statmount: let unset strings be empty

commit e52e97f09fb66fd868260d05bd6b74a9a3db39ee upstream.

Just like it's normal for unset values to be zero, unset strings should be
empty instead of containing random values.

It seems to be a typical mistake that the mask returned by statmount is not
checked, which can result in various bugs.

With this fix, these bugs are prevented, since it is highly likely that
userspace would just want to turn the missing mask case into an empty
string anyway (most of the recently found cases are of this type).

Link: https://lore.kernel.org/all/CAJfpegsVCPfCn2DpM8iiYSS5DpMsLB8QBUCHecoj6s0Vxf4jzg@mail.gmail.com/
Fixes: 68385d77c05b ("statmount: simplify string option retrieval")
Fixes: 46eae99ef733 ("add statmount(2) syscall")
Cc: stable@vger.kernel.org # v6.8
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250130121500.113446-1-mszeredi@redhat.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index 5e678b8c89ad1a80f6abc5b6b6b15c56c55daf3b..73da51ac5a0349f2cebb300aeb979574e6cc37cf 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5053,22 +5053,29 @@ static int statmount_string(struct kstatmount *s, u64 flag)
        size_t kbufsize;
        struct seq_file *seq = &s->seq;
        struct statmount *sm = &s->sm;
+       u32 start, *offp;
+
+       /* Reserve an empty string at the beginning for any unset offsets */
+       if (!seq->count)
+               seq_putc(seq, 0);
+
+       start = seq->count;
 
        switch (flag) {
        case STATMOUNT_FS_TYPE:
-               sm->fs_type = seq->count;
+               offp = &sm->fs_type;
                ret = statmount_fs_type(s, seq);
                break;
        case STATMOUNT_MNT_ROOT:
-               sm->mnt_root = seq->count;
+               offp = &sm->mnt_root;
                ret = statmount_mnt_root(s, seq);
                break;
        case STATMOUNT_MNT_POINT:
-               sm->mnt_point = seq->count;
+               offp = &sm->mnt_point;
                ret = statmount_mnt_point(s, seq);
                break;
        case STATMOUNT_MNT_OPTS:
-               sm->mnt_opts = seq->count;
+               offp = &sm->mnt_opts;
                ret = statmount_mnt_opts(s, seq);
                break;
        default:
@@ -5090,6 +5097,7 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 
        seq->buf[seq->count++] = '\0';
        sm->mask |= flag;
+       *offp = start;
        return 0;
 }