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

[Ross Philipson]

On 12/02/2019 12:29 PM, 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>

Yes this looks correct. The length should be based off the jmp offset
byte and not the size of the structure defined in GRUB.

Reviewed-by: Ross Philipson <ross.philipson@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,
>