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Subject[PATCH 60/73] union-mount: Implement union-aware access()/faccessat() [ver #2]
Date
From: Valerie Aurora <vaurora@redhat.com>

For union mounts, a file located on the lower layer will incorrectly
return EROFS on an access check. To fix this, use the new
path_permission() call, which ignores a read-only lower layer file
system if the target will be copied up to the topmost file system.

Original-author: Valerie Aurora <vaurora@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/open.c | 41 +++++++++++++++++++++++++++++++++++------
1 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 3c44148..d3be9e3 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -32,6 +32,7 @@
#include <linux/dnotify.h>

#include "internal.h"
+#include "union.h"

int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
struct file *filp)
@@ -301,7 +302,11 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
const struct cred *old_cred;
struct cred *override_cred;
struct path path;
+ struct nameidata nd;
+ struct vfsmount *mnt;
struct inode *inode;
+ umode_t i_mode;
+ char *tmp;
int res;

if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */
@@ -325,25 +330,47 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)

old_cred = override_creds(override_cred);

- res = user_path_at(dfd, filename, LOOKUP_FOLLOW, &path);
+ res = user_path_nd(dfd, filename, LOOKUP_FOLLOW, &nd, &path, &tmp);
if (res)
goto out;

+ /* For union mounts, use the topmost mnt's permissions */
+ mnt = path.mnt;
+ if (IS_MNT_LOWER(mnt))
+ mnt = nd.path.mnt;
+
inode = path.dentry->d_inode;
+ i_mode = inode->i_mode;

- if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
+ if ((mode & MAY_EXEC) && S_ISREG(i_mode)) {
/*
* MAY_EXEC on regular files is denied if the fs is mounted
* with the "noexec" flag.
*/
res = -EACCES;
- if (path.mnt->mnt_flags & MNT_NOEXEC)
+ if (mnt->mnt_flags & MNT_NOEXEC)
+ goto out_path_release;
+ }
+
+ mode |= MAY_ACCESS;
+ if ((mode & MAY_WRITE) && unlikely(IS_MNT_LOWER(path.mnt))) {
+ /* If we need to copy up, then the upperfs of a union must be
+ * writable. The lowerfs must be mounted read-only for the
+ * union to exist, but we don't care about that.
+ */
+ res = -EROFS;
+ if ((mnt->mnt_sb->s_flags & MS_RDONLY) &&
+ (S_ISREG(i_mode) || S_ISDIR(i_mode) || S_ISLNK(i_mode)))
goto out_path_release;
+
+ /* We do need write permission on the lower inode, however */
+ res = __inode_permission(inode, mode);
+ } else {
+ res = inode_permission(inode, mode);
}

- res = inode_permission(inode, mode | MAY_ACCESS);
/* SuS v2 requires we report a read only fs too */
- if (res || !(mode & S_IWOTH) || special_file(inode->i_mode))
+ if (res || !(mode & MAY_WRITE) || special_file(inode->i_mode))
goto out_path_release;
/*
* This is a rare case where using __mnt_is_readonly()
@@ -355,11 +382,13 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
* inherently racy and know that the fs may change
* state before we even see this result.
*/
- if (__mnt_is_readonly(path.mnt))
+ if (__mnt_is_readonly(mnt))
res = -EROFS;

out_path_release:
path_put(&path);
+ path_put(&nd.path);
+ putname(tmp);
out:
revert_creds(old_cred);
put_cred(override_cred);


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