[SRU,DISCO,EOAN,FOCAL] UBUNTU: SAUCE: shiftfs: prevent lower dentries from going negative during unlink
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Message ID 20200116213641.24064-1-christian.brauner@ubuntu.com
State New
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  • [SRU,DISCO,EOAN,FOCAL] UBUNTU: SAUCE: shiftfs: prevent lower dentries from going negative during unlink
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Commit Message

Christian Brauner Jan. 16, 2020, 9:36 p.m. UTC
BugLink: https://bugs.launchpad.net/bugs/1860041

All non-special files (For shiftfs this only includes fifos and - for
this case - unix sockets - since we don't allow character and block
devices to be created.) go through shiftfs_open() and have their dentry
pinned through this codepath preventing it from going negative. But
fifos don't use the shiftfs fops but rather use the pipefifo_fops which
means they do not go through shiftfs_open() and thus don't have their
dentry pinned that way. Thus, the lower dentries for such files can go
negative on unlink causing segfaults. The following C program can be
used to reproduce the crash:

 #include <stdio.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <stdlib.h>

 int main(int argc, char *argv[])
 {
        struct stat stat;

        unlink("./bbb");

        int ret = mknod("./bbb", S_IFIFO|0666, 0);
        if (ret < 0)
                exit(1);

        int fd = open("./bbb", O_RDWR);
        if (fd < 0)
                exit(2);

        if (unlink("./bbb"))
                exit(4);

        fstat(fd, &stat);

        return 0;
 }

Similar to ecryptfs we need to dget() the lower dentry before calling
vfs_unlink() on it and dput() it afterwards.

Link: https://travis-ci.community/t/arm64-ppc64le-segfaults/6158/3
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/shiftfs.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Stefan Bader Jan. 17, 2020, 8:29 a.m. UTC | #1
On 16.01.20 22:36, Christian Brauner wrote:
> BugLink: https://bugs.launchpad.net/bugs/1860041
> 
> All non-special files (For shiftfs this only includes fifos and - for
> this case - unix sockets - since we don't allow character and block
> devices to be created.) go through shiftfs_open() and have their dentry
> pinned through this codepath preventing it from going negative. But
> fifos don't use the shiftfs fops but rather use the pipefifo_fops which
> means they do not go through shiftfs_open() and thus don't have their
> dentry pinned that way. Thus, the lower dentries for such files can go
> negative on unlink causing segfaults. The following C program can be
> used to reproduce the crash:
> 
>  #include <stdio.h>
>  #include <fcntl.h>
>  #include <unistd.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
>  #include <unistd.h>
>  #include <stdlib.h>
> 
>  int main(int argc, char *argv[])
>  {
>         struct stat stat;
> 
>         unlink("./bbb");
> 
>         int ret = mknod("./bbb", S_IFIFO|0666, 0);
>         if (ret < 0)
>                 exit(1);
> 
>         int fd = open("./bbb", O_RDWR);
>         if (fd < 0)
>                 exit(2);
> 
>         if (unlink("./bbb"))
>                 exit(4);
> 
>         fstat(fd, &stat);
> 
>         return 0;
>  }
> 
> Similar to ecryptfs we need to dget() the lower dentry before calling
> vfs_unlink() on it and dput() it afterwards.
> 
> Link: https://travis-ci.community/t/arm64-ppc64le-segfaults/6158/3
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
> ---
>  fs/shiftfs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/shiftfs.c b/fs/shiftfs.c
> index 04fba4689eb6..897e9f1b90db 100644
> --- a/fs/shiftfs.c
> +++ b/fs/shiftfs.c
> @@ -583,6 +583,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>  	int err;
>  	const struct cred *oldcred;
>  
> +	dget(lowerd)
>  	oldcred = shiftfs_override_creds(dentry->d_sb);
>  	inode_lock_nested(loweri, I_MUTEX_PARENT);
>  	if (rmdir)
> @@ -602,6 +603,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>  	inode_unlock(loweri);
>  
>  	shiftfs_copyattr(loweri, dir);
> +	dput(lowerd);
>  
>  	return err;
>  }
>
Stefan Bader Jan. 20, 2020, 1:48 p.m. UTC | #2
On 16.01.20 23:36, Christian Brauner wrote:
> BugLink: https://bugs.launchpad.net/bugs/1860041
> 
> All non-special files (For shiftfs this only includes fifos and - for
> this case - unix sockets - since we don't allow character and block
> devices to be created.) go through shiftfs_open() and have their dentry
> pinned through this codepath preventing it from going negative. But
> fifos don't use the shiftfs fops but rather use the pipefifo_fops which
> means they do not go through shiftfs_open() and thus don't have their
> dentry pinned that way. Thus, the lower dentries for such files can go
> negative on unlink causing segfaults. The following C program can be
> used to reproduce the crash:
> 
>  #include <stdio.h>
>  #include <fcntl.h>
>  #include <unistd.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
>  #include <unistd.h>
>  #include <stdlib.h>
> 
>  int main(int argc, char *argv[])
>  {
>         struct stat stat;
> 
>         unlink("./bbb");
> 
>         int ret = mknod("./bbb", S_IFIFO|0666, 0);
>         if (ret < 0)
>                 exit(1);
> 
>         int fd = open("./bbb", O_RDWR);
>         if (fd < 0)
>                 exit(2);
> 
>         if (unlink("./bbb"))
>                 exit(4);
> 
>         fstat(fd, &stat);
> 
>         return 0;
>  }
> 
> Similar to ecryptfs we need to dget() the lower dentry before calling
> vfs_unlink() on it and dput() it afterwards.
> 
> Link: https://travis-ci.community/t/arm64-ppc64le-segfaults/6158/3
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> ---
>  fs/shiftfs.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/shiftfs.c b/fs/shiftfs.c
> index 04fba4689eb6..897e9f1b90db 100644
> --- a/fs/shiftfs.c
> +++ b/fs/shiftfs.c
> @@ -583,6 +583,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>  	int err;
>  	const struct cred *oldcred;
>  
> +	dget(lowerd)
>  	oldcred = shiftfs_override_creds(dentry->d_sb);
>  	inode_lock_nested(loweri, I_MUTEX_PARENT);
>  	if (rmdir)
> @@ -602,6 +603,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>  	inode_unlock(loweri);
>  
>  	shiftfs_copyattr(loweri, dir);
> +	dput(lowerd);
>  
>  	return err;
>  }
> 
There is a v2 that was sent.

Patch
diff mbox series

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index 04fba4689eb6..897e9f1b90db 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -583,6 +583,7 @@  static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
 	int err;
 	const struct cred *oldcred;
 
+	dget(lowerd)
 	oldcred = shiftfs_override_creds(dentry->d_sb);
 	inode_lock_nested(loweri, I_MUTEX_PARENT);
 	if (rmdir)
@@ -602,6 +603,7 @@  static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
 	inode_unlock(loweri);
 
 	shiftfs_copyattr(loweri, dir);
+	dput(lowerd);
 
 	return err;
 }