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Subject[PATCH v6 3/7] exec: Move path_noexec() check earlier
Date
From: Kees Cook <keescook@chromium.org>

The path_noexec() check, like the regular file check, was happening too
late, letting LSMs see impossible execve()s. Check it earlier as well
in may_open() and collect the redundant fs/exec.c path_noexec() test
under the same robustness comment as the S_ISREG() check.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
struct open_flags open_exec_flags = {
.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
.acc_mode = MAY_EXEC,
...
do_filp_open(dfd, filename, open_flags)
path_openat(nameidata, open_flags, flags)
file = alloc_empty_file(open_flags, current_cred());
do_open(nameidata, file, open_flags)
may_open(path, acc_mode, open_flag)
/* new location of MAY_EXEC vs path_noexec() test */
inode_permission(inode, MAY_OPEN | acc_mode)
security_inode_permission(inode, acc_mode)
vfs_open(path, file)
do_dentry_open(file, path->dentry->d_inode, open)
security_file_open(f)
open()
/* old location of path_noexec() test */

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
---
fs/exec.c | 12 ++++--------
fs/namei.c | 4 ++++
2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index bdc6a6eb5dce..4eea20c27b01 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
* and check again at the very end too.
*/
error = -EACCES;
- if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
- goto exit;
-
- if (path_noexec(&file->f_path))
+ if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+ path_noexec(&file->f_path)))
goto exit;

fsnotify_open(file);
@@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
* and check again at the very end too.
*/
err = -EACCES;
- if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
- goto exit;
-
- if (path_noexec(&file->f_path))
+ if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
+ path_noexec(&file->f_path)))
goto exit;

err = deny_write_access(file);
diff --git a/fs/namei.c b/fs/namei.c
index a559ad943970..ddc9b25540fe 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
return -EACCES;
flag &= ~O_TRUNC;
break;
+ case S_IFREG:
+ if ((acc_mode & MAY_EXEC) && path_noexec(path))
+ return -EACCES;
+ break;
}

error = inode_permission(inode, MAY_OPEN | acc_mode);
--
2.27.0
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