Re: [GRUB PATCH 1/1] loader/i386/linux: Fix an underflow in the setup_header length calculation

[hpa]

On December 8, 2019 2:43:57 PM PST, Daniel Kiper <daniel.kiper@xxxxxxxxxx> wrote:
>Hmmm... Nobody cares? Strange...
>
>Adding 3mdeb folks...
>
>Daniel
>
>On Mon, Dec 02, 2019 at 06:29:39PM +0100, Daniel Kiper wrote:
>> Recent work around x86 Linux kernel loader revealed an underflow in
>the
>> setup_header length calculation and another related issue. Both lead
>to
>> the memory overwrite and later machine crash.
>>
>> Currently when the GRUB copies the setup_header into the linux_params
>> (struct boot_params, traditionally known as "zero page") it assumes
>the
>> setup_header size as sizeof(linux_i386_kernel_header/lh). This is
>> incorrect. It should use the value calculated accordingly to the
>Linux
>> kernel boot protocol. Otherwise in case of pretty old kernel, to be
>> exact Linux kernel boot protocol, the GRUB may write more into
>> linux_params than it was expected to. Fortunately this is not very
>big
>> issue. Though it has to be fixed. However, there is also an underflow
>> which is grave. It happens when
>>
>>   sizeof(linux_i386_kernel_header/lh) > "real size of the
>setup_header".
>>
>> Then len value wraps around and grub_file_read() reads whole kernel
>into
>> the linux_params overwriting memory past it. This leads to the GRUB
>> memory allocator breakage and finally to its crash during boot.
>>
>> The patch fixes both issues. Additionally, it moves the code not
>related to
>>
>grub_memset(linux_params)/grub_memcpy(linux_params)/grub_file_read(linux_params)
>> section outside of it to not confuse the reader.
>>
>> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
>> ---
>>  grub-core/loader/i386/linux.c | 15 ++++++++-------
>>  1 file changed, 8 insertions(+), 7 deletions(-)
>>
>> diff --git a/grub-core/loader/i386/linux.c
>b/grub-core/loader/i386/linux.c
>> index d0501e229..ee95cd374 100644
>> --- a/grub-core/loader/i386/linux.c
>> +++ b/grub-core/loader/i386/linux.c
>> @@ -761,17 +761,12 @@ grub_cmd_linux (grub_command_t cmd
>__attribute__ ((unused)),
>>      goto fail;
>>
>>    grub_memset (&linux_params, 0, sizeof (linux_params));
>> -  grub_memcpy (&linux_params.setup_sects, &lh.setup_sects, sizeof
>(lh) - 0x1F1);
>> -
>> -  linux_params.code32_start = prot_mode_target + lh.code32_start -
>GRUB_LINUX_BZIMAGE_ADDR;
>> -  linux_params.kernel_alignment = (1 << align);
>> -  linux_params.ps_mouse = linux_params.padding10 =  0;
>>
>>    /*
>>     * The Linux 32-bit boot protocol defines the setup header end
>>     * to be at 0x202 + the byte value at 0x201.
>>     */
>> -  len = 0x202 + *((char *) &linux_params.jump + 1);
>> +  len = 0x202 + *((char *) &lh.jump + 1);
>>
>>    /* Verify the struct is big enough so we do not write past the
>end. */
>>    if (len > (char *) &linux_params.edd_mbr_sig_buffer - (char *)
>&linux_params) {
>> @@ -779,10 +774,13 @@ grub_cmd_linux (grub_command_t cmd
>__attribute__ ((unused)),
>>      goto fail;
>>    }
>>
>> +  grub_memcpy (&linux_params.setup_sects, &lh.setup_sects, len -
>0x1F1);
>> +
>>    /* 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 ((len > 0) &&
>> +      (grub_file_read (file, (char *) &linux_params + sizeof (lh),
>len) != len))
>>      {
>>        if (!grub_errno)
>>  	grub_error (GRUB_ERR_BAD_OS, N_("premature end of file %s"),
>> @@ -790,6 +788,9 @@ grub_cmd_linux (grub_command_t cmd __attribute__
>((unused)),
>>        goto fail;
>>      }
>>
>> +  linux_params.code32_start = prot_mode_target + lh.code32_start -
>GRUB_LINUX_BZIMAGE_ADDR;
>> +  linux_params.kernel_alignment = (1 << align);
>> +  linux_params.ps_mouse = linux_params.padding10 = 0;
>>    linux_params.type_of_loader = GRUB_LINUX_BOOT_LOADER_TYPE;
>>
>>    /* These two are used (instead of cmd_line_ptr) by older versions
>of Linux,
>> --
>> 2.11.0

Please fix these problems. It is a huge headache for us in the kennel community.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.