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

Alexander Lochmann
2018-12-14T10:56:10Z

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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@tu-dortmund.de>
Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-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