[PATCH 1/3] coredump: Fixes core_pipe_limit sysctl proc_handler

[nicolas.bouchinet@clip-os.org]

From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>

proc_dointvec converts a string to a vector of signed int, which is
stored in the unsigned int .data core_pipe_limit.
It was thus authorized to write a negative value to core_pipe_limit
sysctl which once stored in core_pipe_limit, leads to the signed int
dump_count check against core_pipe_limit never be true. The same can be
achieved with core_pipe_limit set to INT_MAX.

Any negative write or >= to INT_MAX in core_pipe_limit sysctl would
hypothetically allow a user to create very high load on the system by
running processes that produces a coredump in case the core_pattern
sysctl is configured to pipe core files to user space helper.
Memory or PID exhaustion should happen before but it anyway breaks the
core_pipe_limit semantic

This commit fixes this by changing core_pipe_limit sysctl's proc_handler
to proc_dointvec_minmax and bound checking between SYSCTL_ZERO and
SYSCTL_INT_MAX.

Fixes: a293980c2e26 ("exec: let do_coredump() limit the number of concurrent dumps to pipes")
Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
---
 fs/coredump.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 7f12ff6ad1d3e..8ea5896e518dd 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -616,7 +616,8 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 		cprm.limit = RLIM_INFINITY;
 
 		dump_count = atomic_inc_return(&core_dump_count);
-		if (core_pipe_limit && (core_pipe_limit < dump_count)) {
+		if ((core_pipe_limit && (core_pipe_limit < dump_count)) ||
+		    (core_pipe_limit && dump_count == INT_MAX)) {
 			printk(KERN_WARNING "Pid %d(%s) over core_pipe_limit\n",
 			       task_tgid_vnr(current), current->comm);
 			printk(KERN_WARNING "Skipping core dump\n");
@@ -1024,7 +1025,9 @@ static struct ctl_table coredump_sysctls[] = {
 		.data		= &core_pipe_limit,
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_INT_MAX,
 	},
 	{
 		.procname       = "core_file_note_size_limit",
-- 
2.47.0