On Fri, Apr 24, 2020 at 08:43:38AM +0200, Christoph Hellwig wrote:
> Instead of having all the sysctl handlers deal with user pointers, which
> is rather hairy in terms of the BPF interaction, copy the input to and
> from userspace in common code. This also means that the strings are
> always NUL-terminated by the common code, making the API a little bit
> safer.
>
> As most handler just pass through the data to one of the common handlers
> a lot of the changes are mechnical.
This is a lovely cleanup; thank you!
Tiny notes below...
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index b6f5d459b087d..df2143e05c571 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -539,13 +539,13 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
> return err;
> }
>
> -static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
> +static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
> size_t count, loff_t *ppos, int write)
> {
> struct inode *inode = file_inode(filp);
> struct ctl_table_header *head = grab_header(inode);
> struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> - void *new_buf = NULL;
> + void *kbuf;
> ssize_t error;
>
> if (IS_ERR(head))
> @@ -564,27 +564,38 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
> if (!table->proc_handler)
> goto out;
>
> - error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, &count,
> - ppos, &new_buf);
> + if (write) {
> + kbuf = memdup_user_nul(ubuf, count);
> + if (IS_ERR(kbuf)) {
> + error = PTR_ERR(kbuf);
> + goto out;
> + }
> + } else {
> + error = -ENOMEM;
> + kbuf = kzalloc(count, GFP_KERNEL);
> + if (!kbuf)
> + goto out;
> + }
> +
> + error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, &kbuf, &count,
> + ppos);
> if (error)
> - goto out;
> + goto out_free_buf;
>
> /* careful: calling conventions are nasty here */
Is this comment still valid after doing these cleanups?
> - if (new_buf) {
> - mm_segment_t old_fs;
> -
> - old_fs = get_fs();
> - set_fs(KERNEL_DS);
> - error = table->proc_handler(table, write, (void __user *)new_buf,
> - &count, ppos);
> - set_fs(old_fs);
> - kfree(new_buf);
> - } else {
> - error = table->proc_handler(table, write, buf, &count, ppos);
> + error = table->proc_handler(table, write, kbuf, &count, ppos);
> + if (error)
> + goto out_free_buf;
> +
> + if (!write) {
> + error = -EFAULT;
> + if (copy_to_user(ubuf, kbuf, count))
> + goto out_free_buf;
> }
Something I noticed here that existed in the original code, but might be
nice to improve while we're here is to make sure that the "count"
returned from proc_handler() cannot grow _larger_, since then we might
expose heap memory beyond the end of the allocation.
I'll send a patch for this...
>
> - if (!error)
> - error = count;
> + error = count;
> +out_free_buf:
> + kfree(kbuf);
> out:
> sysctl_head_finish(head);
>
> [...]
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 511543d238794..e26fe7e8e19d7 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> [...]
> @@ -682,7 +661,6 @@ static int do_proc_douintvec_w(unsigned int *tbl_data,
> left -= proc_skip_spaces(&p);
>
> out_free:
> - kfree(kbuf);
> if (err)
> return -EINVAL;
This label name isn't accurate any more... *shrug*
--
Kees Cook
