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SubjectRe: [PATCH] jfs: Fix fortify moan in symlink
On Sat, Oct 22, 2022 at 09:39:14PM +0100, linux@treblig.org wrote:
> From: "Dr. David Alan Gilbert" <linux@treblig.org>
>
> JFS has in jfs_incore.h:
>
> /* _inline may overflow into _inline_ea when needed */
> /* _inline_ea may overlay the last part of
> * file._xtroot if maxentry = XTROOTINITSLOT
> */
> union {
> struct {
> /* 128: inline symlink */
> unchar _inline[128];
> /* 128: inline extended attr */
> unchar _inline_ea[128];
> };
> unchar _inline_all[256];
>
> and currently the symlink code copies into _inline;
> if this is larger than 128 bytes it triggers a fortify warning of the
> form:
>
> memcpy: detected field-spanning write (size 132) of single field
> "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615)

Which compiler are you using for this build? This size report (SIZE_MAX)
should be impossible to reach. But also, the size is just wrong --
i_inline is 128 bytes, not SIZE_MAX. So, the detection is working
(132 > 128), but the report is broken, and I can't see how...

>
> when it's actually OK.
>
> Copy it into _inline_all instead.
>
> Reported-by: syzbot+5fc38b2ddbbca7f5c680@syzkaller.appspotmail.com
> Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
> ---
> fs/jfs/namei.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
> index 9db4f5789c0ec..4fbbf88435e69 100644
> --- a/fs/jfs/namei.c
> +++ b/fs/jfs/namei.c
> @@ -946,7 +946,7 @@ static int jfs_symlink(struct user_namespace *mnt_userns, struct inode *dip,
> if (ssize <= IDATASIZE) {
> ip->i_op = &jfs_fast_symlink_inode_operations;
>
> - ip->i_link = JFS_IP(ip)->i_inline;
> + ip->i_link = JFS_IP(ip)->i_inline_all;
> memcpy(ip->i_link, name, ssize);
> ip->i_size = ssize - 1;
>

Regardless, the fix looks correct to me!

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook

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