| Message ID | 3e998bf87638a442cbc6864cdcd3d8d9e08ce3e3.camel@HansenPartnership.com |
|---|---|
| State | New |
| Headers | show |
| Series | efivarfs: fix NULL dereference on resume | expand |
On Mon, Mar 17, 2025 at 11:06:01PM -0400, James Bottomley wrote: > + /* ensure single superblock is alive and pin it */ > + if (!atomic_inc_not_zero(&s->s_active)) > + return NOTIFY_DONE; > + > pr_info("efivarfs: resyncing variable state\n"); > > - /* O_NOATIME is required to prevent oops on NULL mnt */ > + path.dentry = sfi->sb->s_root; > + > + /* > + * do not add SB_KERNMOUNT which a single superblock could > + * expose to userspace and which also causes MNT_INTERNAL, see > + * below > + */ > + path.mnt = vfs_kern_mount(&efivarfs_type, 0, > + efivarfs_type.name, NULL); Umm... That's probably safe, but not as a long-term solution - it's too intimately dependent upon fs/super.c internals. The reasons why you can't run into ->s_umount deadlock here are non-trivial...
On Tue, Mar 18, 2025 at 08:04:59AM +0100, Ard Biesheuvel wrote: > the latter is only needed when it is mounted to begin with, and as a > VFS non-expert, I struggle to understand why it is a) ok and b) > preferred to create a new mount to pass to kernel_file_open(). Could > we add a paragraph to the commit log that explains this? I'm not at all convinced that iterate_dir() is the right thing to use there, but *IF* we go that way, yes, we need a reference to struct mount. We are not going to introduce a very special kind of struct file, along with the arseloads of checking for that crap all over the place - not for the sake of one weird case in one weird filesystem. file->f_path is a valid struct path, which means that ->mnt->mnt_sb == ->dentry->d_sb and refcount of ->mnt is positive as long as struct file exists. Keeping a persistent internal struct mount is, of course, possible, but it will make the damn thing impossible to rmmod, etc. - it will remain in place until the reboot. It might be possible to put together something like "grab a reference to superblock and allocate a temporary struct mount refering to it" (which is what that vfs_kern_mount() boils down to). But I would very much prefer to have it go over the list of children of ->s_root manually, instead of playing silly buggers with iterate_dir(). And yes, it would require exclusion with dissolving dentry tree on umount, for obvious reasons. Which might be done with ->s_active or simply by unregistering that notifier chain as the very first step in ->kill_sb() there.
diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c index 6eae8cf655c1..81b3c6b7e100 100644 --- a/fs/efivarfs/super.c +++ b/fs/efivarfs/super.c @@ -474,12 +474,25 @@ static int efivarfs_check_missing(efi_char16_t *name16, efi_guid_t vendor, return err; } +static void efivarfs_deactivate_super_work(struct work_struct *work) +{ + struct super_block *s = container_of(work, struct super_block, + destroy_work); + /* + * note: here s->destroy_work is free for reuse (which + * will happen in deactivate_super) + */ + deactivate_super(s); +} + +static struct file_system_type efivarfs_type; + static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action, void *ptr) { struct efivarfs_fs_info *sfi = container_of(nb, struct efivarfs_fs_info, pm_nb); - struct path path = { .mnt = NULL, .dentry = sfi->sb->s_root, }; + struct path path; struct efivarfs_ctx ectx = { .ctx = { .actor = efivarfs_actor, @@ -487,6 +500,7 @@ static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action, .sb = sfi->sb, }; struct file *file; + struct super_block *s = sfi->sb; static bool rescan_done = true; if (action == PM_HIBERNATION_PREPARE) { @@ -499,11 +513,43 @@ static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action, if (rescan_done) return NOTIFY_DONE; + /* ensure single superblock is alive and pin it */ + if (!atomic_inc_not_zero(&s->s_active)) + return NOTIFY_DONE; + pr_info("efivarfs: resyncing variable state\n"); - /* O_NOATIME is required to prevent oops on NULL mnt */ + path.dentry = sfi->sb->s_root; + + /* + * do not add SB_KERNMOUNT which a single superblock could + * expose to userspace and which also causes MNT_INTERNAL, see + * below + */ + path.mnt = vfs_kern_mount(&efivarfs_type, 0, + efivarfs_type.name, NULL); + if (IS_ERR(path.mnt)) { + pr_err("efivarfs: internal mount failed\n"); + /* + * We may be the last pinner of the superblock but + * calling efivarfs_kill_sb from within the notifier + * here would deadlock trying to unregister it + */ + INIT_WORK(&s->destroy_work, efivarfs_deactivate_super_work); + schedule_work(&s->destroy_work); + return PTR_ERR(path.mnt); + } + + /* path.mnt now has pin on superblock, so this must be above one */ + atomic_dec(&s->s_active); + file = kernel_file_open(&path, O_RDONLY | O_DIRECTORY | O_NOATIME, current_cred()); + /* + * safe even if last put because no MNT_INTERNAL means this + * will do delayed deactivate_super and not deadlock + */ + mntput(path.mnt); if (IS_ERR(file)) return NOTIFY_DONE;
LSMs often inspect the path.mnt of files in the security hooks which causes a NULL deref in efivarfs_pm_notify because the path is constructed with a NULL path.mnt. Fix by obtaining from vfs_kern_mount() instead and being very careful to ensure that deactivate_super() (potentially caused by a racing userspace umount) is not called directly from the notifier because it would deadlock when efivarfs_kill_sb tried to unregister the notifier chain. Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com> --- fs/efivarfs/super.c | 50 +++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 48 insertions(+), 2 deletions(-)