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From: Al Viro <viro@zeniv.linux.org.uk>
To: Steve French <smfrench@gmail.com>
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 16:41:45 +0000	[thread overview]
Message-ID: <20240204164145.GK2087318@ZenIV> (raw)
In-Reply-To: <20240204162558.GJ2087318@ZenIV>

On Sun, Feb 04, 2024 at 04:25:58PM +0000, Al Viro wrote:

> *IF* you can tell the symlink body without blocking (e.g. you
> have some cached information from the last time you've asked
> the server and have reasons to trust it to be still valid),
> sure, you can return it without dropping out of RCU mode.
> 
> It would be fairly useless for CIFS, since ->d_revalidate() of
> CIFS dentries would reject RCU mode anyway.  That's what normally
> saves you from having ->get_link() called that way, but it's not
> guaranteed - there are convoluted setups that avoid having
> ->d_revalidate() called first.
> 
> See the description of RCU mode filesystem exposure in the
> last posting in this thread for more details.

PS: if you decide to go for handling RCU pathwalk mode in CIFS,
you definitely want to read the first half of
https://lore.kernel.org/linux-fsdevel/20240204022743.GI2087318@ZenIV/
or whatever it eventually turns into.  It obviously needs quite
a bit of massage before it starts to resemble proper docs -
currently it's just the summary I'd put together while going through
the audit: which methods are exposed, how can they tell,
what is and what is not guaranteed for them, etc., with a bit of
"why does VFS bother with something that unpleasant" thrown into the mix
as an explanation.

Any assistance with turning that into a coherent text would be
very welcome - I think that description of RCU pathwalk regarding
its impact on the filesystems would be useful.

  reply	other threads:[~2024-02-04 16:41 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
2024-02-04 16:25       ` Al Viro
2024-02-04 16:41         ` Al Viro [this message]
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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