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SubjectRe: [PATCH 2/3] exec: let do_coredump limit the number of concurrent dumps to pipes (v7)
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@tuxdriver.com>
Reported-by: Earl Chew <earl_chew@agilent.com>


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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