[openwrt/openwrt] base-files: functions.sh: fix config_get() on invalid identifiers

[LEDE Commits]

jow pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/4a6795409d1520fd3da3e909a8bcf9d7fd0927bb

commit 4a6795409d1520fd3da3e909a8bcf9d7fd0927bb
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Wed Aug 5 09:07:00 2020 +0200

    base-files: functions.sh: fix config_get() on invalid identifiers
    
    When passing a section or option value to config_get() which contains
    characters that happen to be valid variable interpolation expressions,
    the function returns a nonsensical expression result instead of the
    expected empty string.
    
    When the passed section or option name contains other characters which
    are not valid within a shell variable name, a substitution error is
    occuring instead.
    
    The issue can be easily reproduced by one of the following examples:
    
        root at OpenWrt:~# . /lib/functions.sh
        root at OpenWrt:~# config load system
        root at OpenWrt:~# config_get variable invalid-section option
        root at OpenWrt:~# echo "$variable"
        section_option:-
    
        root at OpenWrt:~# . /lib/functions.sh
        root at OpenWrt:~# config load system
        root at OpenWrt:~# config_get variable section invalid-option
        root at OpenWrt:~# echo "$variable"
        option:-
    
        root at OpenWrt:~# . /lib/functions.sh
        root at OpenWrt:~# config load system
        root at OpenWrt:~# config_get variable section invalid at option
        -ash: eval: syntax error: bad substitution
    
    Fix this issue by only performing interpolations when the given section
    and option arguments are free of illegal characters.
    
    Signed-off-by: Jo-Philipp Wich <jo at mein.io>
---
 package/base-files/Makefile               |  2 +-
 package/base-files/files/lib/functions.sh | 11 ++++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/package/base-files/Makefile b/package/base-files/Makefile
index 7e0d341705..c3d2aac2bb 100644
--- a/package/base-files/Makefile
+++ b/package/base-files/Makefile
@@ -12,7 +12,7 @@ include $(INCLUDE_DIR)/version.mk
 include $(INCLUDE_DIR)/feeds.mk
 
 PKG_NAME:=base-files
-PKG_RELEASE:=225
+PKG_RELEASE:=226
 PKG_FLAGS:=nonshared
 
 PKG_FILE_DEPENDS:=$(PLATFORM_DIR)/ $(GENERIC_PLATFORM_DIR)/base-files/
diff --git a/package/base-files/files/lib/functions.sh b/package/base-files/files/lib/functions.sh
index 323e057451..7da0c872fa 100755
--- a/package/base-files/files/lib/functions.sh
+++ b/package/base-files/files/lib/functions.sh
@@ -107,9 +107,14 @@ config_unset() {
 # config_get <variable> <section> <option> [<default>]
 # config_get <section> <option>
 config_get() {
-	case "$3" in
-		"") eval echo "\"\${CONFIG_${1}_${2}:-\${4}}\"";;
-		*)  eval export ${NO_EXPORT:+-n} -- "${1}=\${CONFIG_${2}_${3}:-\${4}}";;
+	case "$2${3:-$1}" in
+		*[^A-Za-z0-9_]*) : ;;
+		*)
+			case "$3" in
+				"") eval echo "\"\${CONFIG_${1}_${2}:-\${4}}\"";;
+				*)  eval export ${NO_EXPORT:+-n} -- "${1}=\${CONFIG_${2}_${3}:-\${4}}";;
+			esac
+		;;
 	esac
 }