coredump: Fixes core_pipe_limit sysctl proc_handler

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: a293980 ("exec: let do_coredump() limit the number of concurrent dumps to pipes")
Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>

fs/coredump.c

@@ -1015,7 +1015,9 @@ static const 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",