Subject: Re: [SECURITY PATCH] git-prompt.sh: don't put unsanitized branch names in $PS1

From: Junio C Hamano <[email-masked]>
Date: Mon, 21 Apr 2014 15:23:00 -0700

Richard Hansen <[email-masked]> writes:

> Both bash and zsh subject the value of PS1 to parameter expansion,
> command substitution, and arithmetic expansion.  Rather than include
> the raw, unescaped branch name in PS1 when running in two- or
> three-argument mode, construct PS1 to reference a variable that holds
> the branch name.  Because the shells do not recursively expand, this
> avoids arbitrary code execution by specially-crafted branch names such
> as '$(IFS=_;cmd=sudo_rm_-rf_/;$cmd)'.
>
> Signed-off-by: Richard Hansen <[email-masked]>

I'd like to see this patch eyeballed by those who have been involved
in the script (shortlog and blame tells me they are SZEDER and
Simon, CC'ed), so that we can hopefully merge it by the time -rc1 is
tagged.

Will queue so that I won't lose it in the meantime.

Thanks.

>  contrib/completion/git-prompt.sh | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
> index 7b732d2..bd7ff29 100644
> --- a/contrib/completion/git-prompt.sh
> +++ b/contrib/completion/git-prompt.sh
> @@ -207,7 +207,18 @@ __git_ps1_show_upstream ()
>  			p=" u+${count#*	}-${count%	*}" ;;
>  		esac
>  		if [[ -n "$count" && -n "$name" ]]; then
> -			p="$p $(git rev-parse --abbrev-ref "$upstream" 2>/dev/null)"
> +			__git_ps1_upstream_name=$(git rev-parse \
> +				--abbrev-ref "$upstream" 2>/dev/null)
> +			if [ $pcmode = yes ]; then
> +				# see the comments around the
> +				# __git_ps1_branch_name variable below
> +				p="$p \${__git_ps1_upstream_name}"
> +			else
> +				p="$p ${__git_ps1_upstream_name}"
> +				# not needed anymore; keep user's
> +				# environment clean
> +				unset __git_ps1_upstream_name
> +			fi
>  		fi
>  	fi
>  
> @@ -438,8 +449,27 @@ __git_ps1 ()
>  		__git_ps1_colorize_gitstring
>  	fi
>  
> +	b=${b##refs/heads/}
> +	if [ $pcmode = yes ]; then
> +		# In pcmode (and only pcmode) the contents of
> +		# $gitstring are subject to expansion by the shell.
> +		# Avoid putting the raw ref name in the prompt to
> +		# protect the user from arbitrary code execution via
> +		# specially crafted ref names (e.g., a ref named
> +		# '$(IFS=_;cmd=sudo_rm_-rf_/;$cmd)' would execute
> +		# 'sudo rm -rf /' when the prompt is drawn).  Instead,
> +		# put the ref name in a new global variable (in the
> +		# __git_ps1_* namespace to avoid colliding with the
> +		# user's environment) and reference that variable from
> +		# PS1.
> +		__git_ps1_branch_name=$b
> +		# note that the $ is escaped -- the variable will be
> +		# expanded later (when it's time to draw the prompt)
> +		b="\${__git_ps1_branch_name}"
> +	fi
> +
>  	local f="$w$i$s$u"
> -	local gitstring="$c${b##refs/heads/}${f:+$z$f}$r$p"
> +	local gitstring="$c$b${f:+$z$f}$r$p"
>  
>  	if [ $pcmode = yes ]; then
>  		if [ "${__git_printf_supports_v-}" != yes ]; then