Re: [PATCH 3/4] xattr: Use dedicated slab buckets for setxattr()

[Dave Chinner <david@fromorbit.com>]

On Mon, Mar 04, 2024 at 10:49:31AM -0800, Kees Cook wrote:
> The setxattr() API can be used for exploiting[1][2][3] use-after-free
> type confusion flaws in the kernel. Avoid having a user-controlled size
> cache share the global kmalloc allocator by using a separate set of
> kmalloc buckets.
> 
> Link: https://duasynt.com/blog/linux-kernel-heap-spray [1]
> Link: https://etenal.me/archives/1336 [2]
> Link: https://github.com/a13xp0p0v/kernel-hack-drill/blob/master/drill_exploit_uaf.c [3]
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Jan Kara <jack@suse.cz>
> Cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/xattr.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 09d927603433..2b06316f1d1f 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -821,6 +821,16 @@ SYSCALL_DEFINE4(fgetxattr, int, fd, const char __user *, name,
>  	return error;
>  }
>  
> +static struct kmem_buckets *xattr_buckets;
> +static int __init init_xattr_buckets(void)
> +{
> +	xattr_buckets = kmem_buckets_create("xattr", 0, 0, 0,
> +					    XATTR_LIST_MAX, NULL);
> +
> +	return 0;
> +}
> +subsys_initcall(init_xattr_buckets);
> +
>  /*
>   * Extended attribute LIST operations
>   */
> @@ -833,7 +843,7 @@ listxattr(struct dentry *d, char __user *list, size_t size)
>  	if (size) {
>  		if (size > XATTR_LIST_MAX)
>  			size = XATTR_LIST_MAX;
> -		klist = kvmalloc(size, GFP_KERNEL);
> +		klist = kmem_buckets_alloc(xattr_buckets, size, GFP_KERNEL);

There's a reason this uses kvmalloc() - allocations can be up to
64kB in size and it's not uncommon for large slab allocation to
fail on long running machines. hence this needs to fall back to
vmalloc() to ensure that large xattrs can always be read.

Essentially, you're trading a heap spraying vector that almost
no-one will ever see for a far more frequent -ENOMEM denial of
service that will be seen on production systems where large xattrs
are used.

-Dave.
-- 
Dave Chinner
david@fromorbit.com