[PATCH v2] vfs: Fix anon_inode triggering VFS_BUG_ON_INODE in may_open()

[Penglei Jiang <superman.xpt@xxxxxxxxx>]

Some anon_inodes do not set the S_IFLNK, S_IFDIR, S_IFBLK, S_IFCHR,
S_IFIFO, S_IFSOCK, or S_IFREG flags after initialization. As a result,
opening these files triggers VFS_BUG_ON_INODE in the may_open() function.

Here is the relevant code snippet:

    static int may_open(struct mnt_idmap *idmap, const struct path *path,
                int acc_mode, int flag)
    {
            ...
            switch (inode->i_mode & S_IFMT) {
            case S_IFLNK:
            case S_IFDIR:
            case S_IFBLK:
            case S_IFCHR:
            case S_IFIFO:
            case S_IFSOCK:
            case S_IFREG:
            default:
                    VFS_BUG_ON_INODE(1, inode);
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~
            }
            ...
    }

To address this, we modify the code so that only non-anon_inodes trigger
VFS_BUG_ON_INODE, and we also check MAY_EXEC.

To determine if an inode is an anon_inode, we consider two cases:

  1. If the inode is the same as anon_inode_inode, it is the default
     anon_inode.
  2. Anonymous inodes created with alloc_anon_inode() set the S_PRIVATE
     flag. If S_PRIVATE is set, we consider it an anonymous inode.

The bug can be reproduced with the following code:

    #include <stdio.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <sys/timerfd.h>
    int main(int argc, char **argv) {
            int fd = timerfd_create(CLOCK_MONOTONIC, 0);
            if (fd != -1) {
                    char path[256];
                    sprintf(path, "/proc/self/fd/%d", fd);
                    open(path, O_RDONLY);
            }
            return 0;
    }

Finally, after testing, anon_inodes no longer trigger VFS_BUG_ON_INODE.

Fixes: af153bb63a336 ("vfs: catch invalid modes in may_open()")
Reported-by: syzbot+5d8e79d323a13aa0b248@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@xxxxxxxxxx";
Signed-off-by: Penglei Jiang <superman.xpt@xxxxxxxxx>
---
V1 -> V2: added MAY_EXEC check, added some comments

 fs/anon_inodes.c            | 12 ++++++++++++
 fs/namei.c                  |  8 +++++++-
 include/linux/anon_inodes.h |  1 +
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 583ac81669c2..bf124d53973e 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -303,6 +303,18 @@ int anon_inode_create_getfd(const char *name, const struct file_operations *fops
 	return __anon_inode_getfd(name, fops, priv, flags, context_inode, true);
 }
 
+/**
+ * is_default_anon_inode - Checks if the given inode is the default
+ * anonymous inode (anon_inode_inode)
+ *
+ * @inode: [in] the inode to be checked
+ *
+ * Returns true if the given inode is anon_inode_inode, otherwise returns false.
+ */
+inline bool is_default_anon_inode(const struct inode *inode)
+{
+	return anon_inode_inode == inode;
+}
 
 static int __init anon_inode_init(void)
 {
diff --git a/fs/namei.c b/fs/namei.c
index 360a86ca1f02..e8cc00a7c31a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -40,6 +40,7 @@
 #include <linux/bitops.h>
 #include <linux/init_task.h>
 #include <linux/uaccess.h>
+#include <linux/anon_inodes.h>
 
 #include "internal.h"
 #include "mount.h"
@@ -3429,7 +3430,12 @@ static int may_open(struct mnt_idmap *idmap, const struct path *path,
 			return -EACCES;
 		break;
 	default:
-		VFS_BUG_ON_INODE(1, inode);
+		if (!is_default_anon_inode(inode)
+			&& !(inode->i_flags & S_PRIVATE))
+			VFS_BUG_ON_INODE(1, inode);
+		if (acc_mode & MAY_EXEC)
+			return -EACCES;
+		break;
 	}
 
 	error = inode_permission(idmap, inode, MAY_OPEN | acc_mode);
diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
index edef565c2a1a..eca4a3913ba7 100644
--- a/include/linux/anon_inodes.h
+++ b/include/linux/anon_inodes.h
@@ -30,6 +30,7 @@ int anon_inode_create_getfd(const char *name,
 			    const struct file_operations *fops,
 			    void *priv, int flags,
 			    const struct inode *context_inode);
+bool is_default_anon_inode(const struct inode *inode);
 
 #endif /* _LINUX_ANON_INODES_H */
 
-- 
2.17.1