[PATCH v2 3/3] prctl: Allow ptrace capable processes to change exe_fd

[Adrian Reber <areber@redhat.com>]

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

The current process is authorized to change its /proc/self/exe link via
two policies:
1) The current user can do checkpoint/restore In other words is
   CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE capable.
2) The current user can use ptrace.

With access to ptrace facilities, a process can do the following: fork a
child, execve() the target executable, and have the child use ptrace()
to replace the memory content of the current process. This technique
makes it possible to masquerade an arbitrary program as any executable,
even setuid ones.

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 | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index fd46865b46ba..2f34108e26e0 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1994,12 +1994,23 @@ 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.
+		 * The current process is authorized to change its
+		 * /proc/self/exe link via two policies:
+		 * 1) The current user can do checkpoint/restore
+		 *    In other words is CAP_SYS_ADMIN or
+		 *    CAP_CHECKPOINT_RESTORE capable.
+		 * 2) The current user can use ptrace.
+		 *
+		 * With access to ptrace facilities, a process can do the
+		 * following: fork a child, execve() the target executable,
+		 * and have the child use ptrace() to replace the memory
+		 * content of the current process. This technique makes it
+		 * possible to masquerade an arbitrary program as the target
+		 * executable, even if it is setuid.
 		 */
-		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
-			return -EINVAL;
+		if (!(checkpoint_restore_ns_capable(current_user_ns()) ||
+		      security_ptrace_access_check(current, PTRACE_MODE_ATTACH_REALCREDS)))
+			return -EPERM;
 
 		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
 		if (error)
-- 
2.26.2