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devpts: fix error handling in devpts_mntget()

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If devpts_ptmx_path() returns an error code, then devpts_mntget()
dereferences an ERR_PTR():

    BUG: unable to handle kernel paging request at fffffffffffffff5
    IP: devpts_mntget+0x13f/0x280 fs/devpts/inode.c:173

Fix it by returning early in the error paths.

Reproducer:

    #define _GNU_SOURCE
    #include <fcntl.h>
    #include <sched.h>
    #include <sys/ioctl.h>
    #define TIOCGPTPEER _IO('T', 0x41)

    int main()
    {
        for (;;) {
            int fd = open("/dev/ptmx", 0);
            unshare(CLONE_NEWNS);
            ioctl(fd, TIOCGPTPEER, 0);
        }
    }

Fixes: 311fc65c9f ("pty: Repair TIOCGPTPEER")
Reported-by: syzbot <[email protected]>
Cc: <[email protected]> # v4.13+
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Eric Biggers 2018-01-31 00:49:18 -08:00 • committed by Linus Torvalds
commit c9cc8d01fb
1 changed files with 2 additions and 2 deletions
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4
fs/devpts/inode.c
View file

@ -168,11 +168,11 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
dput(path.dentry);
if (err) {
mntput(path.mnt);
path.mnt = ERR_PTR(err);
return ERR_PTR(err);
}
if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
mntput(path.mnt);
path.mnt = ERR_PTR(-ENODEV);
return ERR_PTR(-ENODEV);
}
return path.mnt;
}