[PATCH] ucount: check for CAP_SYS_RESOURCE using ns_capable_noaudit()

[Ondrej Mosnacek]

The user.* sysctls implement the ctl_table_root::permissions hook and
they override the file access mode based on the CAP_SYS_RESOURCE
capability (at most rwx if capable, at most r-- if not). The capability
is being checked unconditionally, so if an LSM denies the capability, an
audit record may be logged even when access is in fact granted.

Given the logic in the set_permissions() function in kernel/ucount.c and
the unfortunate way the permission checking is implemented, it doesn't
seem viable to avoid false positive denials by deferring the capability
check. Thus, do the same as in net_ctl_permissions() (net/sysctl_net.c)
- switch from ns_capable() to ns_capable_noaudit(), so that the check
never logs an audit record.

Fixes: dbec28460a89 ("userns: Add per user namespace sysctls.")
Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
---
 kernel/ucount.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 586af49fc03e4..fc4a8f2d30965 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -47,7 +47,7 @@ static int set_permissions(struct ctl_table_header *head,
 	int mode;
 
 	/* Allow users with CAP_SYS_RESOURCE unrestrained access */
-	if (ns_capable(user_ns, CAP_SYS_RESOURCE))
+	if (ns_capable_noaudit(user_ns, CAP_SYS_RESOURCE))
 		mode = (table->mode & S_IRWXU) >> 6;
 	else
 	/* Allow all others at most read-only access */
-- 
2.52.0