linux-ext4 - Re: [PATCH] ext4: Reject on-disk mount options with missing NUL-terminator

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Message-ID: <20260209193945-80d9bfc8aa82b0eb1b764c7f-pchelkin@ispras>
Date: Mon, 9 Feb 2026 20:27:50 +0300
From: Fedor Pchelkin <pchelkin@...ras.ru>
To: Kees Cook <kees@...nel.org>
Cc: Theodore Ts'o <tytso@....edu>, 
	李龙兴 <coregee2000@...il.com>, Andreas Dilger <adilger.kernel@...ger.ca>, 
	Andy Shevchenko <andriy.shevchenko@...el.com>, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] ext4: Reject on-disk mount options with missing
 NUL-terminator

Kees Cook wrote:
> ee5a977b4e77 ("ext4: fix string copying in parse_apply_sb_mount_options()")
>   Notices the loud failures of strscpy_pad() introduced by 8ecb790ea8c3,
>   and attempted to silence them by making the destination 64 and rejecting
>   too-long strings from the on-disk copy of s_mount_opts, but didn't
>   actually solve it at all, since the problem was always the over-read
>   of the source seen by strnlen(). (Note that the report quoted in this
>   commit exactly matches the report today.)
> 

[...]

> Reported-by: 李龙兴 <coregee2000@...il.com>
> Closes: https://lore.kernel.org/lkml/CAHPqNmzBb2LruMA6jymoHXQRsoiAKMFZ1wVEz8JcYKg4U6TBbw@mail.gmail.com/
> Fixes: ee5a977b4e77 ("ext4: fix string copying in parse_apply_sb_mount_options()")

Hi there,

[ I'd better be Cc'ed as the author of the commit in Fixes ]

The mentioned reports are for v6.18.2 kernel while ee5a977b4e77 ("ext4:
fix string copying in parse_apply_sb_mount_options()") landed in v6.18.3.
Back at the time I've tested the patch with different bogus s_mount_opts
values and the fortify warnings should have been gone.

I don't think there is an error in ee5a977b4e77 unless these warnings
actually appear on the latest kernels with ee5a977b4e77 applied.

> @@ -2485,6 +2485,13 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
>  	if (!sbi->s_es->s_mount_opts[0])
>  		return 0;
>  
> +	if (strnlen(sbi->s_es->s_mount_opts, sizeof(sbi->s_es->s_mount_opts)) ==
> +	    sizeof(sbi->s_es->s_mount_opts)) {
> +		ext4_msg(sb, KERN_ERR,
> +			 "Mount options in superblock are not NUL-terminated");
> +		return -EINVAL;
> +	}

strscpy_pad() returns -E2BIG if the source string was truncated.  This
happens for the above condition as well - the last byte is truncated and
replaced with a NUL-terminator.

The check at 3db63d2c2d1d ("ext4: check if mount_opts is NUL-terminated in
ext4_ioctl_set_tune_sb()") was done in that manner as there is currently
no way to propagate strscpy_pad() return value up from ext4_sb_setparams().
So the string is independently checked inside ext4_ioctl_set_tune_sb()
directly.

As for the 64/65 byte length part, now the rationale of the checks works
as Darrick Wong described at the other part of this thread and corresponds
to how relevant userspace stuff treats the s_mount_opts field: the buffer
is at most 63 payload characters long + NUL-terminator.  Jan Kara also
shared similar thoughts during the discussion of ee5a977b4e77 [1].

[1]: https://lore.kernel.org/linux-ext4/yq6rbx54jt4btntsh37urd6u63wwcd3lyhovbrm6w7occaveea@riljfkx5jmhi/

> +
>  	if (strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts) < 0)
>  		return -E2BIG;
>  
> -- 
> 2.34.1