe, rather than directly to the filesystem, we
> create a condition in which a user can create a very high load on the system
> simply by running bad applications.  If the pipe reader specified in
> core_pattern is poorly written, we can have lots of ourstandig resources and
> processes in the system.  This sysctl introduces an ability to limit that
> resource consumption.  core_pipe_limit defines how many in-flight dumps may be
> run in parallel, dumps beyond this value are skipped and a note is made in the
> kernel log.  A special value of 0 in core_pipe_limit denotes unlimited core
> dumps may be handled (this is the default value).
> 
> Signed-off-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
> Reported-by: Earl Chew <earl_chew@xxxxxxxxxxx>
> 
> 
>  Documentation/sysctl/kernel.txt |   22 ++++++++++++++++++++++
>  fs/exec.c                       |   21 +++++++++++++++++----
>  kernel/sysctl.c                 |    9 +++++++++
>  3 files changed, 48 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 322a00b..bb226ba 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -21,6 +21,7 @@ show up in /proc/sys/kernel:
>  - acct
>  - auto_msgmni
>  - core_pattern
> +- core_pipe_limit
>  - core_uses_pid
>  - ctrl-alt-del
>  - dentry-state
> @@ -119,6 +120,27 @@ core_pattern is used to specify a core dumpfile pattern name.
>  
>  ==============================================================
>  
> +core_pipe_limit:
> +
> +This sysctl is only applicable when core_pattern is configured to pipe core
> +files to user space helper a (when the first character of core_pattern is a '|',
> +see above).  When collecting cores via a pipe to an application, it is
> +occasionally usefull for the collecting application to gather data about the
> +crashing process from its /proc/pid directory.  In order to do this safely, the
> +kernel must wait for the collecting process to exit, so as not to remove the
> +crashing processes proc files prematurely.  This in turn creates the possibility
> +that a misbehaving userspace collecting process can block the reaping of a
> +crashed process simply by never exiting.  This sysctl defends against that.  It
> +defines how many concurrent crashing processes may be piped to user space
> +applications in parallel.  If this value is exceeded, then those crashing
> +processes above that value are noted via the kernel log and their cores are
> +skipped.  0 is a special value, indicating that unlimited processes may be
> +captured in parallel, but that no waiting will take place (i.e. the collecting
> +process is not guaranteed access to /proc/<crahing pid>/).  This value defaults
> +to 0.
> +
> +==============================================================
> +
>  core_uses_pid:
>  
>  The default coredump filename is "core".  By setting
> diff --git a/fs/exec.c b/fs/exec.c
> index 9defd20..93ab6eb 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -63,6 +63,7 @@
>  
>  int core_uses_pid;
>  char core_pattern[CORENAME_MAX_SIZE] = "core";
> +unsigned int core_pipe_limit;
>  int suid_dumpable = 0;
>  
>  /* The maximal length of core_pattern is also specified in sysctl.c */
> @@ -1726,7 +1727,8 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  	unsigned long core_limit = current->signal->rlim[RLIMIT_CORE].rlim_cur;
>  	char **helper_argv = NULL;
>  	int helper_argc = 0;
> -	char *delimit;
> +	int dump_count = 0;
> +	static atomic_t core_dump_count = ATOMIC_INIT(0);
>  
>  	audit_core_dumps(signr);
>  
> @@ -1798,21 +1800,29 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  			goto fail_unlock;
>  		}
>  
> +		dump_count = atomic_inc_return(&core_dump_count);
> +		if (core_pipe_limit && (core_pipe_limit < dump_count)) {
> +			printk(KERN_WARNING "Pid %d(%s) over core_pipe_limit\n",
> +			       task_tgid_vnr(current), current->comm);
> +			printk(KERN_WARNING "Skipping core dump\n");
> +			goto fail_dropcount;
> +		}
> +
>  		helper_argv = argv_split(GFP_KERNEL, corename+1, &helper_argc);
>  		if (!helper_argv) {
>  			printk(KERN_WARNING "%s failed to allocate memory\n",
>  			       __func__);
> -			goto fail_unlock;
> +			goto fail_dropcount;
>  		}
>  
>  		core_limit = RLIM_INFINITY;
>  
>  		/* SIGPIPE can happen, but it's just never processed */
> - 		if (call_usermodehelper_pipe(corename+1, helper_argv, NULL,
> + 		if (call_usermodehelper_pipe(helper_argv[0], helper_argv, NULL,
>  				&file)) {
>   			printk(KERN_INFO "Core dump to %s pipe failed\n",
>  			       corename);
> - 			goto fail_unlock;
> + 			goto fail_dropcount;
>   		}
>  	} else {
>  		if (core_limit < binfmt->min_coredump)
> @@ -1853,6 +1863,9 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  
>  close_fail:
>  	filp_close(file, NULL);
> +fail_dropcount:
> +	if (dump_count)
> +		atomic_dec(&core_dump_count);
>  fail_unlock:
>  	if (helper_argv)
>  		argv_free(helper_argv);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 62e4ff9..681052f 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -77,6 +77,7 @@ extern int max_threads;
>  extern int core_uses_pid;
>  extern int suid_dumpable;
>  extern char core_pattern[];
> +extern unsigned int core_pipe_limit;
>  extern int pid_max;
>  extern int min_free_kbytes;
>  extern int pid_max_min, pid_max_max;
> @@ -407,6 +408,14 @@ static struct ctl_table kern_table[] = {
>  		.proc_handler	= &proc_dostring,
>  		.strategy	= &sysctl_string,
>  	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname	= "core_pipe_limit",
> +		.data		= &core_pipe_limit,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= &proc_dointvec,
> +	},
>  #ifdef CONFIG_PROC_SYSCTL
>  	{
>  		.procname	= "tainted",
> --
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