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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Chris Coulson <chris.coulson@canonical.com>
Date: Thu, 7 Jan 2021 19:21:03 +0000
Subject: [PATCH] kern/parser: Fix a stack buffer overflow
grub_parser_split_cmdline() expands variable names present in the supplied
command line in to their corresponding variable contents and uses a 1 kiB
stack buffer for temporary storage without sufficient bounds checking. If
the function is called with a command line that references a variable with
a sufficiently large payload, it is possible to overflow the stack
buffer via tab completion, corrupt the stack frame and potentially
control execution.
Fixes: CVE-2020-27749
Reported-by: Chris Coulson <chris.coulson@canonical.com>
Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
grub-core/kern/parser.c | 110 +++++++++++++++++++++++++++++-------------------
1 file changed, 67 insertions(+), 43 deletions(-)
diff --git a/grub-core/kern/parser.c b/grub-core/kern/parser.c
index e010eaa1f..6ab7aa427 100644
--- a/grub-core/kern/parser.c
+++ b/grub-core/kern/parser.c
@@ -18,6 +18,7 @@
*/
#include <grub/parser.h>
+#include <grub/buffer.h>
#include <grub/env.h>
#include <grub/misc.h>
#include <grub/mm.h>
@@ -107,8 +108,8 @@ check_varstate (grub_parser_state_t s)
}
-static void
-add_var (char *varname, char **bp, char **vp,
+static grub_err_t
+add_var (grub_buffer_t varname, grub_buffer_t buf,
grub_parser_state_t state, grub_parser_state_t newstate)
{
const char *val;
@@ -116,31 +117,41 @@ add_var (char *varname, char **bp, char **vp,
/* Check if a variable was being read in and the end of the name
was reached. */
if (!(check_varstate (state) && !check_varstate (newstate)))
- return;
+ return GRUB_ERR_NONE;
- *((*vp)++) = '\0';
- val = grub_env_get (varname);
- *vp = varname;
+ if (grub_buffer_append_char (varname, '\0') != GRUB_ERR_NONE)
+ return grub_errno;
+
+ val = grub_env_get ((const char *) grub_buffer_peek_data (varname));
+ grub_buffer_reset (varname);
if (!val)
- return;
+ return GRUB_ERR_NONE;
/* Insert the contents of the variable in the buffer. */
- for (; *val; val++)
- *((*bp)++) = *val;
+ return grub_buffer_append_data (buf, val, grub_strlen (val));
}
-static void
-terminate_arg (char *buffer, char **bp, int *argc)
+static grub_err_t
+terminate_arg (grub_buffer_t buffer, int *argc)
{
- if (*bp != buffer && *((*bp) - 1) != '\0')
- {
- *((*bp)++) = '\0';
- (*argc)++;
- }
+ grub_size_t unread = grub_buffer_get_unread_bytes (buffer);
+
+ if (unread == 0)
+ return GRUB_ERR_NONE;
+
+ if (*(const char *) grub_buffer_peek_data_at (buffer, unread - 1) == '\0')
+ return GRUB_ERR_NONE;
+
+ if (grub_buffer_append_char (buffer, '\0') != GRUB_ERR_NONE)
+ return grub_errno;
+
+ (*argc)++;
+
+ return GRUB_ERR_NONE;
}
static grub_err_t
-process_char (char c, char *buffer, char **bp, char *varname, char **vp,
+process_char (char c, grub_buffer_t buffer, grub_buffer_t varname,
grub_parser_state_t state, int *argc,
grub_parser_state_t *newstate)
{
@@ -153,12 +164,13 @@ process_char (char c, char *buffer, char **bp, char *varname, char **vp,
* not describe the variable anymore, write the variable to
* the buffer.
*/
- add_var (varname, bp, vp, state, *newstate);
+ if (add_var (varname, buffer, state, *newstate) != GRUB_ERR_NONE)
+ return grub_errno;
if (check_varstate (*newstate))
{
if (use)
- *((*vp)++) = use;
+ return grub_buffer_append_char (varname, use);
}
else if (*newstate == GRUB_PARSER_STATE_TEXT &&
state != GRUB_PARSER_STATE_ESC && grub_isspace (use))
@@ -167,10 +179,10 @@ process_char (char c, char *buffer, char **bp, char *varname, char **vp,
* Don't add more than one argument if multiple
* spaces are used.
*/
- terminate_arg (buffer, bp, argc);
+ return terminate_arg (buffer, argc);
}
else if (use)
- *((*bp)++) = use;
+ return grub_buffer_append_char (buffer, use);
return GRUB_ERR_NONE;
}
@@ -181,19 +193,22 @@ grub_parser_split_cmdline (const char *cmdline,
int *argc, char ***argv)
{
grub_parser_state_t state = GRUB_PARSER_STATE_TEXT;
- /* XXX: Fixed size buffer, perhaps this buffer should be dynamically
- allocated. */
- char buffer[1024];
- char *bp = buffer;
+ grub_buffer_t buffer, varname;
char *rd = (char *) cmdline;
char *rp = rd;
- char varname[200];
- char *vp = varname;
- char *args;
int i;
*argc = 0;
*argv = NULL;
+
+ buffer = grub_buffer_new (1024);
+ if (buffer == NULL)
+ return grub_errno;
+
+ varname = grub_buffer_new (200);
+ if (varname == NULL)
+ goto fail;
+
do
{
if (rp == NULL || *rp == '\0')
@@ -219,7 +234,7 @@ grub_parser_split_cmdline (const char *cmdline,
{
grub_parser_state_t newstate;
- if (process_char (*rp, buffer, &bp, varname, &vp, state, argc,
+ if (process_char (*rp, buffer, varname, state, argc,
&newstate) != GRUB_ERR_NONE)
goto fail;
@@ -230,10 +245,12 @@ grub_parser_split_cmdline (const char *cmdline,
/* A special case for when the last character was part of a
variable. */
- add_var (varname, &bp, &vp, state, GRUB_PARSER_STATE_TEXT);
+ if (add_var (varname, buffer, state, GRUB_PARSER_STATE_TEXT) != GRUB_ERR_NONE)
+ goto fail;
/* Ensure that the last argument is terminated. */
- terminate_arg (buffer, &bp, argc);
+ if (terminate_arg (buffer, argc) != GRUB_ERR_NONE)
+ goto fail;
/* If there are no args, then we're done. */
if (!*argc)
@@ -242,38 +259,45 @@ grub_parser_split_cmdline (const char *cmdline,
goto out;
}
- /* Reserve memory for the return values. */
- args = grub_malloc (bp - buffer);
- if (!args)
- goto fail;
- grub_memcpy (args, buffer, bp - buffer);
-
*argv = grub_calloc (*argc + 1, sizeof (char *));
if (!*argv)
goto fail;
/* The arguments are separated with 0's, setup argv so it points to
the right values. */
- bp = args;
for (i = 0; i < *argc; i++)
{
- (*argv)[i] = bp;
- while (*bp)
- bp++;
- bp++;
+ char *arg;
+
+ if (i > 0)
+ {
+ if (grub_buffer_advance_read_pos (buffer, 1) != GRUB_ERR_NONE)
+ goto fail;
+ }
+
+ arg = (char *) grub_buffer_peek_data (buffer);
+ if (arg == NULL ||
+ grub_buffer_advance_read_pos (buffer, grub_strlen (arg)) != GRUB_ERR_NONE)
+ goto fail;
+
+ (*argv)[i] = arg;
}
+ /* Keep memory for the return values. */
+ grub_buffer_take_data (buffer);
+
grub_errno = GRUB_ERR_NONE;
out:
if (rd != cmdline)
grub_free (rd);
+ grub_buffer_free (buffer);
+ grub_buffer_free (varname);
return grub_errno;
fail:
grub_free (*argv);
- grub_free (args);
goto out;
}
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