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From: Steve French <smfrench@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org,
	 Linus Torvalds <torvalds@linux-foundation.org>,
	Christian Brauner <brauner@kernel.org>,
	 Miklos Szeredi <miklos@szeredi.hu>,
	linux-cifs@vger.kernel.org
Subject: Re: [PATCH 12/13] cifs_get_link(): bail out in unsafe case
Date: Sun, 4 Feb 2024 09:45:42 -0600	[thread overview]
Message-ID: <CAH2r5muOY-K6AEG_fMgTLfc5LBa1x291kCjb3C4Q_TKS8yn1xw@mail.gmail.com> (raw)
In-Reply-To: <20240204021739.1157830-12-viro@zeniv.linux.org.uk>

I may be missing some additional change or proposed future change -
but it looks like the patch to add check for null dentry in
cifs_get_link causes
an extra call to cifs_get_link in pick_link() (in namei.c - see
below), so would be slightly slower than leaving code as is in
cifs_get_link

                if (nd->flags & LOOKUP_RCU) {
                        res = get(NULL, inode, &last->done);
                        if (res == ERR_PTR(-ECHILD) && try_to_unlazy(nd))
                                res = get(link->dentry, inode, &last->done);

cifs.ko doesn't use or check the dentry in cifs_get_link since the
symlink target is stored in the cifs inode, not  accessed via the
dentry, so wasn't clear to me
from the patch description why we would care if dentry is null in
cifs_get_link()

On Sat, Feb 3, 2024 at 8:18 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> ->d_revalidate() bails out there, anyway.  It's not enough
> to prevent getting into ->get_link() in RCU mode, but that
> could happen only in a very contrieved setup.  Not worth
> trying to do anything fancy here unless ->d_revalidate()
> stops kicking out of RCU mode at least in some cases.
>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/smb/client/cifsfs.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
> index e902de4e475a..630e74628dfe 100644
> --- a/fs/smb/client/cifsfs.c
> +++ b/fs/smb/client/cifsfs.c
> @@ -1172,6 +1172,9 @@ const char *cifs_get_link(struct dentry *dentry, struct inode *inode,
>  {
>         char *target_path;
>
> +       if (!dentry)
> +               return ERR_PTR(-ECHILD);
> +
>         target_path = kmalloc(PATH_MAX, GFP_KERNEL);
>         if (!target_path)
>                 return ERR_PTR(-ENOMEM);
> --
> 2.39.2
>
>


-- 
Thanks,

Steve

  reply	other threads:[~2024-02-04 15:45 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-02-04  2:14 [PATCHES] RCU pathwalk race fixes Al Viro
2024-02-04  2:17 ` [PATCH 01/13] fs/super.c: don't drop ->s_user_ns until we free struct super_block itself Al Viro
2024-02-04  2:17   ` [PATCH 02/13] rcu pathwalk: prevent bogus hard errors from may_lookup() Al Viro
2024-02-05 12:26     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 03/13] affs: free affs_sb_info with kfree_rcu() Al Viro
2024-02-05 12:26     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 04/13] exfat: move freeing sbi, upcase table and dropping nls into rcu-delayed helper Al Viro
2024-02-05 12:27     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 05/13] hfsplus: switch to rcu-delayed unloading of nls and freeing ->s_fs_info Al Viro
2024-02-05 12:27     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 06/13] afs: fix __afs_break_callback() / afs_drop_open_mmap() race Al Viro
2024-02-05 12:28     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 07/13] nfs: make nfs_set_verifier() safe for use in RCU pathwalk Al Viro
2024-02-05 12:29     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 08/13] nfs: fix UAF on pathwalk running into umount Al Viro
2024-02-05 12:29     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 09/13] procfs: move dropping pde and pid from ->evict_inode() to ->free_inode() Al Viro
2024-02-05 12:30     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 10/13] procfs: make freeing proc_fs_info rcu-delayed Al Viro
2024-02-05 12:31     ` Christian Brauner
2024-02-04  2:17   ` [PATCH 11/13] fuse: fix UAF in rcu pathwalks Al Viro
2024-02-05 12:31     ` Christian Brauner
2024-02-05 13:51       ` Miklos Szeredi
2024-03-04 14:36         ` Amir Goldstein
2024-03-05 12:43           ` Miklos Szeredi
2024-03-06 10:18             ` Amir Goldstein
2024-03-06 10:21               ` Miklos Szeredi
2024-02-04  2:17   ` [PATCH 12/13] cifs_get_link(): bail out in unsafe case Al Viro
2024-02-04 15:45     ` Steve French [this message]
2024-02-04 16:25       ` Al Viro
2024-02-04 16:41         ` Al Viro
2024-02-04  2:17   ` [PATCH 13/13] ext4_get_link(): fix breakage in RCU mode Al Viro
2024-02-05 12:24   ` [PATCH 01/13] fs/super.c: don't drop ->s_user_ns until we free struct super_block itself Christian Brauner
2024-02-05 12:36   ` Jeff Layton
2024-02-04  2:27 ` RCU pathwalk audit notes Al Viro
2024-02-05 12:48 ` [PATCHES] RCU pathwalk race fixes Jeff Layton

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