[RFAv2 2/2] gmp-utils: protect gdb_mpz exports against out-of-range values

Joel Brobecker
2020-12-05T08:11:01Z

Hi Simon,

Here is v2 of this patch, taking into account your comments and
suggestions. More precisely:

- Parameter "nails" in gdb_mpz::safe_export has been removed;
- I changed the error message to use "[nnn]-bits" instead of
"[nnn]bits";
- I removed a useless memset in gdb_mpz::safe_export;
- I used the {buf, size} idiom to construct a a gdb::array view
instead of using gdb::make_array_view.

The series was re-tested on x86_64-linux.

OK to push?

---------------------------------------------------------------------------

The gdb_mpz class currently provides a couple of methods which
essentially export an mpz_t value into either a buffer, or an integral
type. The export is based on using the mpz_export function which
we discovered can be a bit treacherous if used without caution.

In particular, the initial motivation for this patch was to catch
situations where the mpz_t value was so large that it would not fit
in the destination area. mpz_export does not know the size of
the buffer, and therefore can happily write past the end of our buffer.

While designing a solution to the above problem, I also discovered
that we also needed to be careful when exporting signed numbers.
In particular, numbers which are larger than the maximum value
for a given signed type size, but no so large as to fit in the
*unsigned* version with the same size, would end up being exported
incorrectly. This is related to the fact that mpz_export ignores
the sign of the value being exportd, and assumes an unsigned export.
Thus, for such large values, the appears as if mpz_export is able
to fit our value into our buffer, but in fact, it does not.

Also, I noticed that gdb_mpz::write wasn't taking its unsigned_p
parameter, which was a hole.

For all these reasons, a new low-level private method called
"safe_export" has been added to class gdb_mpz, whose goal is
to perform all necessary checks and manipulations for a safe
and correct export. As a bonus, this method allows us to factorize
the handling of negative value exports.

The gdb_mpz::as_integer and gdb_mpz::write methods are then simplified
to take advantage of this new safe_export method.

gdb/ChangeLog:

* gmp-utils.h (gdb_mpz::safe_export): New private method.
(gdb_mpz::as_integer): Reimplement using gdb_mpz::safe_export.
* gmp-utils.c (gdb_mpz::write): Rewrite using gdb_mpz::safe_export.
(gdb_mpz::safe_export): New method.
* unittests/gmp-utils-selftests .c (gdb_mpz_as_integer):
Update function description.
(check_as_integer_raises_out_of_range_error): New function.
(gdb_mpz_as_integer_out_of_range): New function.
(_initialize_gmp_utils_selftests): Register
gdb_mpz_as_integer_out_of_range as a selftest.
---
gdb/gmp-utils.c | 79 +++++++++++++++++++++++++++++++++----
gdb/gmp-utils.h | 40 ++++++++++---------
gdb/unittests/gmp-utils-selftests.c | 71 +++++++++++++++++++++++++++++++--
3 files changed, 162 insertions(+), 28 deletions(-)

diff --git a/gdb/gmp-utils.c b/gdb/gmp-utils.c
index e3a3333..6b5c820 100644
--- a/gdb/gmp-utils.c
+++ b/gdb/gmp-utils.c
@@ -68,9 +68,61 @@ void
gdb_mpz::write (gdb::array_view<gdb_byte> buf, enum bfd_endian byte_order,
bool unsigned_p) const
{
+ this->safe_export
+ (buf, byte_order == BFD_ENDIAN_BIG ? 1 : -1 /* endian */, unsigned_p);
+}
+
+/* See gmp-utils.h. */
+
+void
+gdb_mpz::safe_export (gdb::array_view<gdb_byte> buf,
+ int endian, bool unsigned_p) const
+{
+ gdb_assert (buf.size () > 0);
+
+ if (mpz_sgn (val) == 0)
+ {
+ /* Our value is zero, so no need to call mpz_export to do the work,
+ especially since mpz_export's documentation explicitly says
+ that the function is a noop in this case. Just write zero to
+ BUF ourselves. */
+ memset (buf.data (), 0, buf.size ());
+ return;
+ }
+
+ /* Determine the maximum range of values that our buffer can hold,
+ and verify that VAL is within that range. */
+
+ gdb_mpz lo, hi;
+ const size_t max_usable_bits = buf.size () * HOST_CHAR_BIT;
+ if (unsigned_p)
+ {
+ lo = 0;
+
+ mpz_ui_pow_ui (hi.val, 2, max_usable_bits);
+ mpz_sub_ui (hi.val, hi.val, 1);
+ }
+ else
+ {
+ mpz_ui_pow_ui (lo.val, 2, max_usable_bits - 1);
+ mpz_neg (lo.val, lo.val);
+
+ mpz_ui_pow_ui (hi.val, 2, max_usable_bits - 1);
+ mpz_sub_ui (hi.val, hi.val, 1);
+ }
+
+ if (mpz_cmp (val, lo.val) < 0 || mpz_cmp (val, hi.val) > 0)
+ error (_("Cannot export value %s as %zu-bits %s integer"
+ " (must be between %s and %s)"),
+ this->str ().c_str (),
+ max_usable_bits,
+ unsigned_p ? _("unsigned") : _("signed"),
+ lo.str ().c_str (),
+ hi.str ().c_str ());
+
gdb_mpz exported_val (val);

- if (mpz_cmp_ui (val, 0) < 0)
+ if (mpz_cmp_ui (exported_val.val, 0) < 0)
{
/* mpz_export does not handle signed values, so create a positive
value whose bit representation as an unsigned of the same length
@@ -81,13 +133,26 @@ gdb_mpz::write (gdb::array_view<gdb_byte> buf, enum bfd_endian byte_order,
mpz_add (exported_val.val, exported_val.val, neg_offset.val);
}

+ /* Do the export into a buffer allocated by GMP itself; that way,
+ we can detect cases where BUF is not large enough to export
+ our value, and thus avoid a buffer overlow. Normally, this should
+ never happen, since we verified earlier that the buffer is large
+ enough to accomodate our value, but doing this allows us to be
+ extra safe with the export.
+
+ After verification that the export behaved as expected, we will
+ copy the data over to BUF. */
+
+ size_t word_countp;
+ gdb::unique_xmalloc_ptr<void> exported
+ (mpz_export (NULL, &word_countp, -1 /* order */, buf.size () /* size */,
+ endian, 0 /* nails */, exported_val.val));
+
+ gdb_assert (word_countp == 1);
+
/* Start by clearing the buffer, as mpz_export only writes as many
- bytes as it needs (including none, if the value to export is zero. */
- memset (buf.data (), 0, buf.size ());
- mpz_export (buf.data (), NULL /* count */, -1 /* order */,
- buf.size () /* size */,
- byte_order == BFD_ENDIAN_BIG ? 1 : -1 /* endian */,
- 0 /* nails */, exported_val.val);
+ bytes as it needs. */
+ memcpy (buf.data (), exported.get (), buf.size ());
}

/* See gmp-utils.h. */
diff --git a/gdb/gmp-utils.h b/gdb/gmp-utils.h
index 12e4f8e..dae62cf 100644
--- a/gdb/gmp-utils.h
+++ b/gdb/gmp-utils.h
@@ -121,6 +121,24 @@ struct gdb_mpz

/* Helper template for constructor and operator=. */
template<typename T> void set (T src);
+
+ /* Low-level function to export VAL into BUF as a number whose byte size
+ is the size of BUF.
+
+ If UNSIGNED_P is true, then export VAL into BUF as an unsigned value.
+ Otherwise, export it as a signed value.
+
+ The API is inspired from GMP's mpz_export, hence the naming and types
+ of the following parameter:
+ - ENDIAN should be:
+ . 1 for most significant byte first; or
+ . -1 for least significant byte first; or
+ . 0 for native endianness.
+
+ An error is raised if BUF is not large enough to contain the value
+ being exported. */
+ void safe_export (gdb::array_view<gdb_byte> buf,
+ int endian, bool unsigned_p) const;
};

/* A class to make it easier to use GMP's mpq_t values within GDB. */
@@ -258,26 +276,12 @@ template<typename T>
T
gdb_mpz::as_integer () const
{
- /* Initialize RESULT, because mpz_export only write the minimum
- number of bytes, including none if our value is zero! */
- T result = 0;
-
- gdb_mpz exported_val (val);
- if (std::is_signed<T>::value && mpz_cmp_ui (val, 0) < 0)
- {
- /* We want to use mpz_export to set the return value, but
- this function does not handle the sign. So give exported_val
- a value which is at the same time positive, and has the same
- bit representation as our negative value. */
- gdb_mpz neg_offset;
+ T result;

- mpz_ui_pow_ui (neg_offset.val, 2, sizeof (T) * HOST_CHAR_BIT);
- mpz_add (exported_val.val, exported_val.val, neg_offset.val);
- }
+ this->safe_export ({(gdb_byte *) &result, sizeof (result)},
+ 0 /* endian (0 = native) */,
+ !std::is_signed<T>::value /* unsigned_p */);

- mpz_export (&result, NULL /* count */, -1 /* order */,
- sizeof (T) /* size */, 0 /* endian (0 = native) */,
- 0 /* nails */, exported_val.val);
return result;
}

diff --git a/gdb/unittests/gmp-utils-selftests.c b/gdb/unittests/gmp-utils-selftests.c
index 1365905..30c1902 100644
--- a/gdb/unittests/gmp-utils-selftests.c
+++ b/gdb/unittests/gmp-utils-selftests.c
@@ -26,9 +26,10 @@ namespace selftests {

/* Perform a series of general tests of gdb_mpz's as_integer method.

- This function tries to be reasonably exhaustive, by testing the edges,
- as well as a resonable set of values including negative ones, zero,
- and positive values. */
+ This function limits itself to values which are in range (out-of-range
+ values will be tested separately). In doing so, it tries to be reasonably
+ exhaustive, by testing the edges, as well as a resonable set of values
+ including negative ones, zero, and positive values. */

static void
gdb_mpz_as_integer ()
@@ -80,6 +81,68 @@ gdb_mpz_as_integer ()
SELF_CHECK (v.as_integer<ULONGEST> () == ul_expected);
}

+/* A helper function which calls the given gdb_mpz object's as_integer
+ method with the given type T, and verifies that this triggers
+ an error due to VAL's value being out of range for type T. */
+
+template<typename T, typename = gdb::Requires<std::is_integral<T>>>
+static void
+check_as_integer_raises_out_of_range_error (const gdb_mpz &val)
+{
+ try
+ {
+ val.as_integer<T> ();
+ }
+ catch (const gdb_exception_error &ex)
+ {
+ SELF_CHECK (ex.reason == RETURN_ERROR);
+ SELF_CHECK (ex.error == GENERIC_ERROR);
+ SELF_CHECK (strstr (ex.what (), "Cannot export value") != nullptr);
+ return;
+ }
+ /* The expected exception did not get raised. */
+ SELF_CHECK (false);
+}
+
+/* Perform out-of-range tests of gdb_mpz's as_integer method.
+
+ The goal of this function is to verify that gdb_mpz::as_integer
+ handles out-of-range values correctly. */
+
+static void
+gdb_mpz_as_integer_out_of_range ()
+{
+ gdb_mpz v;
+
+ /* Try LONGEST_MIN minus 1. */
+ mpz_ui_pow_ui (v.val, 2, sizeof (LONGEST) * 8 - 1);
+ mpz_neg (v.val, v.val);
+ mpz_sub_ui (v.val, v.val, 1);
+
+ check_as_integer_raises_out_of_range_error<ULONGEST> (v);
+ check_as_integer_raises_out_of_range_error<LONGEST> (v);
+
+ /* Try negative one (-1). */
+ v = -1;
+
+ check_as_integer_raises_out_of_range_error<ULONGEST> (v);
+ SELF_CHECK (v.as_integer<LONGEST> () == (LONGEST) -1);
+
+ /* Try LONGEST_MAX plus 1. */
+ v = LONGEST_MAX;
+ mpz_add_ui (v.val, v.val, 1);
+
+ SELF_CHECK (v.as_integer<ULONGEST> () == (ULONGEST) LONGEST_MAX + 1);
+ check_as_integer_raises_out_of_range_error<LONGEST> (v);
+
+ /* Try ULONGEST_MAX plus 1. */
+ v = ULONGEST_MAX;
+ mpz_add_ui (v.val, v.val, 1);
+
+ check_as_integer_raises_out_of_range_error<ULONGEST> (v);
+ check_as_integer_raises_out_of_range_error<LONGEST> (v);
+}
+
/* A helper function to store the given integer value into a buffer,
before reading it back into a gdb_mpz. Sets ACTUAL to the value
read back, while at the same time setting EXPECTED as the value
@@ -445,6 +508,8 @@ _initialize_gmp_utils_selftests ()
{
selftests::register_test ("gdb_mpz_as_integer",
selftests::gdb_mpz_as_integer);
+ selftests::register_test ("gdb_mpz_as_integer_out_of_range",
+ selftests::gdb_mpz_as_integer_out_of_range);
selftests::register_test ("gdb_mpz_read_all_from_small",
selftests::gdb_mpz_read_all_from_small);
selftests::register_test ("gdb_mpz_read_min_max",
--
2.1.4