Re: [PATCH] Fix security issue when reading username and password

[Daniel Kiper]

Hi Hector,

On Thu, Oct 24, 2019 at 03:22:43PM +0100, Hector Marco wrote:
> Hello Daniel,
>
> Something went wrong in my last email, what I wanted to say is:
>
> The patch prevents that "cur_len" underflows. No negative values for
> "cur_len" so no way to underflow the "cur_len" variable and therefore no
> vulnerability.

First of all cur_len is unsigned. So, it does not get negative values at
all. Though even it was signed I cannot see where in the code it can get
negative value. Am I missing something?

Daniel

> Hector.
>
>
> On 24/10/2019 15:13, Hector Marco wrote:
> > Hello Daniel,
> >
> > The patch prevents that "cur_len" underflows. No negative values for
> > "cur_len" so way to underflow the "cur_len" variable and therefore
> >
> > I hope this helps,
> >
> > Hector.
> >
> >
> >
> > On 23/10/2019 11:14, Daniel Kiper wrote:
> >> On Fri, Oct 18, 2019 at 02:39:01PM +0200, Javier Martinez Canillas wrote:
> >>> From: Hector Marco-Gisbert <address@hidden>
> >>>
> >>>   This patch fixes two integer underflows at:
> >>>     * grub-core/lib/crypto.c
> >>>     * grub-core/normal/auth.c
> >>>
> >>> Resolves: CVE-2015-8370
> >>>
> >>> Signed-off-by: Hector Marco-Gisbert <address@hidden>
> >>> Signed-off-by: Ismael Ripoll-Ripoll <address@hidden>
> >>> Signed-off-by: Javier Martinez Canillas <address@hidden>
> >>> ---
> >>>
> >>>  grub-core/lib/crypto.c  | 2 +-
> >>>  grub-core/normal/auth.c | 2 +-
> >>>  2 files changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/grub-core/lib/crypto.c b/grub-core/lib/crypto.c
> >>> index ca334d5a40e..e6c78d16d39 100644
> >>> --- a/grub-core/lib/crypto.c
> >>> +++ b/grub-core/lib/crypto.c
> >>> @@ -468,7 +468,7 @@ grub_password_get (char buf[], unsigned buf_size)
> >>>     break;
> >>>   }
> >>>
> >>> -      if (key == '\b')
> >>> +      if (key == '\b' && cur_len)
> >>>   {
> >>>     if (cur_len)
> >>>       cur_len--;
> >>> diff --git a/grub-core/normal/auth.c b/grub-core/normal/auth.c
> >>> index 6be678c0de1..c35ce972473 100644
> >>> --- a/grub-core/normal/auth.c
> >>> +++ b/grub-core/normal/auth.c
> >>> @@ -172,7 +172,7 @@ grub_username_get (char buf[], unsigned buf_size)
> >>>     break;
> >>>   }
> >>>
> >>> -      if (key == GRUB_TERM_BACKSPACE)
> >>> +      if (key == GRUB_TERM_BACKSPACE && cur_len)
> >>>   {
> >>>     if (cur_len)
> >>>       {
> >>
> >> TBH, I do not understand how this patch helps. It only delays continue
> >> execution to the next "if (!grub_isprint (key))" if cur_len == 0.
> >>
> >> Daniel