On Fri, Aug 02, 2024 at 02:26:05PM -0500, Eric Blake wrote: > My next patch needs to convert text from an untrusted input into an > output representation that is suitable for display on a terminal is > useful to more than just the json-writer; the text should normally be > UTF-8, but blindly allowing all Unicode code points (including ASCII > ESC) through to a terminal risks remote-code-execution attacks on some > terminals. Extract the existing body of json-writer's quoted_strinto > a new helper routine mod_utf8_sanitize, and generalize it to also work > on data that is length-limited rather than NUL-terminated. [I was > actually surprised that glib does not have such a sanitizer already - > Google turns up lots of examples of rolling your own string > sanitizer.] > > If desired in the future, we may want to tweak whether the output is > guaranteed to be ASCII (using lots of \u escape sequences, including > surrogate pairs for code points outside the BMP) or if we are okay > passing printable Unicode through (we still need to escape control > characters). But for now, I went for minimal code churn, including > the fact that the resulting function allows a non-UTF-8 2-byte synonym > for U+0000. > > Signed-off-by: Eric Blake <ebl...@redhat.com>
I have to admit I'd never heard of "modified UTF-8" before, but it's a thing: https://en.wikipedia.org/wiki/UTF-8#Modified_UTF-8 As the patch is almost a simple code motion: Reviewed-by: Richard W.M. Jones <rjo...@redhat.com> Rich. > --- > include/qemu/unicode.h | 3 ++ > qobject/json-writer.c | 47 +---------------------- > util/unicode.c | 84 ++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 88 insertions(+), 46 deletions(-) > > diff --git a/include/qemu/unicode.h b/include/qemu/unicode.h > index 7fa10b8e604..e1013b29f72 100644 > --- a/include/qemu/unicode.h > +++ b/include/qemu/unicode.h > @@ -4,4 +4,7 @@ > int mod_utf8_codepoint(const char *s, size_t n, char **end); > ssize_t mod_utf8_encode(char buf[], size_t bufsz, int codepoint); > > +void mod_utf8_sanitize(GString *buf, const char *str); > +void mod_utf8_sanitize_len(GString *buf, const char *str, ssize_t len); > + > #endif > diff --git a/qobject/json-writer.c b/qobject/json-writer.c > index 309a31d57a9..5c14574efee 100644 > --- a/qobject/json-writer.c > +++ b/qobject/json-writer.c > @@ -104,53 +104,8 @@ static void pretty_newline_or_space(JSONWriter *writer) > > static void quoted_str(JSONWriter *writer, const char *str) > { > - const char *ptr; > - char *end; > - int cp; > - > g_string_append_c(writer->contents, '"'); > - > - for (ptr = str; *ptr; ptr = end) { > - cp = mod_utf8_codepoint(ptr, 6, &end); > - switch (cp) { > - case '\"': > - g_string_append(writer->contents, "\\\""); > - break; > - case '\\': > - g_string_append(writer->contents, "\\\\"); > - break; > - case '\b': > - g_string_append(writer->contents, "\\b"); > - break; > - case '\f': > - g_string_append(writer->contents, "\\f"); > - break; > - case '\n': > - g_string_append(writer->contents, "\\n"); > - break; > - case '\r': > - g_string_append(writer->contents, "\\r"); > - break; > - case '\t': > - g_string_append(writer->contents, "\\t"); > - break; > - default: > - if (cp < 0) { > - cp = 0xFFFD; /* replacement character */ > - } > - if (cp > 0xFFFF) { > - /* beyond BMP; need a surrogate pair */ > - g_string_append_printf(writer->contents, "\\u%04X\\u%04X", > - 0xD800 + ((cp - 0x10000) >> 10), > - 0xDC00 + ((cp - 0x10000) & 0x3FF)); > - } else if (cp < 0x20 || cp >= 0x7F) { > - g_string_append_printf(writer->contents, "\\u%04X", cp); > - } else { > - g_string_append_c(writer->contents, cp); > - } > - } > - }; > - > + mod_utf8_sanitize(writer->contents, str); > g_string_append_c(writer->contents, '"'); > } > > diff --git a/util/unicode.c b/util/unicode.c > index 8580bc598b3..a419ed4de35 100644 > --- a/util/unicode.c > +++ b/util/unicode.c > @@ -154,3 +154,87 @@ ssize_t mod_utf8_encode(char buf[], size_t bufsz, int > codepoint) > buf[4] = 0; > return 4; > } > + > +/** > + * mod_utf8_sanitize: > + * @buf: Destination buffer > + * @str: Modified UTF-8 string to sanitize > + * > + * Append the contents of the NUL-terminated Modified UTF-8 string > + * @str into @buf, with escape sequences used for non-printable ASCII > + * and for quote characters. The result is therefore safe for output > + * to a terminal. > + * > + * Modified UTF-8 is exactly like UTF-8, except U+0000 is encoded as > + * "\xC0\x80". > + */ > +void mod_utf8_sanitize(GString *buf, const char *str) > +{ > + mod_utf8_sanitize_len(buf, str, -1); > +} > + > +/** > + * mod_utf8_sanitize: > + * @buf: Destination buffer > + * @str: Modified UTF-8 string to sanitize > + * @len: Length of @str, or negative to stop at NUL terminator > + * > + * Append the contents of @len bytes of the Modified UTF-8 string > + * @str into @buf, with escape sequences used for non-printable ASCII > + * and for quote characters. The result is therefore safe for output > + * to a terminal. > + * > + * Modified UTF-8 is exactly like UTF-8, except U+0000 is encoded as > + * "\xC0\x80". > + */ > +void mod_utf8_sanitize_len(GString *buf, const char *str, ssize_t len) > +{ > + const char *ptr; > + char *end; > + int cp; > + > + if (len < 0) { > + len = strlen(str); > + } > + > + for (ptr = str; *ptr; ptr = end) { > + cp = mod_utf8_codepoint(ptr, MIN(6, str + len - ptr), &end); > + switch (cp) { > + case '\"': > + g_string_append(buf, "\\\""); > + break; > + case '\\': > + g_string_append(buf, "\\\\"); > + break; > + case '\b': > + g_string_append(buf, "\\b"); > + break; > + case '\f': > + g_string_append(buf, "\\f"); > + break; > + case '\n': > + g_string_append(buf, "\\n"); > + break; > + case '\r': > + g_string_append(buf, "\\r"); > + break; > + case '\t': > + g_string_append(buf, "\\t"); > + break; > + default: > + if (cp < 0) { > + cp = 0xFFFD; /* replacement character */ > + } > + if (cp > 0xFFFF) { > + /* beyond BMP; need a surrogate pair */ > + g_string_append_printf(buf, "\\u%04X\\u%04X", > + 0xD800 + ((cp - 0x10000) >> 10), > + 0xDC00 + ((cp - 0x10000) & 0x3FF)); > + } else if (cp < 0x20 || cp >= 0x7F) { > + g_string_append_printf(buf, "\\u%04X", cp); > + } else { > + g_string_append_c(buf, cp); > + } > + } > + } > +} > -- > 2.45.2 -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v
