From a8d02e97b6a23ac9648a7a550feb0ed7c7109ec2 Mon Sep 17 00:00:00 2001 From: Al Viro Date: Thu, 16 Jun 2011 15:10:06 +0100 Subject: [PATCH] VFS: Fix vfsmount overput on simultaneous automount BugLink: http://bugs.launchpad.net/bugs/769927 [Kudos to dhowells for tracking that crap down] If two processes attempt to cause automounting on the same mountpoint at the same time, the vfsmount holding the mountpoint will be left with one too few references on it, causing a BUG when the kernel tries to clean up. The problem is that lock_mount() drops the caller's reference to the mountpoint's vfsmount in the case where it finds something already mounted on the mountpoint as it transits to the mounted filesystem and replaces path->mnt with the new mountpoint vfsmount. During a pathwalk, however, we don't take a reference on the vfsmount if it is the same as the one in the nameidata struct, but do_add_mount() doesn't know this. The fix is to make sure we have a ref on the vfsmount of the mountpoint before calling do_add_mount(). However, if lock_mount() doesn't transit, we're then left with an extra ref on the mountpoint vfsmount which needs releasing. We can handle that in follow_managed() by not making assumptions about what we can and what we cannot get from lookup_mnt() as the current code does. The callers of follow_managed() expect that reference to path->mnt will be grabbed iff path->mnt has been changed. follow_managed() and follow_automount() keep track of whether such reference has been grabbed and assume that it'll happen in those and only those cases that'll have us return with changed path->mnt. That assumption is almost correct - it breaks in case of racing automounts and in even harder to hit race between following a mountpoint and a couple of mount --move. The thing is, we don't need to make that assumption at all - after the end of loop in follow_manage() we can check if path->mnt has ended up unchanged and do mntput() if needed. The BUG can be reproduced with the following test program: #include #include #include #include #include int main(int argc, char **argv) { int pid, ws; struct stat buf; pid = fork(); stat(argv[1], &buf); if (pid > 0) wait(&ws); return 0; } and the following procedure: (1) Mount an NFS volume that on the server has something else mounted on a subdirectory. For instance, I can mount / from my server: mount warthog:/ /mnt -t nfs4 -r On the server /data has another filesystem mounted on it, so NFS will see a change in FSID as it walks down the path, and will mark /mnt/data as being a mountpoint. This will cause the automount code to be triggered. !!! Do not look inside the mounted fs at this point !!! (2) Run the above program on a file within the submount to generate two simultaneous automount requests: /tmp/forkstat /mnt/data/testfile (3) Unmount the automounted submount: umount /mnt/data (4) Unmount the original mount: umount /mnt At this point the kernel should throw a BUG with something like the following: BUG: Dentry ffff880032e3c5c0{i=2,n=} still in use (1) [unmount of nfs4 0:12] Note that the bug appears on the root dentry of the original mount, not the mountpoint and not the submount because sys_umount() hasn't got to its final mntput_no_expire() yet, but this isn't so obvious from the call trace: [] shrink_dcache_for_umount+0x69/0x82 [] generic_shutdown_super+0x37/0x15b [] ? nfs_super_return_all_delegations+0x2e/0x1b1 [nfs] [] kill_anon_super+0x1d/0x7e [] nfs4_kill_super+0x60/0xb6 [nfs] [] deactivate_locked_super+0x34/0x83 [] deactivate_super+0x6f/0x7b [] mntput_no_expire+0x18d/0x199 [] mntput+0x3b/0x44 [] release_mounts+0xa2/0xbf [] sys_umount+0x47a/0x4ba [] ? trace_hardirqs_on_caller+0x1fd/0x22f [] system_call_fastpath+0x16/0x1b as do_umount() is inlined. However, you can see release_mounts() in there. Note also that it may be necessary to have multiple CPU cores to be able to trigger this bug. Tested-by: Jeff Layton Tested-by: Ian Kent Signed-off-by: David Howells Signed-off-by: Al Viro (cherry picked from commit 8aef18845266f5c05904c610088f2d1ed58f6be3) Signed-off-by: Chris J Arges --- fs/namei.c | 24 ++++++++++++++++-------- 1 files changed, 16 insertions(+), 8 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 44890c6..7bf6968 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -923,6 +923,11 @@ static int follow_automount(struct path *path, unsigned flags, if (!mnt) /* mount collision */ return 0; + if (!*need_mntput) { + /* lock_mount() may release path->mnt on error */ + mntget(path->mnt); + *need_mntput = true; + } err = finish_automount(mnt, path); switch (err) { @@ -930,12 +935,9 @@ static int follow_automount(struct path *path, unsigned flags, /* Someone else made a mount here whilst we were busy */ return 0; case 0: - dput(path->dentry); - if (*need_mntput) - mntput(path->mnt); + path_put(path); path->mnt = mnt; path->dentry = dget(mnt->mnt_root); - *need_mntput = true; return 0; default: return err; @@ -955,9 +957,10 @@ static int follow_automount(struct path *path, unsigned flags, */ static int follow_managed(struct path *path, unsigned flags) { + struct vfsmount *mnt = path->mnt; /* held by caller, must be left alone */ unsigned managed; bool need_mntput = false; - int ret; + int ret = 0; /* Given that we're not holding a lock here, we retain the value in a * local variable for each dentry as we look at it so that we don't see @@ -973,7 +976,7 @@ static int follow_managed(struct path *path, unsigned flags) ret = path->dentry->d_op->d_manage(path->dentry, false, false); if (ret < 0) - return ret == -EISDIR ? 0 : ret; + break; } /* Transit to a mounted filesystem. */ @@ -999,14 +1002,19 @@ static int follow_managed(struct path *path, unsigned flags) if (managed & DCACHE_NEED_AUTOMOUNT) { ret = follow_automount(path, flags, &need_mntput); if (ret < 0) - return ret == -EISDIR ? 0 : ret; + break; continue; } /* We didn't change the current path point */ break; } - return 0; + + if (need_mntput && path->mnt == mnt) + mntput(path->mnt); + if (ret == -EISDIR) + ret = 0; + return ret; } int follow_down_one(struct path *path) -- 1.7.5.4