[PATCH] shmem: restrict noswap option to initial user namespace

From: Christian Brauner <brauner@kernel.org>
To: Luis Chamberlain <mcgrof@kernel.org>, Hugh Dickins <hughd@google.com>
Cc: Christian Brauner <brauner@kernel.org>,
	akpm@linux-foundation.org, willy@infradead.org,
	linux-mm@kvack.org, p.raghav@samsung.com, da.gomez@samsung.com,
	a.manzanares@samsung.com, dave@stgolabs.net,
	yosryahmed@google.com, keescook@chromium.org,
	patches@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: [PATCH] shmem: restrict noswap option to initial user namespace
Date: Thu, 20 Apr 2023 10:57:43 +0200	[thread overview]
Message-ID: <20230420-faxen-advokat-40abb4c1a152@brauner> (raw)
In-Reply-To: <ZD8Q2SzVr3xDmCgY@bombadil.infradead.org>

Prevent tmpfs instances mounted in an unprivileged namespaces from
evading accounting of locked memory by using the "noswap" mount option.

Cc: Luis Chamberlain <mcgrof@kernel.org>
Reported-by: Hugh Dickins <hughd@google.com>
Link: https://lore.kernel.org/lkml/79eae9fe-7818-a65c-89c6-138b55d609a@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 mm/shmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 787e83791eb5..21ce9b26bb4d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3571,6 +3571,10 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 		ctx->seen |= SHMEM_SEEN_INUMS;
 		break;
 	case Opt_noswap:
+		if ((fc->user_ns != &init_user_ns) || !capable(CAP_SYS_ADMIN)) {
+			return invalfc(fc,
+				       "Turning off swap in unprivileged tmpfs mounts unsupported");
+		}
 		ctx->noswap = true;
 		ctx->seen |= SHMEM_SEEN_NOSWAP;
 		break;
-- 
2.34.1


