651c992437
Backport upstream commit 668fea81 to fix CVE-2026-59679, an
out-of-bounds read/write vulnerability in fs_read_glyphs().
The fix validates that num_chars from the FS_QueryXBitmaps16
reply does not exceed the encoding array size allocated during
FS_QueryXExtents16 handling, preventing exploitation by a
malicious font server. A regression test is included.
CVE: CVE-2026-59679
Upstream patches:
- 668fea81f4.patch
Resolves: RHEL-221956
This commit was backported by Ymir, a Red Hat Enterprise Linux software maintenance AI agent.
Assisted-by: Ymir
118 lines
3.8 KiB
Diff
118 lines
3.8 KiB
Diff
From 76a453c43a7fb74f1e6258d452b3d3df51b97af4 Mon Sep 17 00:00:00 2001
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From: Peter Hutterer <peter.hutterer@who-t.net>
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Date: Mon, 13 Jul 2026 15:48:06 +1000
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Subject: [PATCH] fserve: validate num_chars against encoding array size in
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fs_read_glyphs
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FS_QueryXExtents16 causes us to allocate the encoding[] array, later
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during the FS_QueryXBitmaps16 reply handling we fill in that array.
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There is no verification that the allocation is large enough, a
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malicious font server could send us a small numExtents and a
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large num_chars to force underallocation and OOB read/rwrite.
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A regression test is included that constructs a crafted
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FS_QueryXBitmaps16 reply with num_chars > num_encoding and verifies
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the library rejects it.
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CVE-2026-59679
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Found-by: Zhixi "Jace" Sun, independent security researcher
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Assisted-by: Claude:claude-opus-4-6
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Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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Part-of: <https://gitlab.freedesktop.org/xorg/lib/libxfont/-/merge_requests/36>
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---
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Makefile.am | 16 ++++++++++++++++
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src/fc/fserve.c | 22 ++++++++++++++++++++++
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src/fc/fservestr.h | 1 +
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3 files changed, 39 insertions(+)
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diff --git a/Makefile.am b/Makefile.am
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index c1a3db2..ef9bc40 100644
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--- a/Makefile.am
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+++ b/Makefile.am
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@@ -157,6 +157,22 @@ endif
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EXTRA_DIST = src/builtins/buildfont
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+# Security regression tests
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+TESTS =
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+check_PROGRAMS =
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+
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+if XFONT_FC
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+TESTS += test-fserve-read-glyphs
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+check_PROGRAMS += test-fserve-read-glyphs
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+
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+# The test #includes fserve.c directly to access static functions so
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+# we statically link against libXfont2.a.
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+test_fserve_read_glyphs_SOURCES = test/test-fserve-read-glyphs.c
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+test_fserve_read_glyphs_CFLAGS = $(AM_CFLAGS) -I$(top_srcdir)/include -I$(top_srcdir)/src/fc
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+test_fserve_read_glyphs_LDFLAGS = -static
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+test_fserve_read_glyphs_LDADD = libXfont2.la $(LTLIBOBJS)
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+endif XFONT_FC
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+
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MAINTAINERCLEANFILES = ChangeLog INSTALL
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.PHONY: ChangeLog INSTALL
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diff --git a/src/fc/fserve.c b/src/fc/fserve.c
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index 708fc35..4141c3c 100644
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--- a/src/fc/fserve.c
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+++ b/src/fc/fserve.c
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@@ -1097,6 +1097,7 @@ fs_read_extent_info(FontPathElementPtr fpe, FSBlockDataPtr blockrec)
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return AllocError;
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}
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fsfont->encoding = pCI;
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+ fsfont->num_encoding = numExtents;
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if (haveInk)
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fsfont->inkMetrics = pCI + numExtents;
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else
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@@ -2004,6 +2005,17 @@ fs_read_glyphs(FontPathElementPtr fpe, FSBlockDataPtr blockrec)
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{
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minchar = 0;
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maxchar = rep->num_chars;
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+
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+ /* Reject replies where num_chars exceeds the encoding array
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+ size allocated in fs_read_extent_info() to prevent
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+ out-of-bounds access on encoding[]. */
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+ if (rep->num_chars > (CARD32)fsdata->num_encoding)
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+ {
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+ ErrorF("fserve: num_chars (%u) > num_encoding (%d)\n",
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+ (unsigned) rep->num_chars, fsdata->num_encoding);
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+ err = AllocError;
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+ goto bail;
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+ }
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}
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off_adr = (char *)ppbits;
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@@ -2025,6 +2037,16 @@ fs_read_glyphs(FontPathElementPtr fpe, FSBlockDataPtr blockrec)
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for (i = 0; i < rep->num_chars; i++)
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{
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memcpy(&local_off, off_adr, SIZEOF(fsOffset32)); /* align it */
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+ /* Bounds-check minchar against the encoding array size to
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+ prevent out-of-bounds access from a malicious font server
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+ reply with more num_chars than num_extents. */
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+ if (minchar >= (unsigned long)fsdata->num_encoding)
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+ {
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+ ErrorF("fserve: glyph index %lu >= num_encoding (%d)\n",
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+ minchar, fsdata->num_encoding);
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+ err = AllocError;
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+ goto bail;
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+ }
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if (blockrec->type == FS_OPEN_FONT ||
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fsdata->encoding[minchar].bits == &_fs_glyph_requested)
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{
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diff --git a/src/fc/fservestr.h b/src/fc/fservestr.h
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index 29ae46e..da95e41 100644
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--- a/src/fc/fservestr.h
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+++ b/src/fc/fservestr.h
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@@ -43,6 +43,7 @@ typedef struct _fs_glyph {
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typedef struct _fs_font {
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CharInfoPtr pDefault;
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CharInfoPtr encoding;
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+ int num_encoding;
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CharInfoPtr inkMetrics;
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FSGlyphPtr glyphs;
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} FSFontRec, *FSFontPtr;
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--
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2.39.5
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