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Re: [PATCH] loader/i386/linux: calculate the size of the setup header

From: Andrew Jeddeloh
Subject: Re: [PATCH] loader/i386/linux: calculate the size of the setup header
Date: Fri, 25 May 2018 12:02:03 -0700

Oops, my bad, didn't mean to drop the ML.

So you're saying the setup header could expand to include some of pad7
in the future, but we error if it goes beyond that (into the e820
map)? Looking at bootparam.h it looks like the _most_ correct thing
would be to stop at edd_mbr_sig_buffer, but grub doesn't have a
corresponding field so e820_map seems good enough. Thanks for working
through this.

Here's the revised patch

>From d46c75b4a9151c10e7f7809637f9f42a2c393c25 Mon Sep 17 00:00:00 2001
From: Andrew Jeddeloh <address@hidden>
Date: Tue, 8 May 2018 14:39:01 -0700
Subject: [PATCH] loader/i386/linux: calculate setup header length

Previously the length was just assumed to be the size of the
linux_params struct. The linux x86 32 bit boot protocol says the size of
the setup header is 0x202 + the byte at 0x201 in the boot params.
Calculate the size of the header, rather than assume it is the size of
the linux_params struct.

Signed-off-by: Andrew Jeddeloh <address@hidden>
 grub-core/loader/i386/linux.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/grub-core/loader/i386/linux.c b/grub-core/loader/i386/linux.c
index 44301e126..b5b65ea36 100644
--- a/grub-core/loader/i386/linux.c
+++ b/grub-core/loader/i386/linux.c
@@ -805,7 +805,19 @@ grub_cmd_linux (grub_command_t cmd __attribute__
   linux_params.kernel_alignment = (1 << align);
   linux_params.ps_mouse = linux_params.padding10 =  0;

-  len = sizeof (linux_params) - sizeof (lh);
+  /* The Linux 32 bit boot protocol defines the setup header size to
be 0x202 + the byte value
+   * at 0x201. */
+  len = *((char *)&linux_params.jump + 1) + 0x202;
+  /* Verify the struct is big enough so we do not write past the end */
+  if (len > (char *)&linux_params.e820_map - (char *)&linux_params) {
+    grub_error (GRUB_ERR_BAD_OS, "boot params setup header too big");
+    goto fail;
+  }
+  /* We've already read lh so there is no need to read it second time. */
+  len -= sizeof(lh);
   if (grub_file_read (file, (char *) &linux_params + sizeof (lh), len) != len)
       if (!grub_errno)

On Fri, May 18, 2018 at 4:53 AM, Daniel Kiper <address@hidden> wrote:
> Re-added grub-devel. Next time please do not drop GRUB ML from the addresses.
> On Thu, May 17, 2018 at 03:40:32PM -0700, Andrew Jeddeloh wrote:
>> Sorry about the long delay, I agree with all the sugguestions, except 
>> shouldn't
> No problem.
>> if (len > &linux_params.e820_map - &linux_params)
>> be
>> if (len > sizeof(linux_params))
>> since at that point len is the total size of the header which
>> linux_params represents?
> Please take a look at arch/x86/include/uapi/asm/bootparam.h in latest
> Linux kernel source. My proposal is better but it seems to me right now
> that it is still too much. I have a feeling that we should not go beyond
> the end of boot_params._pad7. Am I right?
> Daniel

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