Web lists-archives.com

[PATCH] http.c: shell command evaluation for extraheader

The http.extraHeader config parameter currently only supports storing
constant values. There are two main use cases where this fails:

  0. Sensitive payloads: frequently this config parameter is used to pass
     authentication credentials in place of or in addition to the
     Authorization header, however since this value is required to be in
     the clear this can create security issues.

  1. Mutating headers: some headers, especially new authentication
     schemes, leverage short lived tokens that change over time.

There do exist solutions with current tools for these use cases, however
none are optimal:

  0. Shell alias: by aliasing over git with a call to git that includes the
     config directive and evaluates the header value inline, you can
     fake the desired mutability:
       `alias git='git -c http.extraHeader="$(gpg -d crypt.gpg)"'`
     This presents two problems:
     a. aliasing over commands can be confusing to new users, since git
        config information is stored in shell configs
     b. this solution scales only to your shell, not all shells

  1. Global hook: you could implement a hook that writes the config
     entry before fetch / pull actions, so that it is up to date, but
     this does nothing to secure it.

  2. git-credential-helper: the credential helper interface already
     supports shelling out to arbitrary binaries or scripts, however
     this interface can only be used to populate the Authorization

The optimal solution involves extending the current implementation of
http.extraHeader parsing to allow for arbitrary shell command execution.
There seem to be two paradigms for such features:

  0. Overloading with '!' prefixes: seen in alias.* and credential.helper

  1. New "Cmd" suffix parameters: seen in sendemail.toCmd sendemail.ccCmd

While the latter may be more clear without documentation, the addition
of a new config parameter seemed more complex for the codebase. As such,
new documentation is included.

Several edge cases came up during implementation and the following
design decisions were made:

  0. Stdin and stderr for the child_process are exposed to the user:
     this allows commands that print status information via stderr, and
     accept input to function. The use case considered is text input for
     decryption and error handling that is out of scope for git.

  1. Failure to exec: if either the file does not exist, or any other
     exec related failure occurs, no error is presented to the user,
     and the header is not included

  2. Non-zero return code: if the child_process returns a non-zero
     value, no error is presented to the user, the return value is
     consumed, and the header is not included in the request.

  3. Headers starting with the '!' character require a shell command to
     create: because no escaping syntax was implemented, the following
     is required for such headers: "!printf '!magic: abra'"

Signed-off-by: Colin Arnott <colin@xxxxxxxxxxxxx>
 Documentation/config.txt    |  7 +++++++
 http.c                      | 20 ++++++++++++++++++++
 t/t5551-http-fetch-smart.sh |  6 ++++--
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index f57e9cf10..4b2171d60 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1918,6 +1918,13 @@ http.extraHeader::
 	more than one such entry exists, all of them are added as extra
 	headers.  To allow overriding the settings inherited from the system
 	config, an empty value will reset the extra headers to the empty list.
+	If the value is prefixed with an exclamation point, it will
+	be treated as a shell command.  For example, defining
+	"http.extraHeader = !gpg -d < secure_header.gpg", will pass the
+	decrypted header, if the command does not exec cleanly or has a
+	non-zero return value, no header will be added.  Note that shell
+	commands will be executed from the top-level directory of a
+	repository, which may not necessarily be the current directory.
 	The pathname of a file containing previously stored cookie lines,
diff --git a/http.c b/http.c
index 31755023a..11103df41 100644
--- a/http.c
+++ b/http.c
@@ -380,6 +380,26 @@ static int http_options(const char *var, const char *value, void *cb)
 		} else if (!*value) {
 			extra_http_headers = NULL;
+		} else if (value[0] == '!') {
+			struct child_process cp = CHILD_PROCESS_INIT;
+			cp.git_cmd = 0;
+			cp.in = 0;
+			cp.out = -1;
+			cp.err = 0;
+			cp.use_shell = 1;
+			argv_array_push(&cp.args, value + 1);
+			if (!start_command(&cp)) {
+				struct strbuf output;
+				strbuf_init(&output, 0);
+				strbuf_read(&output, cp.out, 0);
+				close(cp.out);
+				cp.out = -1;
+				if (!finish_command(&cp)) {
+					extra_http_headers =
+						curl_slist_append(extra_http_headers, output.buf);
+				}
+				strbuf_release(&output);
+			}
 		} else {
 			extra_http_headers =
 				curl_slist_append(extra_http_headers, value);
diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index f5721b4a5..039afc76a 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -351,7 +351,9 @@ test_expect_success 'custom http headers' '
 	test_must_fail git -c http.extraheader="x-magic-two: cadabra" \
 		fetch "$HTTPD_URL/smart_headers/repo.git" &&
 	git -c http.extraheader="x-magic-one: abra" \
-	    -c http.extraheader="x-magic-two: cadabra" \
+	    -c http.extraheader="!printf \"x-magic-two: cadabra\"" \
+	    -c http.extraheader="!printf \"x-magic-three: alakazam; exit 2\"" \
+	    -c http.extraheader="!shellcommanddoesnotexist" \
 	    fetch "$HTTPD_URL/smart_headers/repo.git" &&
 	git update-index --add --cacheinfo 160000,$(git rev-parse HEAD),sub &&
 	git config -f .gitmodules submodule.sub.path sub &&
@@ -360,7 +362,7 @@ test_expect_success 'custom http headers' '
 	git submodule init sub &&
 	test_must_fail git submodule update sub &&
 	git -c http.extraheader="x-magic-one: abra" \
-	    -c http.extraheader="x-magic-two: cadabra" \
+	    -c http.extraheader="!printf \"x-magic-two: cadabra\"" \
 		submodule update sub