Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks
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From: Aleksa Sarai <hidden>
Date: 2020-01-01 14:44:34
Also in:
linux-fsdevel, lkml, stable
On 2020-01-01, Al Viro [off-list ref] wrote:
On Wed, Jan 01, 2020 at 12:54:46AM +0000, Al Viro wrote:quoted
Note, BTW, that lookup_last() (aka walk_component()) does just that - we only hit step_into() on LAST_NORM. The same goes for do_last(). mountpoint_last() not doing the same is _not_ intentional - it's definitely a bug. Consider your testcase; link points to . here. So the only thing you could expect from trying to follow it would be the directory 'link' lives in. And you don't have it when you reach the fscker via /proc/self/fd/3; what happens instead is nd->path set to ./link (by nd_jump_link()) *AND* step_into() called, pushing the same ./link onto stack. It violates all kinds of assumptions made by fs/namei.c - when pushing a symlink onto stack nd->path is expected to contain the base directory for resolving it. I'm fairly sure that this is the cause of at least some of the insanity you've caught; there always could be something else, of course, but this hole needs to be closed in any case.... and with removal of now unused local variable, that's mountpoint_last(): fix the treatment of LAST_BIND step_into() should be attempted only in LAST_NORM case, when we have the parent directory (in nd->path). We get away with that for LAST_DOT and LOST_DOTDOT, since those can't be symlinks, making step_init() and equivalent of path_to_nameidata() - we do a bit of useless work, but that's it. For LAST_BIND (i.e. the case when we'd just followed a procfs-style symlink) we really can't go there - result might be a symlink and we really can't attempt following it. lookup_last() and do_last() do handle that properly; mountpoint_last() should do the same. Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Thanks, this fixes the issue for me (and also fixes another reproducer I found -- mounting a symlink on top of itself then trying to umount it). Reported-by: Aleksa Sarai <redacted> Tested-by: Aleksa Sarai <redacted> As for the original topic of bind-mounting symlinks -- given this is a supported feature, would you be okay with me sending an updated O_EMPTYPATH series?
quoted hunk ↗ jump to hunk
---diff --git a/fs/namei.c b/fs/namei.c index d6c91d1e88cb..13f9f973722b 100644 --- a/fs/namei.c +++ b/fs/namei.c@@ -2643,7 +2643,6 @@ EXPORT_SYMBOL(user_path_at_empty); static int mountpoint_last(struct nameidata *nd) { - int error = 0; struct dentry *dir = nd->path.dentry; struct path path;@@ -2656,10 +2655,7 @@ mountpoint_last(struct nameidata *nd) nd->flags &= ~LOOKUP_PARENT; if (unlikely(nd->last_type != LAST_NORM)) { - error = handle_dots(nd, nd->last_type); - if (error) - return error; - path.dentry = dget(nd->path.dentry); + return handle_dots(nd, nd->last_type); } else { path.dentry = d_lookup(dir, &nd->last); if (!path.dentry) {
-- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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