linux-kernel - [PATCH] x86, boot: skip relocs when load address unchanged

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Message-ID: <20150116005146.GA4212@www.outflux.net>
Date:	Thu, 15 Jan 2015 16:51:46 -0800
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Vivek Goyal <vgoyal@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	Junjie Mao <eternal.n08@...il.com>,
	Andi Kleen <ak@...ux.intel.com>, Baoquan He <bhe@...hat.com>,
	"Thomas D." <whissi@...ssi.de>
Subject: [PATCH] x86, boot: skip relocs when load address unchanged

On 64-bit, relocation is not required unless the load address gets
changed. Without this, relocations do unexpected things when the kernel
is above 4G.

Reported-by: Baoquan He <bhe@...hat.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Cc: stable@...r.kernel.org
---
This is a reimplementation of Baoquan's "kaslr: check if kernel location is
changed", which performs the check without needing to change the function
declaration. This should have exactly the same effect, but I dropped Vivek's
Ack and Thomas's Test, since it's technically a different patch.
---
 arch/x86/boot/compressed/misc.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index dcc1c536cc21..a950864a64da 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -373,6 +373,8 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
 				  unsigned long output_len,
 				  unsigned long run_size)
 {
+	unsigned char *output_orig = output;
+
 	real_mode = rmode;
 
 	sanitize_boot_params(real_mode);
@@ -421,7 +423,12 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
 	debug_putstr("\nDecompressing Linux... ");
 	decompress(input_data, input_len, NULL, NULL, output, NULL, error);
 	parse_elf(output);
-	handle_relocations(output, output_len);
+	/*
+	 * 32-bit always performs relocations. 64-bit relocations are only
+	 * needed if kASLR has chosen a different load address.
+	 */
+	if (!IS_ENABLED(CONFIG_X86_64) || output != output_orig)
+		handle_relocations(output, output_len);
 	debug_putstr("done.\nBooting the kernel.\n");
 	return output;
 }
-- 
1.9.1


-- 
Kees Cook
Chrome OS Security
--
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