[PATCH] r/o bind mounts: elevate write count for xattr_permission() callers

This basically audits the callers of xattr_permission(), which calls
permission() and can perform writes to the filesystem.

[AV: add missing parts - removexattr() and nfsd posix acls, plug for a leak
spotted by Miklos]

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 42 insertions, 9 deletions

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index c593db047d8b..c309c881bd4e 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -658,14 +658,19 @@ nfsd4_setattr(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
                         return status;
                 }
         }
+        status = mnt_want_write(cstate->current_fh.fh_export->ex_path.mnt);
+        if (status)
+                return status;
         status = nfs_ok;
         if (setattr->sa_acl != NULL)
                 status = nfsd4_set_nfs4_acl(rqstp, &cstate->current_fh,
                                             setattr->sa_acl);
         if (status)
-                return status;
+                goto out;
         status = nfsd_setattr(rqstp, &cstate->current_fh, &setattr->sa_iattr,
                                 0, (time_t)0);
+out:
+        mnt_drop_write(cstate->current_fh.fh_export->ex_path.mnt);
         return status;
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 18a4cc9feeb3..626dfd38528f 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -2086,6 +2086,9 @@ nfsd_set_posix_acl(struct svc_fh *fhp, int type, struct posix_acl *acl)
         } else
                 size = 0;
 
+        error = mnt_want_write(fhp->fh_export->ex_path.mnt);
+        if (error)
+                goto getout;
         if (size)
                 error = vfs_setxattr(fhp->fh_dentry, name, value, size, 0);
         else {
@@ -2097,6 +2100,7 @@ nfsd_set_posix_acl(struct svc_fh *fhp, int type, struct posix_acl *acl)
                                 error = 0;
                 }
         }
+        mnt_drop_write(fhp->fh_export->ex_path.mnt);
 
 getout:
         kfree(value);
diff --git a/fs/xattr.c b/fs/xattr.c
index 3acab1615460..f7062da505d4 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -11,6 +11,7 @@
 #include <linux/slab.h>
 #include <linux/file.h>
 #include <linux/xattr.h>
+#include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
@@ -32,8 +33,6 @@ xattr_permission(struct inode *inode, const char *name, int mask)
          * filesystem  or on an immutable / append-only inode.
          */
         if (mask & MAY_WRITE) {
-                if (IS_RDONLY(inode))
-                        return -EROFS;
                 if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
                         return -EPERM;
         }
@@ -262,7 +261,11 @@ sys_setxattr(char __user *path, char __user *name, void __user *value,
         error = user_path_walk(path, &nd);
         if (error)
                 return error;
-        error = setxattr(nd.path.dentry, name, value, size, flags);
+        error = mnt_want_write(nd.path.mnt);
+        if (!error) {
+                error = setxattr(nd.path.dentry, name, value, size, flags);
+                mnt_drop_write(nd.path.mnt);
+        }
         path_put(&nd.path);
         return error;
 }
@@ -277,7 +280,11 @@ sys_lsetxattr(char __user *path, char __user *name, void __user *value,
         error = user_path_walk_link(path, &nd);
         if (error)
                 return error;
-        error = setxattr(nd.path.dentry, name, value, size, flags);
+        error = mnt_want_write(nd.path.mnt);
+        if (!error) {
+                error = setxattr(nd.path.dentry, name, value, size, flags);
+                mnt_drop_write(nd.path.mnt);
+        }
         path_put(&nd.path);
         return error;
 }
@@ -295,7 +302,12 @@ sys_fsetxattr(int fd, char __user *name, void __user *value,
                 return error;
         dentry = f->f_path.dentry;
         audit_inode(NULL, dentry);
-        error = setxattr(dentry, name, value, size, flags);
+        error = mnt_want_write(f->f_path.mnt);
+        if (!error) {
+                error = setxattr(dentry, name, value, size, flags);
+                mnt_drop_write(f->f_path.mnt);
+        }
+out_fput:
         fput(f);
         return error;
 }
@@ -482,7 +494,11 @@ sys_removexattr(char __user *path, char __user *name)
         error = user_path_walk(path, &nd);
         if (error)
                 return error;
-        error = removexattr(nd.path.dentry, name);
+        error = mnt_want_write(nd.path.mnt);
+        if (!error) {
+                error = removexattr(nd.path.dentry, name);
+                mnt_drop_write(nd.path.mnt);
+        }
         path_put(&nd.path);
         return error;
 }
@@ -496,7 +512,11 @@ sys_lremovexattr(char __user *path, char __user *name)
         error = user_path_walk_link(path, &nd);
         if (error)
                 return error;
-        error = removexattr(nd.path.dentry, name);
+        error = mnt_want_write(nd.path.mnt);
+        if (!error) {
+                error = removexattr(nd.path.dentry, name);
+                mnt_drop_write(nd.path.mnt);
+        }
         path_put(&nd.path);
         return error;
 }
@@ -513,7 +533,11 @@ sys_fremovexattr(int fd, char __user *name)
                 return error;
         dentry = f->f_path.dentry;
         audit_inode(NULL, dentry);
-        error = removexattr(dentry, name);
+        error = mnt_want_write(f->f_path.mnt);
+        if (!error) {
+                error = removexattr(dentry, name);
+                mnt_drop_write(f->f_path.mnt);
+        }
         fput(f);
         return error;
 }