Re: [PATCH AUTOSEL 5.2 66/76] ceph: fix buffer free while holding i_ceph_lock in __ceph_setxattr()

[Ilya Dryomov]

On Thu, Aug 29, 2019 at 11:16 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> On Thu, Aug 29, 2019 at 10:51:04PM +0200, Ilya Dryomov wrote:
> >On Thu, Aug 29, 2019 at 8:15 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:
> >>
> >> From: Luis Henriques <lhenriques@xxxxxxxx>
> >>
> >> [ Upstream commit 86968ef21596515958d5f0a40233d02be78ecec0 ]
> >>
> >> Calling ceph_buffer_put() in __ceph_setxattr() may end up freeing the
> >> i_xattrs.prealloc_blob buffer while holding the i_ceph_lock.  This can be
> >> fixed by postponing the call until later, when the lock is released.
> >>
> >> The following backtrace was triggered by fstests generic/117.
> >>
> >>   BUG: sleeping function called from invalid context at mm/vmalloc.c:2283
> >>   in_atomic(): 1, irqs_disabled(): 0, pid: 650, name: fsstress
> >>   3 locks held by fsstress/650:
> >>    #0: 00000000870a0fe8 (sb_writers#8){.+.+}, at: mnt_want_write+0x20/0x50
> >>    #1: 00000000ba0c4c74 (&type->i_mutex_dir_key#6){++++}, at: vfs_setxattr+0x55/0xa0
> >>    #2: 000000008dfbb3f2 (&(&ci->i_ceph_lock)->rlock){+.+.}, at: __ceph_setxattr+0x297/0x810
> >>   CPU: 1 PID: 650 Comm: fsstress Not tainted 5.2.0+ #437
> >>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-prebuilt.qemu.org 04/01/2014
> >>   Call Trace:
> >>    dump_stack+0x67/0x90
> >>    ___might_sleep.cold+0x9f/0xb1
> >>    vfree+0x4b/0x60
> >>    ceph_buffer_release+0x1b/0x60
> >>    __ceph_setxattr+0x2b4/0x810
> >>    __vfs_setxattr+0x66/0x80
> >>    __vfs_setxattr_noperm+0x59/0xf0
> >>    vfs_setxattr+0x81/0xa0
> >>    setxattr+0x115/0x230
> >>    ? filename_lookup+0xc9/0x140
> >>    ? rcu_read_lock_sched_held+0x74/0x80
> >>    ? rcu_sync_lockdep_assert+0x2e/0x60
> >>    ? __sb_start_write+0x142/0x1a0
> >>    ? mnt_want_write+0x20/0x50
> >>    path_setxattr+0xba/0xd0
> >>    __x64_sys_lsetxattr+0x24/0x30
> >>    do_syscall_64+0x50/0x1c0
> >>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >>   RIP: 0033:0x7ff23514359a
> >>
> >> Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
> >> Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> >> Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
> >> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> >> ---
> >>  fs/ceph/xattr.c | 8 ++++++--
> >>  1 file changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/fs/ceph/xattr.c b/fs/ceph/xattr.c
> >> index 0619adbcbe14c..8382299fc2d84 100644
> >> --- a/fs/ceph/xattr.c
> >> +++ b/fs/ceph/xattr.c
> >> @@ -1028,6 +1028,7 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> >>         struct ceph_inode_info *ci = ceph_inode(inode);
> >>         struct ceph_mds_client *mdsc = ceph_sb_to_client(inode->i_sb)->mdsc;
> >>         struct ceph_cap_flush *prealloc_cf = NULL;
> >> +       struct ceph_buffer *old_blob = NULL;
> >>         int issued;
> >>         int err;
> >>         int dirty = 0;
> >> @@ -1101,13 +1102,15 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> >>                 struct ceph_buffer *blob;
> >>
> >>                 spin_unlock(&ci->i_ceph_lock);
> >> -               dout(" preaallocating new blob size=%d\n", required_blob_size);
> >> +               ceph_buffer_put(old_blob); /* Shouldn't be required */
> >> +               dout(" pre-allocating new blob size=%d\n", required_blob_size);
> >>                 blob = ceph_buffer_new(required_blob_size, GFP_NOFS);
> >>                 if (!blob)
> >>                         goto do_sync_unlocked;
> >>                 spin_lock(&ci->i_ceph_lock);
> >> +               /* prealloc_blob can't be released while holding i_ceph_lock */
> >>                 if (ci->i_xattrs.prealloc_blob)
> >> -                       ceph_buffer_put(ci->i_xattrs.prealloc_blob);
> >> +                       old_blob = ci->i_xattrs.prealloc_blob;
> >>                 ci->i_xattrs.prealloc_blob = blob;
> >>                 goto retry;
> >>         }
> >> @@ -1123,6 +1126,7 @@ int __ceph_setxattr(struct inode *inode, const char *name,
> >>         }
> >>
> >>         spin_unlock(&ci->i_ceph_lock);
> >> +       ceph_buffer_put(old_blob);
> >>         if (lock_snap_rwsem)
> >>                 up_read(&mdsc->snap_rwsem);
> >>         if (dirty)
> >
> >Hi Sasha,
> >
> >I didn't tag i_ceph_lock series for stable because this is a very old
> >bug which no one ever hit in real life, at least to my knowledge.
>
> I can drop it if you prefer.

Either is fine with me.  I just wanted to explain my rationale for not
tagging them for stable in the first place and point out that there is
a prerequisite.

Thanks,

                Ilya