[PATCH] proc: fix inode uid/gid writeback race

[Alexey Dobriyan <adobriyan@gmail.com>]

(euid, egid) pair is snapshotted correctly from task under RCU,
but writeback to inode can be done in any order.

Fix by doing writeback under inode->i_lock where necessary
(/proc/* , /proc/*/fd/* , /proc/*/map_files/* revalidate).

Reported-by: syzbot+e392f8008a294fdf8891@syzkaller.appspotmail.com
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/proc/base.c     |   25 +++++++++++++++++++++++--
 fs/proc/fd.c       |    2 +-
 fs/proc/internal.h |    2 ++
 3 files changed, 26 insertions(+), 3 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1743,6 +1743,25 @@ void task_dump_owner(struct task_struct *task, umode_t mode,
 	*rgid = gid;
 }
 
+/* use if inode is live */
+void task_dump_owner_to_inode(struct task_struct *task, umode_t mode,
+			      struct inode *inode)
+{
+	kuid_t uid;
+	kgid_t gid;
+
+	task_dump_owner(task, mode, &uid, &gid);
+	/*
+	 * There is no atomic "change all credentials at once" system call,
+	 * guaranteeing more than _some_ snapshot from "struct cred" ends up
+	 * in inode is not possible.
+	 */
+	spin_lock(&inode->i_lock);
+	inode->i_uid = uid;
+	inode->i_gid = gid;
+	spin_unlock(&inode->i_lock);
+}
+
 struct inode *proc_pid_make_inode(struct super_block * sb,
 				  struct task_struct *task, umode_t mode)
 {
@@ -1769,6 +1788,7 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
 	if (!ei->pid)
 		goto out_unlock;
 
+	/* fresh inode -- no races */
 	task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
 	security_task_to_inode(task, inode);
 
@@ -1802,6 +1822,7 @@ int pid_getattr(const struct path *path, struct kstat *stat,
 			 */
 			return -ENOENT;
 		}
+		/* "struct kstat" is thread local, atomic snapshot is enough */
 		task_dump_owner(task, inode->i_mode, &stat->uid, &stat->gid);
 	}
 	rcu_read_unlock();
@@ -1815,7 +1836,7 @@ int pid_getattr(const struct path *path, struct kstat *stat,
  */
 void pid_update_inode(struct task_struct *task, struct inode *inode)
 {
-	task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
+	task_dump_owner_to_inode(task, inode->i_mode, inode);
 
 	inode->i_mode &= ~(S_ISUID | S_ISGID);
 	security_task_to_inode(task, inode);
@@ -1990,7 +2011,7 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
 	mmput(mm);
 
 	if (exact_vma_exists) {
-		task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
+		task_dump_owner_to_inode(task, 0, inode);
 
 		security_task_to_inode(task, inode);
 		status = 1;
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -101,7 +101,7 @@ static bool tid_fd_mode(struct task_struct *task, unsigned fd, fmode_t *mode)
 static void tid_fd_update_inode(struct task_struct *task, struct inode *inode,
 				fmode_t f_mode)
 {
-	task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
+	task_dump_owner_to_inode(task, 0, inode);
 
 	if (S_ISLNK(inode->i_mode)) {
 		unsigned i_mode = S_IFLNK;
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -123,6 +123,8 @@ static inline struct task_struct *get_proc_task(const struct inode *inode)
 
 void task_dump_owner(struct task_struct *task, umode_t mode,
 		     kuid_t *ruid, kgid_t *rgid);
+void task_dump_owner_to_inode(struct task_struct *task, umode_t mode,
+			      struct inode *inode);
 
 unsigned name_to_int(const struct qstr *qstr);
 /*