Move the syslog syscall to be gated by CAP_SYS_ADMIN or CAP_SYSLOG

This call is what is used to implement `dmesg` to get kernel messages
about the host. This can leak substantial information about the host.
It is normally available to unprivileged users on the host, unless
the sysctl `kernel.dmesg_restrict = 1` is set, but this is not set
by standard on the majority of distributions. Blocking this to restrict
leaks about the configuration seems correct.

Fix moby#37897

See also https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

profiles/seccomp/default.json

@@ -329,7 +329,6 @@
 				"sync_file_range",
 				"syncfs",
 				"sysinfo",
-				"syslog",
 				"tee",
 				"tgkill",
 				"time",
@@ -561,6 +560,7 @@
 				"setdomainname",
 				"sethostname",
 				"setns",
+				"syslog",
 				"umount",
 				"umount2",
 				"unshare"
@@ -762,6 +762,20 @@
 				]
 			},
 			"excludes": {}
+		},
+		{
+			"names": [
+				"syslog"
+			],
+			"action": "SCMP_ACT_ALLOW",
+			"args": [],
+			"comment": "",
+			"includes": {
+				"caps": [
+					"CAP_SYSLOG"
+				]
+			},
+			"excludes": {}
 		}
 	]
 }

profiles/seccomp/seccomp_default.go

@@ -322,7 +322,6 @@ func DefaultProfile() *types.Seccomp {
 				"sync_file_range",
 				"syncfs",
 				"sysinfo",
-				"syslog",
 				"tee",
 				"tgkill",
 				"time",
@@ -492,6 +491,7 @@ func DefaultProfile() *types.Seccomp {
 				"setdomainname",
 				"sethostname",
 				"setns",
+				"syslog",
 				"umount",
 				"umount2",
 				"unshare",
@@ -642,6 +642,16 @@ func DefaultProfile() *types.Seccomp {
 				Caps: []string{"CAP_SYS_NICE"},
 			},
 		},
+		{
+			Names: []string{
+				"syslog",
+			},
+			Action: types.ActAllow,
+			Args:   []*types.Arg{},
+			Includes: types.Filter{
+				Caps: []string{"CAP_SYSLOG"},
+			},
+		},
 	}
 
 	return &types.Seccomp{