linux-kernel - Re: [PATCH] bcachefs: fix use-after-free in bch2_dirent_to

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Message-ID: <suouhqf3so43qperydsoykxiqglasg45boxjimbsarsz72jqr2@lf47oxtlcand>
Date: Sat, 4 Oct 2025 12:21:35 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Nirbhay Sharma <nirbhay.lkd@...il.com>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+7f176adb30b21606c5fc@...kaller.appspotmail.com, skhan@...uxfoundation.org, david.hunter.linux@...il.com, 
	linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] bcachefs: fix use-after-free in bch2_dirent_to_text()

On Fri, Oct 03, 2025 at 04:40:33AM +0530, Nirbhay Sharma wrote:
> Add bounds checking before calculating pointer offsets in dirent name
> accessor functions to prevent out-of-bounds memory access when
> processing corrupted filesytem metadata.
> 
> When d_name_len contains a corrupted value, the pointer calculation
> &d.v->d_cf_name_block.d_names[name_len] results in an offset far
> outside the dirent structure, triggering KASAN use-after-free erors.

We validate d_name_len in bch2_dirent_validate(), so... this looks
entirely bogus.

Did you test any of this?

> 
> While bch2_dirent_validate() detects such corruption,
> bch2_dirent_to_text() may still be called for debug output, so the
> accessor functions must handle invalid data gracefully.
> 
> Fixes: c21f41f6905be4fc5059a10a5bba94105ba87269 ("bcachefs: bch2_dirent_to_text() shows casefolded dirents")
> Reported-by: syzbot+7f176adb30b21606c5fc@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7f176adb30b21606c5fc
> Signed-off-by: Nirbhay Sharma <nirbhay.lkd@...il.com>
> ---
>  fs/bcachefs/dirent.c | 39 ++++++++++++++++++++++++++++++---------
>  1 file changed, 30 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/bcachefs/dirent.c b/fs/bcachefs/dirent.c
> index d198001838f3..8be31b41c32b 100644
> --- a/fs/bcachefs/dirent.c
> +++ b/fs/bcachefs/dirent.c
> @@ -58,8 +58,16 @@ static unsigned bch2_dirent_name_bytes(struct bkey_s_c_dirent d)
>  
>  struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
>  {
> +	unsigned int name_len, max_len;
> +
>  	if (d.v->d_casefold) {
> -		unsigned name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
> +		name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
> +		max_len = bkey_val_bytes(d.k) -
> +			offsetof(struct bch_dirent, d_cf_name_block.d_names);
> +
> +		if (name_len > max_len)
> +			return (struct qstr) QSTR_INIT(NULL, 0);
> +
>  		return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[0], name_len);
>  	} else {
>  		return (struct qstr) QSTR_INIT(d.v->d_name, bch2_dirent_name_bytes(d));
> @@ -68,13 +76,19 @@ struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
>  
>  static struct qstr bch2_dirent_get_casefold_name(struct bkey_s_c_dirent d)
>  {
> -	if (d.v->d_casefold) {
> -		unsigned name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
> -		unsigned cf_name_len = le16_to_cpu(d.v->d_cf_name_block.d_cf_name_len);
> -		return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[name_len], cf_name_len);
> -	} else {
> +	unsigned int name_len, cf_name_len, max_len;
> +
> +	if (!d.v->d_casefold)
>  		return (struct qstr) QSTR_INIT(NULL, 0);
> -	}
> +
> +	name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
> +	cf_name_len = le16_to_cpu(d.v->d_cf_name_block.d_cf_name_len);
> +	max_len = bkey_val_bytes(d.k) - offsetof(struct bch_dirent, d_cf_name_block.d_names);
> +
> +	if (name_len > max_len || cf_name_len > max_len || name_len + cf_name_len > max_len)
> +		return (struct qstr) QSTR_INIT(NULL, 0);
> +
> +	return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[name_len], cf_name_len);
>  }
>  
>  static inline struct qstr bch2_dirent_get_lookup_name(struct bkey_s_c_dirent d)
> @@ -212,11 +226,18 @@ void bch2_dirent_to_text(struct printbuf *out, struct bch_fs *c, struct bkey_s_c
>  	struct bkey_s_c_dirent d = bkey_s_c_to_dirent(k);
>  	struct qstr d_name = bch2_dirent_get_name(d);
>  
> +	if (!d_name.name || !d_name.len) {
> +		prt_str(out, "(invalid)");
> +		return;
> +	}
> +
>  	prt_printf(out, "%.*s", d_name.len, d_name.name);
>  
>  	if (d.v->d_casefold) {
> -		struct qstr d_name = bch2_dirent_get_lookup_name(d);
> -		prt_printf(out, " (casefold %.*s)", d_name.len, d_name.name);
> +		struct qstr d_cf_name = bch2_dirent_get_lookup_name(d);
> +
> +		if (d_cf_name.name && d_cf_name.len)
> +			prt_printf(out, " (casefold %.*s)", d_name.len, d_name.name);
>  	}
>  
>  	prt_str(out, " ->");
> -- 
> 2.51.0
>