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>

diff --git a/mm/shmem.c b/mm/shmem.c
index 3531ebb..1a6263f 100644 (file)
--- 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;