6.15-stable review patch. If anyone has any objections, please let me know.
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From: Christian Brauner brauner@kernel.org
commit 22bdf3d6581af6d06ed8a46c6835648421cca0ea upstream.
It is currently possible to change the mode and owner of the single anonymous inode in the kernel:
int main(int argc, char *argv[]) { int ret, sfd; sigset_t mask; struct signalfd_siginfo fdsi;
sigemptyset(&mask); sigaddset(&mask, SIGINT); sigaddset(&mask, SIGQUIT);
ret = sigprocmask(SIG_BLOCK, &mask, NULL); if (ret < 0) _exit(1);
sfd = signalfd(-1, &mask, 0); if (sfd < 0) _exit(2);
ret = fchown(sfd, 5555, 5555); if (ret < 0) _exit(3);
ret = fchmod(sfd, 0777); if (ret < 0) _exit(3);
_exit(4); }
This is a bug. It's not really a meaningful one because anonymous inodes don't really figure into path lookup and they cannot be reopened via /proc/<pid>/fd/<nr> and can't be used for lookup itself. So they can only ever serve as direct references.
But it is still completely bogus to allow the mode and ownership or any of the properties of the anonymous inode to be changed. Block this!
Link: https://lore.kernel.org/20250407-work-anon_inode-v1-3-53a44c20d44e@kernel.or... Reviewed-by: Jeff Layton jlayton@kernel.org Cc: stable@vger.kernel.org # all LTS kernels Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/anon_inodes.c | 7 +++++++ fs/internal.h | 2 ++ 2 files changed, 9 insertions(+)
--- a/fs/anon_inodes.c +++ b/fs/anon_inodes.c @@ -57,8 +57,15 @@ int anon_inode_getattr(struct mnt_idmap return 0; }
+int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry, + struct iattr *attr) +{ + return -EOPNOTSUPP; +} + static const struct inode_operations anon_inode_operations = { .getattr = anon_inode_getattr, + .setattr = anon_inode_setattr, };
/* --- a/fs/internal.h +++ b/fs/internal.h @@ -346,3 +346,5 @@ int statmount_mnt_idmap(struct mnt_idmap int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path, struct kstat *stat, u32 request_mask, unsigned int query_flags); +int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry, + struct iattr *attr);