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

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Message-Id: <20230420-faxen-advokat-40abb4c1a152@brauner>
Date:   Thu, 20 Apr 2023 10:57:43 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     Christian Brauner <brauner@...nel.org>, akpm@...ux-foundation.org,
        willy@...radead.org, linux-mm@...ck.org, p.raghav@...sung.com,
        da.gomez@...sung.com, a.manzanares@...sung.com, dave@...olabs.net,
        yosryahmed@...gle.com, keescook@...omium.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: [PATCH] shmem: restrict noswap option to initial user namespace

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

Cc: Luis Chamberlain <mcgrof@...nel.org>
Reported-by: Hugh Dickins <hughd@...gle.com>
Link: https://lore.kernel.org/lkml/79eae9fe-7818-a65c-89c6-138b55d609a@google.com
Signed-off-by: Christian Brauner <brauner@...nel.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