[SECURITY PATCH 12/28] term: Fix overflow on user inputs

[Daniel Kiper]

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

This requires a very weird input from the serial interface but can cause
an overflow in input_buf (keys) overwriting the next variable (npending)
with the user choice:

(pahole output)

struct grub_terminfo_input_state {
        int                        input_buf[6];         /*     0    24 */
        int                        npending;             /*    24     4 */ <- 
CORRUPT
        ...snip...

The magic string requires causing this is "ESC,O,],0,1,2,q" and we overflow
npending with "q" (aka increase npending to 161). The simplest fix is to
just to disallow overwrites input_buf, which exactly what this patch does.

Fixes: CID 292449

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/term/terminfo.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/grub-core/term/terminfo.c b/grub-core/term/terminfo.c
index 0e9de7f8f..cd7200803 100644
--- a/grub-core/term/terminfo.c
+++ b/grub-core/term/terminfo.c
@@ -398,7 +398,7 @@ grub_terminfo_getwh (struct grub_term_output *term)
 }
 
 static void
-grub_terminfo_readkey (struct grub_term_input *term, int *keys, int *len,
+grub_terminfo_readkey (struct grub_term_input *term, int *keys, int *len, int 
max_len,
                       int (*readkey) (struct grub_term_input *term))
 {
   int c;
@@ -414,6 +414,9 @@ grub_terminfo_readkey (struct grub_term_input *term, int 
*keys, int *len,
     if (c == -1)                                               \
       return;                                                  \
                                                                \
+    if (*len >= max_len)                                       \
+      return;                                                   \
+                                                                \
     keys[*len] = c;                                            \
     (*len)++;                                                  \
   }
@@ -602,8 +605,8 @@ grub_terminfo_getkey (struct grub_term_input *termi)
       return ret;
     }
 
-  grub_terminfo_readkey (termi, data->input_buf,
-                        &data->npending, data->readkey);
+  grub_terminfo_readkey (termi, data->input_buf, &data->npending,
+                        GRUB_TERMINFO_READKEY_MAX_LEN, data->readkey);
 
 #if defined(__powerpc__) && defined(GRUB_MACHINE_IEEE1275)
   if (data->npending == 1 && data->input_buf[0] == GRUB_TERM_ESC
-- 
2.11.0