[PATCH v3 1/2] prctl_set_mm: Refactor checks from validate_prctl_map

[Michal Koutný <mkoutny@xxxxxxxx>]

Despite comment of validate_prctl_map claims there are no capability
checks, it is not completely true since commit 4d28df6152aa ("prctl:
Allow local CAP_SYS_ADMIN changing exe_file"). Extract the check out of
the function and make the function perform purely arithmetic checks.

This patch should not change any behavior, it is mere refactoring for
following patch.

v1, v2: ---
v3: Remove unused mm variable from validate_prctl_map_addr

CC: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
CC: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 kernel/sys.c | 46 ++++++++++++++++++++--------------------------
 1 file changed, 20 insertions(+), 26 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..5e0a5edf47f8 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1882,13 +1882,14 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 }
 
 /*
+ * Check arithmetic relations of passed addresses.
+ *
  * WARNING: we don't require any capability here so be very careful
  * in what is allowed for modification from userspace.
  */
-static int validate_prctl_map(struct prctl_mm_map *prctl_map)
+static int validate_prctl_map_addr(struct prctl_mm_map *prctl_map)
 {
 	unsigned long mmap_max_addr = TASK_SIZE;
-	struct mm_struct *mm = current->mm;
 	int error = -EINVAL, i;
 
 	static const unsigned char offsets[] = {
@@ -1949,24 +1950,6 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map)
 			      prctl_map->start_data))
 			goto out;
 
-	/*
-	 * Someone is trying to cheat the auxv vector.
-	 */
-	if (prctl_map->auxv_size) {
-		if (!prctl_map->auxv || prctl_map->auxv_size > sizeof(mm->saved_auxv))
-			goto out;
-	}
-
-	/*
-	 * Finally, make sure the caller has the rights to
-	 * change /proc/pid/exe link: only local sys admin should
-	 * be allowed to.
-	 */
-	if (prctl_map->exe_fd != (u32)-1) {
-		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
-			goto out;
-	}
-
 	error = 0;
 out:
 	return error;
@@ -1993,11 +1976,17 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 	if (copy_from_user(&prctl_map, addr, sizeof(prctl_map)))
 		return -EFAULT;
 
-	error = validate_prctl_map(&prctl_map);
+	error = validate_prctl_map_addr(&prctl_map);
 	if (error)
 		return error;
 
 	if (prctl_map.auxv_size) {
+		/*
+		 * Someone is trying to cheat the auxv vector.
+		 */
+		if (!prctl_map.auxv || prctl_map.auxv_size > sizeof(mm->saved_auxv))
+			return -EINVAL;
+
 		memset(user_auxv, 0, sizeof(user_auxv));
 		if (copy_from_user(user_auxv,
 				   (const void __user *)prctl_map.auxv,
@@ -2010,6 +1999,14 @@ 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.
+		 */
+		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+			return -EINVAL;
+
 		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
 		if (error)
 			return error;
@@ -2097,7 +2094,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
 			unsigned long arg4, unsigned long arg5)
 {
 	struct mm_struct *mm = current->mm;
-	struct prctl_mm_map prctl_map;
+	struct prctl_mm_map prctl_map = { .auxv = NULL, .auxv_size = 0, .exe_fd = -1 };
 	struct vm_area_struct *vma;
 	int error;
 
@@ -2139,9 +2136,6 @@ static int prctl_set_mm(int opt, unsigned long addr,
 	prctl_map.arg_end	= mm->arg_end;
 	prctl_map.env_start	= mm->env_start;
 	prctl_map.env_end	= mm->env_end;
-	prctl_map.auxv		= NULL;
-	prctl_map.auxv_size	= 0;
-	prctl_map.exe_fd	= -1;
 
 	switch (opt) {
 	case PR_SET_MM_START_CODE:
@@ -2181,7 +2175,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
 		goto out;
 	}
 
-	error = validate_prctl_map(&prctl_map);
+	error = validate_prctl_map_addr(&prctl_map);
 	if (error)
 		goto out;
 
-- 
2.16.4