[pacman-dev] [PATCH] libmakepkg/integrity: check for invalid tags

[Allan McRae]

On 06/07/17 03:48, Eli Schwartz wrote:
> As per https://lists.archlinux.org/pipermail/arch-general/2017-July/043876.html
> git doesn't check that the tag name matches what an annotated tag object
> *thinks* it should be called. This is a bit of a theoretical attack and
> some would argue that we should always use commits since upstream can
> legitimately change a tag, but nevertheless this can result in a
> downgrade attack if the git download transport was manipulated or the
> upstream repository hacked.
> 
> So, check the tag blob to make sure the tag actually matches the name we
> used for `git checkout`
> 

For reference, I'd like to see some links in the commit message to where
this is discussed with git developers and with them saying that this is
not an issue to be fixed in git.

> Signed-off-by: Eli Schwartz <eschwartz93 at gmail.com>
> ---
> 
> v2: use git's built-in format specifier to obtain the real tagname with
> a single command. I didn't realize in v1 that this was possible.
> 
>  scripts/libmakepkg/integrity/verify_signature.sh.in | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/scripts/libmakepkg/integrity/verify_signature.sh.in b/scripts/libmakepkg/integrity/verify_signature.sh.in
> index 5468f977..93d88006 100644
> --- a/scripts/libmakepkg/integrity/verify_signature.sh.in
> +++ b/scripts/libmakepkg/integrity/verify_signature.sh.in
> @@ -187,6 +187,13 @@ verify_git_signature() {
>  
>  	printf "    %s git repo ... " "${dir##*/}" >&2
>  
> +	tagname="$(git -C "$dir" tag -l --format='%(tag)' "$fragval")"
> +	if [[ $fragtype = tag && -n $tagname && $tagname != $fragval ]]; then
> +		printf "%s (%s)" "$(gettext "FAILED")" "$(gettext "forged tag, you have been hacked!")" >&2

Just:
"the git tag has been forged"

It is not necessarily the person running makepkg that has been hacked.

> +		errors=1
> +		return 1
> +	fi
> +
>  	git -C "$dir" verify-$fragtype --raw "$fragval" > "$statusfile" 2>&1
>  	if ! grep -qs NEWSIG "$statusfile"; then
>  		printf '%s\n' "$(gettext "SIGNATURE NOT FOUND")" >&2
>