linux-kernel - [PATCH] Abort file_remove_privs() for non-reg. files

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Date:   Fri, 14 Dec 2018 11:55:52 +0100
From:   Alexander Lochmann <alexander.lochmann@...dortmund.de>
To:     Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Horst Schirmeier <horst.schirmeier@...dortmund.de>
Subject: [PATCH] Abort file_remove_privs() for non-reg. files

file_remove_privs() might be called for non-regular files, e.g.
blkdev inode. There is no reason to do its job on things
like blkdev inodes, pipes, or cdevs. Hence, abort if
file does not refer to a regular inode.
The following stacktrace shows how to get there:
13: entry_SYSENTER_32:460
12: do_fast_syscall_32:410
11: _static_cpu_has:146
10: do_syscall_32_irqs_on:322
09: SyS_pwrite64:636
08: SYSC_pwrite64:650
07: fdput:38
06: vfs_write:560
05: __vfs_write:512
04: new_sync_write:500
03: blkdev_write_iter:1977
02: __generic_file_write_iter:2897
01: file_remove_privs:1818
00: inode_has_no_xattr:3163

Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf
Spinczyk)

Signed-off-by: Alexander Lochmann <alexander.lochmann@...dortmund.de>
Signed-off-by: Horst Schirmeier <horst.schirmeier@...dortmund.de>
---
 fs/inode.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 35d2108d567c..682088190413 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1820,8 +1820,13 @@ int file_remove_privs(struct file *file)
 	int kill;
 	int error = 0;

-	/* Fast path for nothing security related */
-	if (IS_NOSEC(inode))
+	/*
+	 * Fast path for nothing security related.
+	 * As well for non-regular files, e.g. blkdev inodes.
+	 * For example, blkdev_write_iter() might get here
+	 * trying to remove privs which it is not allowed to.
+	 */
+	if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
 		return 0;

 	kill = dentry_needs_remove_privs(dentry);
-- 
2.19.2

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