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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@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.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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