linux-kernel - [PATCH v3 102/110] namei: make may_follow

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Message-Id: <1431367690-5223-102-git-send-email-viro@ZenIV.linux.org.uk>
Date:	Mon, 11 May 2015 19:08:02 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH v3 102/110] namei: make may_follow_link() safe in RCU mode

From: Al Viro <viro@...iv.linux.org.uk>

We *can't* call that audit garbage in RCU mode - it's doing a weird
mix of allocations (GFP_NOFS, immediately followed by GFP_KERNEL)
and I'm not touching that... thing again.

So if this security sclero^Whardening feature gets triggered when
we are in RCU mode, tough - we'll fail with -ECHILD and have
everything restarted in non-RCU mode.  Only to hit the same test
and fail, this time with EACCES and with (oh, rapture) an audit spew
produced.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/namei.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 998c3c2..20bf494 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -794,6 +794,9 @@ static inline int may_follow_link(struct nameidata *nd)
 	if (uid_eq(parent->i_uid, inode->i_uid))
 		return 0;
 
+	if (nd->flags & LOOKUP_RCU)
+		return -ECHILD;
+
 	audit_log_link_denied("follow_link", &nd->stack[0].link);
 	return -EACCES;
 }
-- 
2.1.4

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