[PATCH 3.16 325/366] afs: Fix directory permissions check

Ben Hutchings
2018-10-14T15:25:41Z

From: David Howells <dhowells@redhat.com>

commit 378831e4daec75fbba6d3612bcf3b4dd00ddbf08 upstream.

Doing faccessat("/afs/some/directory", 0) triggers a BUG in the permissions
check code.

Fix this by just removing the BUG section. If no permissions are asked
for, just return okay if the file exists.

Also:

(1) Split up the directory check so that it has separate if-statements
rather than if-else-if (e.g. checking for MAY_EXEC shouldn't skip the
check for MAY_READ and MAY_WRITE).

(2) Check for MAY_CHDIR as MAY_EXEC.

Without the main fix, the following BUG may occur:

kernel BUG at fs/afs/security.c:386!
invalid opcode: 0000 [#1] SMP PTI
...
RIP: 0010:afs_permission+0x19d/0x1a0 [kafs]
...
Call Trace:
? inode_permission+0xbe/0x180
? do_faccessat+0xdc/0x270
? do_syscall_64+0x60/0x1f0
? entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 00d3b7a4533e ("[AFS]: Add security support.")
Reported-by: Jonathan Billings <jsbillings@jsbillings.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/afs/security.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)

--- a/fs/afs/security.c
+++ b/fs/afs/security.c
@@ -323,18 +323,14 @@ int afs_permission(struct inode *inode,
mask, access, S_ISDIR(inode->i_mode) ? "dir" : "file");

if (S_ISDIR(inode->i_mode)) {
- if (mask & MAY_EXEC) {
+ if (mask & (MAY_EXEC | MAY_READ | MAY_CHDIR)) {
if (!(access & AFS_ACE_LOOKUP))
goto permission_denied;
- } else if (mask & MAY_READ) {
- if (!(access & AFS_ACE_LOOKUP))
- goto permission_denied;
- } else if (mask & MAY_WRITE) {
+ }
+ if (mask & MAY_WRITE) {
if (!(access & (AFS_ACE_DELETE | /* rmdir, unlink, rename from */
AFS_ACE_INSERT))) /* create, mkdir, symlink, rename to */
goto permission_denied;
- } else {
- BUG();
}
} else {
if (!(access & AFS_ACE_LOOKUP))