Re: [PATCH 2/3] ext4: fix mount parameters check for empty values

[Christian Brauner <brauner@kernel.org>]

On Thu, Feb 29, 2024 at 04:30:09PM +0000, Luis Henriques wrote:
> Now that parameters that have the flag 'fs_param_can_be_empty' set and
> their value is NULL are handled as 'flag' type, we need to properly check
> for empty (NULL) values.
> 
> Signed-off-by: Luis Henriques <lhenriques@suse.de>
> ---
>  fs/ext4/super.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 0f931d0c227d..44ba2212dfb3 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2183,12 +2183,12 @@ static int ext4_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  	switch (token) {
>  #ifdef CONFIG_QUOTA
>  	case Opt_usrjquota:
> -		if (!*param->string)
> +		if (!param->string)
>  			return unnote_qf_name(fc, USRQUOTA);

I fail to understand how that can happen. Currently both of these
options are parsed as strings via:

#define fsparam_string_empty(NAME, OPT) \
        __fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL)


So if someone sets fsconfig(..., FSCONFIG_SET_STRING, "usrquota", NULL, ...)
we give an immediate

        case FSCONFIG_SET_STRING:
                if (!_key || !_value || aux) return -EINVAL;

from fsconfig() so we know that param->string cannot be NULL. If that
were the case we'd NULL deref in fs_param_is_string():

int fs_param_is_string(struct p_log *log, const struct fs_parameter_spec *p,
                       struct fs_parameter *param, struct fs_parse_result *result)
{
        if (param->type != fs_value_is_string ||
            (!*param->string && !(p->flags & fs_param_can_be_empty)))

So you're check above seems wrong. If I'm mistaken, please explain, how
this can happen in detail.