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Message-ID: <2910678.e9J7NaK4W3@oscar>
Date: Thu, 16 Nov 2023 17:48:20 +0100
From: Anders Larsen <al@...rsen.net>
To: Kees Cook <keescook@...omium.org>
Cc: Ronald Monthero <debug.penguin32@...il.com>, gustavoars@...nel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] qnx4: fix to avoid panic due to buffer overflow
On 2023-11-16 15:58 Kees Cook wrote:
> On Thu, Nov 16, 2023 at 06:29:59AM -0800, Kees Cook wrote:
> > On Sun, Nov 12, 2023 at 07:53:53PM +1000, Ronald Monthero wrote:
> > > qnx4 dir name length can vary to be of maximum size
> > > QNX4_NAME_MAX or QNX4_SHORT_NAME_MAX depending on whether
> > > 'link info' entry is stored and the status byte is set.
> > > So to avoid buffer overflow check di_fname length
> > > fetched from (struct qnx4_inode_entry *)
> > > before use in strlen to avoid buffer overflow.
> > >
> > > panic context
> > > [ 4849.636861] detected buffer overflow in strlen
> > > [ 4849.636897] ------------[ cut here ]------------
> > > [ 4849.636902] kernel BUG at lib/string.c:1165!
> > > [ 4849.636917] invalid opcode: 0000 [#2] SMP PTI
> > > ..
> > > [ 4849.637047] Call Trace:
> > > [ 4849.637053] <TASK>
> > > [ 4849.637059] ? show_trace_log_lvl+0x1d6/0x2ea
> > > [ 4849.637075] ? show_trace_log_lvl+0x1d6/0x2ea
> > > [ 4849.637095] ? qnx4_find_entry.cold+0xc/0x18 [qnx4]
> > > [ 4849.637111] ? show_regs.part.0+0x23/0x29
> > > [ 4849.637123] ? __die_body.cold+0x8/0xd
> > > [ 4849.637135] ? __die+0x2b/0x37
> > > [ 4849.637147] ? die+0x30/0x60
> > > [ 4849.637161] ? do_trap+0xbe/0x100
> > > [ 4849.637171] ? do_error_trap+0x6f/0xb0
> > > [ 4849.637180] ? fortify_panic+0x13/0x15
> > > [ 4849.637192] ? exc_invalid_op+0x53/0x70
> > > [ 4849.637203] ? fortify_panic+0x13/0x15
> > > [ 4849.637215] ? asm_exc_invalid_op+0x1b/0x20
> > > [ 4849.637228] ? fortify_panic+0x13/0x15
> > > [ 4849.637240] ? fortify_panic+0x13/0x15
> > > [ 4849.637251] qnx4_find_entry.cold+0xc/0x18 [qnx4]
> > > [ 4849.637264] qnx4_lookup+0x3c/0xa0 [qnx4]
> > > [ 4849.637275] __lookup_slow+0x85/0x150
> > > [ 4849.637291] walk_component+0x145/0x1c0
> > > [ 4849.637304] ? path_init+0x2c0/0x3f0
> > > [ 4849.637316] path_lookupat+0x6e/0x1c0
> > > [ 4849.637330] filename_lookup+0xcf/0x1d0
> > > [ 4849.637341] ? __check_object_size+0x1d/0x30
> > > [ 4849.637354] ? strncpy_from_user+0x44/0x150
> > > [ 4849.637365] ? getname_flags.part.0+0x4c/0x1b0
> > > [ 4849.637375] user_path_at_empty+0x3f/0x60
> > > [ 4849.637383] vfs_statx+0x7a/0x130
> > > [ 4849.637393] do_statx+0x45/0x80
> > > ..
> > >
> > > Signed-off-by: Ronald Monthero <debug.penguin32@...il.com>
> > > ---
> > >
> > > fs/qnx4/namei.c | 7 +++++++
> > > 1 file changed, 7 insertions(+)
> > >
> > > diff --git a/fs/qnx4/namei.c b/fs/qnx4/namei.c
> > > index 8d72221735d7..825b891a52b3 100644
> > > --- a/fs/qnx4/namei.c
> > > +++ b/fs/qnx4/namei.c
> > > @@ -40,6 +40,13 @@ static int qnx4_match(int len, const char *name,
> > >
> > > } else {
> > >
> > > namelen = QNX4_SHORT_NAME_MAX;
> > >
> > > }
> > >
> > > +
> > > + /** qnx4 dir name length can vary, check the di_fname
> > > + * fetched from (struct qnx4_inode_entry *) before use in
> > > + * strlen to avoid panic due to buffer overflow"
> > > + */
> >
> > Style nit: this comment should start with just "/*" alone, like:
> > /*
> >
> > * qnx4 dir name ...
> >
> > > + if (strnlen(de->di_fname, namelen) >= sizeof(de->di_fname))
> > > + return -ENAMETOOLONG;
> > >
> > > thislen = strlen( de->di_fname );
> >
> > de->di_fname is:
> >
> > struct qnx4_inode_entry {
> >
> > char di_fname[QNX4_SHORT_NAME_MAX];
> >
> > ...
> >
> > #define QNX4_SHORT_NAME_MAX 16
> > #define QNX4_NAME_MAX 48
> >
> > It's always going to have a max of QNX4_SHORT_NAME_MAX. Is any of this
> > code correct if namelen ends up being QNX4_NAME_MAX? It'll be reading
> > past the end of di_fname.
> >
> > Is bh->b_data actually struct qnx4_inode_entry ?
>
> Ah-ha, it looks like it's _not_:
>
> if (!(bh = qnx4_find_entry(len, dir, name, &de, &ino)))
> goto out;
> /* The entry is linked, let's get the real info */
> if ((de->di_status & QNX4_FILE_LINK) == QNX4_FILE_LINK) {
> lnk = (struct qnx4_link_info *) de;
>
> It seems that entries may be either struct qnx4_inode_entry or struct
> qnx4_link_info but it's not captured in a union.
>
> This needs to be fixed by not lying to the compiler about what is there.
>
> How about this?
The switch won't work since the _status field is a bit-field, so we should
rather reuse the similar union-logic already present in fs/qnx4/dir.c
> diff --git a/fs/qnx4/namei.c b/fs/qnx4/namei.c
> index 8d72221735d7..3cd20065bcfa 100644
> --- a/fs/qnx4/namei.c
> +++ b/fs/qnx4/namei.c
> @@ -26,31 +26,39 @@
> static int qnx4_match(int len, const char *name,
> struct buffer_head *bh, unsigned long *offset)
> {
> - struct qnx4_inode_entry *de;
> - int namelen, thislen;
> + union qnx4_dir_entry *de;
> + char *entry_fname;
> + int entry_len, entry_max_len;
>
> if (bh == NULL) {
> printk(KERN_WARNING "qnx4: matching unassigned buffer !
\n");
> return 0;
> }
> - de = (struct qnx4_inode_entry *) (bh->b_data + *offset);
> + de = (union qnx4_dir_entry *) (bh->b_data + *offset);
> *offset += QNX4_DIR_ENTRY_SIZE;
> - if ((de->di_status & QNX4_FILE_LINK) != 0) {
> - namelen = QNX4_NAME_MAX;
> - } else {
> - namelen = QNX4_SHORT_NAME_MAX;
> - }
> - thislen = strlen( de->di_fname );
> - if ( thislen > namelen )
> - thislen = namelen;
> - if (len != thislen) {
> +
> + switch (de->inode.di_status) {
> + case QNX4_FILE_LINK:
> + entry_fname = de->link.dl_fname;
> + entry_max_len = sizeof(de->link.dl_fname);
> + break;
> + case QNX4_FILE_USED:
> + entry_fname = de->inode.di_fname;
> + entry_max_len = sizeof(de->inode.di_fname);
> + break;
> + default:
> return 0;
> }
> - if (strncmp(name, de->di_fname, len) == 0) {
> - if ((de->di_status & (QNX4_FILE_USED|QNX4_FILE_LINK)) !
= 0) {
> - return 1;
> - }
> - }
> +
> + /* Directory entry may not be %NUL-terminated. */
> + entry_len = strnlen(entry_fname, entry_max_len);
> +
> + if (len != entry_len)
> + return 0;
> +
> + if (strncmp(name, entry_fname, len) == 0)
> + return 1;
> +
> return 0;
> }
>
> diff --git a/include/uapi/linux/qnx4_fs.h b/include/uapi/linux/qnx4_fs.h
> index 31487325d265..e033dbe1e009 100644
> --- a/include/uapi/linux/qnx4_fs.h
> +++ b/include/uapi/linux/qnx4_fs.h
> @@ -68,6 +68,13 @@ struct qnx4_link_info {
> __u8 dl_status;
> };
>
> +union qnx4_dir_entry {
> + struct qnx4_inode_entry inode;
> + struct qnx4_link_info link;
> +};
> +_Static_assert(offsetof(struct qnx4_inode_entry, di_status) ==
> + offsetof(struct qnx4_link_info, dl_status));
> +
> struct qnx4_xblk {
> __le32 xblk_next_xblk;
> __le32 xblk_prev_xblk;
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