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

[Christian Brauner]

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

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