From 873fe23b565b370dd14eba391d137fe425d479a0 Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Wed, 6 Nov 2024 20:34:50 +0100 Subject: [PATCH 1/3] sideband: mask control characters The output of `git clone` is a vital component for understanding what has happened when things go wrong. However, these logs are partially under the control of the remote server (via the "sideband", which typically contains what the remote `git pack-objects` process sends to `stderr`), and is currently not sanitized by Git. This makes Git susceptible to ANSI escape sequence injection (see CWE-150, https://cwe.mitre.org/data/definitions/150.html), which allows attackers to corrupt terminal state, to hide information, and even to insert characters into the input buffer (i.e. as if the user had typed those characters). To plug this vulnerability, disallow any control character in the sideband, replacing them instead with the common `^` (e.g. `^[` for `\x1b`, `^A` for `\x01`). There is likely a need for more fine-grained controls instead of using a "heavy hammer" like this, which will be introduced subsequently. Signed-off-by: Johannes Schindelin --- sideband.c | 17 +++++++++++++++-- t/t5409-colorize-remote-messages.sh | 12 ++++++++++++ 2 files changed, 27 insertions(+), 2 deletions(-) diff --git a/sideband.c b/sideband.c index ea7c25211e..d2e6023e60 100644 --- a/sideband.c +++ b/sideband.c @@ -66,6 +66,19 @@ void list_config_color_sideband_slots(struct string_list *list, const char *pref list_config_item(list, prefix, keywords[i].keyword); } +static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n) +{ + strbuf_grow(dest, n); + for (; n && *src; src++, n--) { + if (!iscntrl(*src) || *src == '\t' || *src == '\n') + strbuf_addch(dest, *src); + else { + strbuf_addch(dest, '^'); + strbuf_addch(dest, 0x40 + *src); + } + } +} + /* * Optionally highlight one keyword in remote output if it appears at the start * of the line. This should be called for a single line only, which is @@ -81,7 +94,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n) int i; if (!want_color_stderr(use_sideband_colors())) { - strbuf_add(dest, src, n); + strbuf_add_sanitized(dest, src, n); return; } @@ -114,7 +127,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n) } } - strbuf_add(dest, src, n); + strbuf_add_sanitized(dest, src, n); } diff --git a/t/t5409-colorize-remote-messages.sh b/t/t5409-colorize-remote-messages.sh index fa5de4500a..d0745c391b 100755 --- a/t/t5409-colorize-remote-messages.sh +++ b/t/t5409-colorize-remote-messages.sh @@ -98,4 +98,16 @@ test_expect_success 'fallback to color.ui' ' grep "error: error" decoded ' +test_expect_success 'disallow (color) control sequences in sideband' ' + write_script .git/color-me-surprised <<-\EOF && + printf "error: Have you \\033[31mread\\033[m this?\\n" >&2 + exec "$@" + EOF + test_config_global uploadPack.packObjectshook ./color-me-surprised && + test_commit need-at-least-one-commit && + git clone --no-local . throw-away 2>stderr && + test_decode_color decoded && + test_grep ! RED decoded +' + test_done From 268465a5f749cccd5c983ad4453b0f7d41d673cc Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Wed, 6 Nov 2024 21:07:51 +0100 Subject: [PATCH 2/3] sideband: introduce an "escape hatch" to allow control characters The preceding commit fixed the vulnerability whereas sideband messages (that are under the control of the remote server) could contain ANSI escape sequences that would be sent to the terminal verbatim. However, this fix may not be desirable under all circumstances, e.g. when remote servers deliberately add coloring to their messages to increase their urgency. To help with those use cases, give users a way to opt-out of the protections: `sideband.allowControlCharacters`. Signed-off-by: Johannes Schindelin --- Documentation/config.adoc | 2 ++ Documentation/config/sideband.adoc | 5 +++++ sideband.c | 10 ++++++++++ t/t5409-colorize-remote-messages.sh | 8 +++++++- 4 files changed, 24 insertions(+), 1 deletion(-) create mode 100644 Documentation/config/sideband.adoc diff --git a/Documentation/config.adoc b/Documentation/config.adoc index 62eebe7c54..dcea3c0c15 100644 --- a/Documentation/config.adoc +++ b/Documentation/config.adoc @@ -523,6 +523,8 @@ include::config/sequencer.adoc[] include::config/showbranch.adoc[] +include::config/sideband.adoc[] + include::config/sparse.adoc[] include::config/splitindex.adoc[] diff --git a/Documentation/config/sideband.adoc b/Documentation/config/sideband.adoc new file mode 100644 index 0000000000..3fb5045cd7 --- /dev/null +++ b/Documentation/config/sideband.adoc @@ -0,0 +1,5 @@ +sideband.allowControlCharacters:: + By default, control characters that are delivered via the sideband + are masked, to prevent potentially unwanted ANSI escape sequences + from being sent to the terminal. Use this config setting to override + this behavior. diff --git a/sideband.c b/sideband.c index d2e6023e60..ecba71e661 100644 --- a/sideband.c +++ b/sideband.c @@ -26,6 +26,8 @@ static struct keyword_entry keywords[] = { { "error", GIT_COLOR_BOLD_RED }, }; +static int allow_control_characters; + /* Returns a color setting (GIT_COLOR_NEVER, etc). */ static enum git_colorbool use_sideband_colors(void) { @@ -39,6 +41,9 @@ static enum git_colorbool use_sideband_colors(void) if (use_sideband_colors_cached != GIT_COLOR_UNKNOWN) return use_sideband_colors_cached; + repo_config_get_bool(the_repository, "sideband.allowcontrolcharacters", + &allow_control_characters); + if (!repo_config_get_string_tmp(the_repository, key, &value)) use_sideband_colors_cached = git_config_colorbool(key, value); else if (!repo_config_get_string_tmp(the_repository, "color.ui", &value)) @@ -68,6 +73,11 @@ void list_config_color_sideband_slots(struct string_list *list, const char *pref static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n) { + if (allow_control_characters) { + strbuf_add(dest, src, n); + return; + } + strbuf_grow(dest, n); for (; n && *src; src++, n--) { if (!iscntrl(*src) || *src == '\t' || *src == '\n') diff --git a/t/t5409-colorize-remote-messages.sh b/t/t5409-colorize-remote-messages.sh index d0745c391b..fb31e85254 100755 --- a/t/t5409-colorize-remote-messages.sh +++ b/t/t5409-colorize-remote-messages.sh @@ -105,9 +105,15 @@ test_expect_success 'disallow (color) control sequences in sideband' ' EOF test_config_global uploadPack.packObjectshook ./color-me-surprised && test_commit need-at-least-one-commit && + git clone --no-local . throw-away 2>stderr && test_decode_color decoded && - test_grep ! RED decoded + test_grep ! RED decoded && + + rm -rf throw-away && + git -c sideband.allowControlCharacters clone --no-local . throw-away 2>stderr && + test_decode_color decoded && + test_grep RED decoded ' test_done From e6a90de08e2804c3d7c46decd7e738800ce7f15b Mon Sep 17 00:00:00 2001 From: Johannes Schindelin Date: Mon, 18 Nov 2024 21:42:57 +0100 Subject: [PATCH 3/3] sideband: do allow ANSI color sequences by default The preceding two commits introduced special handling of the sideband channel to neutralize ANSI escape sequences before sending the payload to the terminal, and `sideband.allowControlCharacters` to override that behavior. However, some `pre-receive` hooks that are actively used in practice want to color their messages and therefore rely on the fact that Git passes them through to the terminal. In contrast to other ANSI escape sequences, it is highly unlikely that coloring sequences can be essential tools in attack vectors that mislead Git users e.g. by hiding crucial information. Therefore we can have both: Continue to allow ANSI coloring sequences to be passed to the terminal, and neutralize all other ANSI escape sequences. Signed-off-by: Johannes Schindelin --- Documentation/config/sideband.adoc | 17 ++++++-- sideband.c | 61 ++++++++++++++++++++++++++--- t/t5409-colorize-remote-messages.sh | 16 +++++++- 3 files changed, 84 insertions(+), 10 deletions(-) diff --git a/Documentation/config/sideband.adoc b/Documentation/config/sideband.adoc index 3fb5045cd7..f347fd6b33 100644 --- a/Documentation/config/sideband.adoc +++ b/Documentation/config/sideband.adoc @@ -1,5 +1,16 @@ sideband.allowControlCharacters:: By default, control characters that are delivered via the sideband - are masked, to prevent potentially unwanted ANSI escape sequences - from being sent to the terminal. Use this config setting to override - this behavior. + are masked, except ANSI color sequences. This prevents potentially + unwanted ANSI escape sequences from being sent to the terminal. Use + this config setting to override this behavior: ++ +-- + color:: + Allow ANSI color sequences, line feeds and horizontal tabs, + but mask all other control characters. This is the default. + false:: + Mask all control characters other than line feeds and + horizontal tabs. + true:: + Allow all control characters to be sent to the terminal. +-- diff --git a/sideband.c b/sideband.c index ecba71e661..17d0d5b719 100644 --- a/sideband.c +++ b/sideband.c @@ -26,7 +26,11 @@ static struct keyword_entry keywords[] = { { "error", GIT_COLOR_BOLD_RED }, }; -static int allow_control_characters; +static enum { + ALLOW_NO_CONTROL_CHARACTERS = 0, + ALLOW_ALL_CONTROL_CHARACTERS = 1, + ALLOW_ANSI_COLOR_SEQUENCES = 2 +} allow_control_characters = ALLOW_ANSI_COLOR_SEQUENCES; /* Returns a color setting (GIT_COLOR_NEVER, etc). */ static enum git_colorbool use_sideband_colors(void) @@ -41,8 +45,24 @@ static enum git_colorbool use_sideband_colors(void) if (use_sideband_colors_cached != GIT_COLOR_UNKNOWN) return use_sideband_colors_cached; - repo_config_get_bool(the_repository, "sideband.allowcontrolcharacters", - &allow_control_characters); + switch (repo_config_get_maybe_bool(the_repository, "sideband.allowcontrolcharacters", &i)) { + case 0: /* Boolean value */ + allow_control_characters = i ? ALLOW_ALL_CONTROL_CHARACTERS : + ALLOW_NO_CONTROL_CHARACTERS; + break; + case -1: /* non-Boolean value */ + if (repo_config_get_string_tmp(the_repository, "sideband.allowcontrolcharacters", + &value)) + ; /* huh? `get_maybe_bool()` returned -1 */ + else if (!strcmp(value, "color")) + allow_control_characters = ALLOW_ANSI_COLOR_SEQUENCES; + else + warning(_("unrecognized value for `sideband." + "allowControlCharacters`: '%s'"), value); + break; + default: + break; /* not configured */ + } if (!repo_config_get_string_tmp(the_repository, key, &value)) use_sideband_colors_cached = git_config_colorbool(key, value); @@ -71,9 +91,37 @@ void list_config_color_sideband_slots(struct string_list *list, const char *pref list_config_item(list, prefix, keywords[i].keyword); } +static int handle_ansi_color_sequence(struct strbuf *dest, const char *src, int n) +{ + int i; + + /* + * Valid ANSI color sequences are of the form + * + * ESC [ [ [; ]*] m + */ + + if (allow_control_characters != ALLOW_ANSI_COLOR_SEQUENCES || + n < 3 || src[0] != '\x1b' || src[1] != '[') + return 0; + + for (i = 2; i < n; i++) { + if (src[i] == 'm') { + strbuf_add(dest, src, i + 1); + return i; + } + if (!isdigit(src[i]) && src[i] != ';') + break; + } + + return 0; +} + static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n) { - if (allow_control_characters) { + int i; + + if (allow_control_characters == ALLOW_ALL_CONTROL_CHARACTERS) { strbuf_add(dest, src, n); return; } @@ -82,7 +130,10 @@ static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n) for (; n && *src; src++, n--) { if (!iscntrl(*src) || *src == '\t' || *src == '\n') strbuf_addch(dest, *src); - else { + else if ((i = handle_ansi_color_sequence(dest, src, n))) { + src += i; + n -= i; + } else { strbuf_addch(dest, '^'); strbuf_addch(dest, 0x40 + *src); } diff --git a/t/t5409-colorize-remote-messages.sh b/t/t5409-colorize-remote-messages.sh index fb31e85254..a755c49a74 100755 --- a/t/t5409-colorize-remote-messages.sh +++ b/t/t5409-colorize-remote-messages.sh @@ -100,7 +100,7 @@ test_expect_success 'fallback to color.ui' ' test_expect_success 'disallow (color) control sequences in sideband' ' write_script .git/color-me-surprised <<-\EOF && - printf "error: Have you \\033[31mread\\033[m this?\\n" >&2 + printf "error: Have you \\033[31mread\\033[m this?\\a\\n" >&2 exec "$@" EOF test_config_global uploadPack.packObjectshook ./color-me-surprised && @@ -108,12 +108,24 @@ test_expect_success 'disallow (color) control sequences in sideband' ' git clone --no-local . throw-away 2>stderr && test_decode_color decoded && + test_grep RED decoded && + test_grep "\\^G" stderr && + tr -dc "\\007" actual && + test_must_be_empty actual && + + rm -rf throw-away && + git -c sideband.allowControlCharacters=false \ + clone --no-local . throw-away 2>stderr && + test_decode_color decoded && test_grep ! RED decoded && + test_grep "\\^G" stderr && rm -rf throw-away && git -c sideband.allowControlCharacters clone --no-local . throw-away 2>stderr && test_decode_color decoded && - test_grep RED decoded + test_grep RED decoded && + tr -dc "\\007" actual && + test_file_not_empty actual ' test_done