linux-kernel - Re: [PATCH] fs/xattr.c: fix simple_xattr_list to always include security.* xattrs

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Message-ID: <CAEjxPJ6V8aApDHaiU2+8pfDzf7S-bpgdFO3X6UHjRdCNFUNhXw@mail.gmail.com>
Date: Thu, 24 Apr 2025 11:40:13 -0400
From: Stephen Smalley <stephen.smalley.work@...il.com>
Cc: paul@...l-moore.com, omosnace@...hat.com, selinux@...r.kernel.org, 
	linux-security-module@...r.kernel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/xattr.c: fix simple_xattr_list to always include
 security.* xattrs

On Thu, Apr 24, 2025 at 11:28 AM Stephen Smalley
<stephen.smalley.work@...il.com> wrote:
>
> The vfs has long had a fallback to obtain the security.* xattrs from the
> LSM when the filesystem does not implement its own listxattr, but
> shmem/tmpfs and kernfs later gained their own xattr handlers to support
> other xattrs. Unfortunately, as a side effect, tmpfs and kernfs-based
> filesystems like sysfs no longer return the synthetic security.* xattr
> names via listxattr unless they are explicitly set by userspace or
> initially set upon inode creation after policy load. coreutils has
> recently switched from unconditionally invoking getxattr for security.*
> for ls -Z via libselinux to only doing so if listxattr returns the xattr
> name, breaking ls -Z of such inodes.
>
> Before:
> $ getfattr -m.* /run/initramfs
> <no output>
> $ getfattr -m.* /sys/kernel/fscaps
> <no output>
> $ setfattr -n user.foo /run/initramfs
> $ getfattr -m.* /run/initramfs
> user.foo
>
> After:
> $ getfattr -m.* /run/initramfs
> security.selinux
> $ getfattr -m.* /sys/kernel/fscaps
> security.selinux
> $ setfattr -n user.foo /run/initramfs
> $ getfattr -m.* /run/initramfs
> security.selinux
> user.foo
>
> Link: https://lore.kernel.org/selinux/CAFqZXNtF8wDyQajPCdGn=iOawX4y77ph0EcfcqcUUj+T87FKyA@mail.gmail.com/
> Link: https://lore.kernel.org/selinux/20250423175728.3185-2-stephen.smalley.work@gmail.com/
> Signed-off-by: Stephen Smalley <stephen.smalley.work@...il.com>

Fixes: b09e0fa4b4ea66266058ee ("tmpfs: implement generic xattr support")

> ---
>  fs/xattr.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 02bee149ad96..2fc314b27120 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -1428,6 +1428,15 @@ static bool xattr_is_trusted(const char *name)
>         return !strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN);
>  }
>
> +static bool xattr_is_maclabel(const char *name)
> +{
> +       const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
> +
> +       return !strncmp(name, XATTR_SECURITY_PREFIX,
> +                       XATTR_SECURITY_PREFIX_LEN) &&
> +               security_ismaclabel(suffix);
> +}
> +
>  /**
>   * simple_xattr_list - list all xattr objects
>   * @inode: inode from which to get the xattrs
> @@ -1460,6 +1469,17 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
>         if (err)
>                 return err;
>
> +       err = security_inode_listsecurity(inode, buffer, remaining_size);
> +       if (err < 0)
> +               return err;
> +
> +       if (buffer) {
> +               if (remaining_size < err)
> +                       return -ERANGE;
> +               buffer += err;
> +       }
> +       remaining_size -= err;
> +
>         read_lock(&xattrs->lock);
>         for (rbp = rb_first(&xattrs->rb_root); rbp; rbp = rb_next(rbp)) {
>                 xattr = rb_entry(rbp, struct simple_xattr, rb_node);
> @@ -1468,6 +1488,10 @@ ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs *xattrs,
>                 if (!trusted && xattr_is_trusted(xattr->name))
>                         continue;
>
> +               /* skip MAC labels; these are provided by LSM above */
> +               if (xattr_is_maclabel(xattr->name))
> +                       continue;
> +
>                 err = xattr_list_one(&buffer, &remaining_size, xattr->name);
>                 if (err)
>                         break;
> --
> 2.49.0
>