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Commit 7a4dec53 authored by Al Viro's avatar Al Viro
Browse files

Fix sget() race with failing mount 



If sget() finds a matching superblock being set up, it'll
grab an active reference to it and grab s_umount.  That's
fine - we'll wait for completion of foofs_get_sb() that way.
However, if said foofs_get_sb() fails we'll end up holding
the halfway-created superblock.  deactivate_locked_super()
called by foofs_get_sb() will just unlock the sucker since
we are holding another active reference to it.

What we need is a way to tell if superblock has been successfully
set up.  Unfortunately, neither ->s_root nor the check for
MS_ACTIVE quite fit.  Cheap and easy way, suitable for backport:
new flag set by the (only) caller of ->get_sb().  If that flag
isn't present by the time sget() grabbed s_umount on preexisting
superblock it has found, it's seeing a stillborn and should
just bury it with deactivate_locked_super() (and repeat the search).

Longer term we want to set that flag in ->get_sb() instances (and
check for it to distinguish between "sget() found us a live sb"
and "sget() has allocated an sb, we need to set it up" in there,
instead of checking ->s_root as we do now).

Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org
parent 4f331f01

fs/namespace.c

+1 −1
Original line number Diff line number Diff line
@@ -1984,7 +1984,7 @@ long do_mount(char *dev_name, char *dir_name, char *type_page, 
	if (flags & MS_RDONLY)

		mnt_flags |= MNT_READONLY;



	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |

	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |

		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |

		   MS_STRICTATIME);

fs/super.c

+6 −0
Original line number Diff line number Diff line
@@ -305,8 +305,13 @@ struct super_block *sget(struct file_system_type *type, 
			if (s) {

				up_write(&s->s_umount);

				destroy_super(s);

				s = NULL;

			}

			down_write(&old->s_umount);

			if (unlikely(!(old->s_flags & MS_BORN))) {

				deactivate_locked_super(old);

				goto retry;

			}

			return old;

		}

	}
@@ -918,6 +923,7 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void 
		goto out_free_secdata;

	BUG_ON(!mnt->mnt_sb);

	WARN_ON(!mnt->mnt_sb->s_bdi);

	mnt->mnt_sb->s_flags |= MS_BORN;



	error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);

	if (error)

include/linux/fs.h

+1 −0
Original line number Diff line number Diff line
@@ -209,6 +209,7 @@ struct inodes_stat_t { 
#define MS_KERNMOUNT	(1<<22) /* this is a kern_mount call */

#define MS_I_VERSION	(1<<23) /* Update inode I_version field */

#define MS_STRICTATIME	(1<<24) /* Always perform atime updates */

#define MS_BORN		(1<<29)

#define MS_ACTIVE	(1<<30)

#define MS_NOUSER	(1<<31)