Messages in this thread | | | Date | Sun, 8 Dec 2019 23:43:57 +0100 | From | Daniel Kiper <> | Subject | Re: [GRUB PATCH 1/1] loader/i386/linux: Fix an underflow in the setup_header length calculation |
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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@oracle.com> > --- > 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
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