From 01106e140835ff94fdef79d66f166b87b0c89824 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Thu, 20 Apr 2023 10:57:43 +0200
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.

Link: https://lkml.kernel.org/r/20230420-faxen-advokat-40abb4c1a152@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>
Reported-by: Hugh Dickins <hughd@google.com>
  Link: https://lore.kernel.org/lkml/79eae9fe-7818-a65c-89c6-138b55d609a@google.com
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/shmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 3531ebb3494e..1a6263f54ada 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3582,6 +3582,10 @@ 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;