exec: let do_coredump() limit the number of concurrent dumps to pipes

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).

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Earl Chew <earl_chew@agilent.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 49 insertions, 5 deletions

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index b3d8b4922740..a028b92001ed 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -22,6 +22,7 @@ show up in /proc/sys/kernel:
 - callhome                   [ S390 only ]
 - auto_msgmni
 - core_pattern
+- core_pipe_limit
 - core_uses_pid
 - ctrl-alt-del
 - dentry-state
@@ -135,6 +136,27 @@ 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 735d9c18ec71..dc022dd15d51 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 */
@@ -1744,7 +1745,8 @@ 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);
 
@@ -1826,28 +1828,36 @@ 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
                 file = filp_open(corename,
                                  O_CREAT | 2 | O_NOFOLLOW | O_LARGEFILE | flag,
                                  0600);
         if (IS_ERR(file))
-                goto fail_unlock;
+                goto fail_dropcount;
         inode = file->f_path.dentry->d_inode;
         if (inode->i_nlink > 1)
                 goto close_fail;        /* multiple links - don't dump */
@@ -1877,6 +1887,9 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
                 current->signal->group_exit_code |= 0x80;
 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 7f4f57bea4ce..37abb8c3995b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -76,6 +76,7 @@ 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;
@@ -423,6 +424,14 @@ 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",