From 78da5faccb3e065116b75b3ff87ff55381da6c76 Mon Sep 17 00:00:00 2001 From: Philip Withnall Date: Thu, 15 Dec 2022 13:00:39 +0000 Subject: [PATCH 1/2] =?UTF-8?q?gvariant:=20Check=20offset=20table=20doesn?= =?UTF-8?q?=E2=80=99t=20fall=20outside=20variant=20bounds?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit When dereferencing the first entry in the offset table for a tuple, check that it doesn’t fall outside the bounds of the variant first. This prevents an out-of-bounds read from some non-normal tuples. This bug was introduced in commit 73d0aa81c2575a5c9ae77d. Includes a unit test, although the test will likely only catch the original bug if run with asan enabled. Signed-off-by: Philip Withnall Fixes: #2840 oss-fuzz#54302 --- glib/gvariant-serialiser.c | 12 ++++++-- glib/tests/gvariant.c | 63 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 72 insertions(+), 3 deletions(-) diff --git a/glib/gvariant-serialiser.c b/glib/gvariant-serialiser.c index f443c2eb85..4e4a73ad17 100644 --- a/glib/gvariant-serialiser.c +++ b/glib/gvariant-serialiser.c @@ -984,7 +984,8 @@ gvs_tuple_get_member_bounds (GVariantSerialised value, member_info = g_variant_type_info_member_info (value.type_info, index_); - if (member_info->i + 1) + if (member_info->i + 1 && + offset_size * (member_info->i + 1) <= value.size) member_start = gvs_read_unaligned_le (value.data + value.size - offset_size * (member_info->i + 1), offset_size); @@ -995,7 +996,8 @@ gvs_tuple_get_member_bounds (GVariantSerialised value, member_start &= member_info->b; member_start |= member_info->c; - if (member_info->ending_type == G_VARIANT_MEMBER_ENDING_LAST) + if (member_info->ending_type == G_VARIANT_MEMBER_ENDING_LAST && + offset_size * (member_info->i + 1) <= value.size) member_end = value.size - offset_size * (member_info->i + 1); else if (member_info->ending_type == G_VARIANT_MEMBER_ENDING_FIXED) @@ -1006,11 +1008,15 @@ gvs_tuple_get_member_bounds (GVariantSerialised value, member_end = member_start + fixed_size; } - else /* G_VARIANT_MEMBER_ENDING_OFFSET */ + else if (member_info->ending_type == G_VARIANT_MEMBER_ENDING_OFFSET && + offset_size * (member_info->i + 2) <= value.size) member_end = gvs_read_unaligned_le (value.data + value.size - offset_size * (member_info->i + 2), offset_size); + else /* invalid */ + member_end = G_MAXSIZE; + if (out_member_start != NULL) *out_member_start = member_start; if (out_member_end != NULL) diff --git a/glib/tests/gvariant.c b/glib/tests/gvariant.c index b360888e4d..98c51a1d75 100644 --- a/glib/tests/gvariant.c +++ b/glib/tests/gvariant.c @@ -5576,6 +5576,67 @@ test_normal_checking_tuple_offsets4 (void) g_variant_unref (variant); } +/* This is a regression test that dereferencing the first element in the offset + * table doesn’t dereference memory before the start of the GVariant. The first + * element in the offset table gives the offset of the final member in the + * tuple (the offset table is stored in reverse), and the position of this final + * member is needed to check that none of the tuple members overlap with the + * offset table + * + * See https://gitlab.gnome.org/GNOME/glib/-/issues/2840 */ +static void +test_normal_checking_tuple_offsets5 (void) +{ + /* A tuple of type (sss) in normal form would have an offset table with two + * entries: + * - The first entry (lowest index in the table) gives the offset of the + * third `s` in the tuple, as the offset table is reversed compared to the + * tuple members. + * - The second entry (highest index in the table) gives the offset of the + * second `s` in the tuple. + * - The offset of the first `s` in the tuple is always 0. + * + * See §2.5.4 (Structures) of the GVariant specification for details, noting + * that the table is only layed out this way because all three members of the + * tuple have non-fixed sizes. + * + * It’s not clear whether the 0xaa data of this variant is part of the strings + * in the tuple, or part of the offset table. It doesn’t really matter. This + * is a regression test to check that the code to validate the offset table + * doesn’t unconditionally try to access the first entry in the offset table + * by subtracting the table size from the end of the GVariant data. + * + * In this non-normal case, that would result in an address off the start of + * the GVariant data, and an out-of-bounds read, because the GVariant is one + * byte long, but the offset table is calculated as two bytes long (with 1B + * sized entries) from the tuple’s type. + */ + const GVariantType *data_type = G_VARIANT_TYPE ("(sss)"); + const guint8 data[] = { 0xaa }; + gsize size = sizeof (data); + GVariant *variant = NULL; + GVariant *normal_variant = NULL; + GVariant *expected = NULL; + + g_test_bug ("https://gitlab.gnome.org/GNOME/glib/-/issues/2840"); + + variant = g_variant_new_from_data (data_type, data, size, FALSE, NULL, NULL); + g_assert_nonnull (variant); + + g_assert_false (g_variant_is_normal_form (variant)); + + normal_variant = g_variant_get_normal_form (variant); + g_assert_nonnull (normal_variant); + + expected = g_variant_new_parsed ("('', '', '')"); + g_assert_cmpvariant (expected, variant); + g_assert_cmpvariant (expected, normal_variant); + + g_variant_unref (expected); + g_variant_unref (normal_variant); + g_variant_unref (variant); +} + /* Test that an otherwise-valid serialised GVariant is considered non-normal if * its offset table entries are too wide. * @@ -5827,6 +5888,8 @@ main (int argc, char **argv) test_normal_checking_tuple_offsets3); g_test_add_func ("/gvariant/normal-checking/tuple-offsets4", test_normal_checking_tuple_offsets4); + g_test_add_func ("/gvariant/normal-checking/tuple-offsets5", + test_normal_checking_tuple_offsets5); g_test_add_func ("/gvariant/normal-checking/tuple-offsets/minimal-sized", test_normal_checking_tuple_offsets_minimal_sized); g_test_add_func ("/gvariant/normal-checking/empty-object-path", -- GitLab From 21a204147b16539b3eda3143b32844c49e29f4d4 Mon Sep 17 00:00:00 2001 From: Philip Withnall Date: Thu, 15 Dec 2022 16:49:28 +0000 Subject: [PATCH 2/2] gvariant: Propagate trust when getting a child of a serialised variant If a variant is trusted, that means all its children are trusted, so ensure that their checked offsets are set as such. This allows a lot of the offset table checks to be avoided when getting children from trusted serialised tuples, which speeds things up. No unit test is included because this is just a performance fix. If there are other slownesses, or regressions, in serialised `GVariant` performance, the fuzzing setup will catch them like it did this one. This change does reduce the time to run the oss-fuzz reproducer from 80s to about 0.7s on my machine. Signed-off-by: Philip Withnall Fixes: #2841 oss-fuzz#54314 --- glib/gvariant-core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/glib/gvariant-core.c b/glib/gvariant-core.c index f441c4757e..4778022829 100644 --- a/glib/gvariant-core.c +++ b/glib/gvariant-core.c @@ -1198,8 +1198,8 @@ g_variant_get_child_value (GVariant *value, child->contents.serialised.bytes = g_bytes_ref (value->contents.serialised.bytes); child->contents.serialised.data = s_child.data; - child->contents.serialised.ordered_offsets_up_to = s_child.ordered_offsets_up_to; - child->contents.serialised.checked_offsets_up_to = s_child.checked_offsets_up_to; + child->contents.serialised.ordered_offsets_up_to = (value->state & STATE_TRUSTED) ? G_MAXSIZE : s_child.ordered_offsets_up_to; + child->contents.serialised.checked_offsets_up_to = (value->state & STATE_TRUSTED) ? G_MAXSIZE : s_child.checked_offsets_up_to; return child; } -- GitLab