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Date:   Tue, 20 Sep 2022 12:21:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-hardening@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>, Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        xen-devel@...ts.xenproject.org, llvm@...ts.linux.dev,
        Siddhesh Poyarekar <siddhesh@...plt.org>,
        Arnd Bergmann <arnd@...db.de>, Tom Rix <trix@...hat.com>,
        Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] x86/entry: Work around Clang __bdos() bug

After expanding bounds checking to use __builtin_dynamic_object_size(),
Clang produces a false positive when building with CONFIG_FORTIFY_SOURCE=y
and CONFIG_UBSAN_BOUNDS=y when operating on an array with a dynamic
offset. Work around this by using a direct assignment of an empty
instance. Avoids this warning:

../include/linux/fortify-string.h:309:4: warning: call to __write_overflow_field declared with 'warn
ing' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wat
tribute-warning]
                        __write_overflow_field(p_size_field, size);
                        ^

which was isolated to the memset() call in xen_load_idt().

Note that this looks very much like another bug that was worked around:
https://github.com/ClangBuiltLinux/linux/issues/1592

Cc: Juergen Gross <jgross@...e.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: xen-devel@...ts.xenproject.org
Cc: llvm@...ts.linux.dev
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/x86/xen/enlighten_pv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 0ed2e487a693..9b1a58dda935 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -765,6 +765,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
 {
 	static DEFINE_SPINLOCK(lock);
 	static struct trap_info traps[257];
+	static const struct trap_info zero = { };
 	unsigned out;
 
 	trace_xen_cpu_load_idt(desc);
@@ -774,7 +775,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
 	memcpy(this_cpu_ptr(&idt_desc), desc, sizeof(idt_desc));
 
 	out = xen_convert_trap_info(desc, traps, false);
-	memset(&traps[out], 0, sizeof(traps[0]));
+	traps[out] = zero;
 
 	xen_mc_flush();
 	if (HYPERVISOR_set_trap_table(traps))
-- 
2.34.1

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