[PATCH v5 5/6] prctl: Allow checkpoint/restore capable processes to change exe link

[Adrian Reber <areber@redhat.com>]

From: Nicolas Viennot <Nicolas.Viennot@twosigma.com>

Allow CAP_CHECKPOINT_RESTORE capable users to change /proc/self/exe.

This commit also changes the permission error code from -EINVAL to
-EPERM for consistency with the rest of the prctl() syscall when
checking capabilities.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
Signed-off-by: Adrian Reber <areber@redhat.com>
---
 kernel/sys.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 00a96746e28a..dd59b9142b1d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2007,12 +2007,14 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 
 	if (prctl_map.exe_fd != (u32)-1) {
 		/*
-		 * Make sure the caller has the rights to
-		 * change /proc/pid/exe link: only local sys admin should
-		 * be allowed to.
+		 * Check if the current user is checkpoint/restore capable.
+		 * At the time of this writing, it checks for CAP_SYS_ADMIN
+		 * or CAP_CHECKPOINT_RESTORE.
+		 * Note that a user with access to ptrace can masquerade an
+		 * arbitrary program as any executable, even setuid ones.
 		 */
-		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
-			return -EINVAL;
+		if (!checkpoint_restore_ns_capable(current_user_ns()))
+			return -EPERM;
 
 		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
 		if (error)
-- 
2.26.2