[PATCH 2/3] 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.

CC: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
CC: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 kernel/sys.c | 45 ++++++++++++++++++++-------------------------
 1 file changed, 20 insertions(+), 25 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..e1acb444d7b0 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1882,10 +1882,12 @@ 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;
@@ -1949,24 +1951,6 @@ 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 +1977,17 @@ 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 +2000,14 @@ 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 +2095,7 @@ 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 +2137,6 @@ 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 +2176,7 @@ 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