CVE-2020-25656: keyboard, reorder user buffer handling in vt_do_kdgkb_ioctl

From

Subject

[PATCH 11/17] vt: keyboard, reorder user buffer handling in vt_do_kdgkb_ioctl

Date

Thu, 29 Oct 2020 12:32:16 +0100

KDGKBSENT (the getter) needs only ‘user_kdgkb->kb_func’ from the
userspace, i.e. the index. Then it needs a buffer for a local copy of
'kb_string’.

KDSKBSENT (the setter) needs a copy up to the length of
'user_kdgkb->kb_string’.

That means, we obtain the index before the switch-case and use it in
both paths and:

1. allocate full space in the getter case, and
2. copy the string only in the setter case. We do it by strndup_user
   helper now which was not available when this function was written.

Given we copy the two members of ‘struct kbsentry’ separately, we no
longer need a local definition. Hence we need to change all the sizeofs
here too.

Signed-off-by: Jiri Slaby [email protected]
—
drivers/tty/vt/keyboard.c | 42 ++++++++++++++++++±-------------------
1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 55014f57a3de…81afe0438b34 100644
— a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -2021,7 +2021,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm,
/* FIXME: This one needs untangling */
int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
{
- struct kbsentry *kbs;

• char *kbs;
  u_char *q;
  int sz, fnw_sz;
  int delta;
  @@ -2034,39 +2034,37 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
  if (!capable(CAP_SYS_TTY_CONFIG))
  perm = 0;

• kbs = kmalloc(sizeof(*kbs), GFP_KERNEL);
  - if (!kbs) {
  - ret = -ENOMEM;
  - goto reterr;
  - }

• if (get_user(kb_func, &user_kdgkb->kb_func))

•   return -EFAULT;
  

• /* we mostly copy too much here (512bytes), but who cares ;) */
  - if (copy_from_user(kbs, user_kdgkb, sizeof(struct kbsentry))) {
  - ret = -EFAULT;
  - goto reterr;
  - }
  - kbs->kb_string[sizeof(kbs->kb_string)-1] = '\0’;
  - kb_func = array_index_nospec(kbs->kb_func, MAX_NR_FUNC);

• kb_func = array_index_nospec(kb_func, MAX_NR_FUNC);

  switch (cmd) {
  case KDGKBSENT: {
  /* size should have been a struct member */
  ssize_t len = sizeof(user_kdgkb->kb_string);

•  kbs = kmalloc(len, GFP\_KERNEL);  
  

•   if (!kbs)  
  

•       return -ENOMEM;  
  

•   spin\_lock\_irqsave(&func\_buf\_lock, flags);  
  

- len = strlcpy(kbs->kb_string, func_table[kb_func] ? : "", len);

•   len = strlcpy(kbs, func\_table\[kb\_func\] ? : "", len);  
    spin\_unlock\_irqrestore(&func\_buf\_lock, flags);
  

•  ret = copy\_to\_user(user\_kdgkb->kb\_string, kbs->kb\_string,  
  

- len + 1) ? -EFAULT : 0;

•   ret = copy\_to\_user(user\_kdgkb->kb\_string, kbs, len + 1) ?  
  

•       -EFAULT : 0;
  
    goto reterr;  
  

  }
  case KDSKBSENT:
  - if (!perm) {
  - ret = -EPERM;
  - goto reterr;
  - }

•   if (!perm)  
  

•       return -EPERM;  
  

•   kbs = strndup\_user(user\_kdgkb->kb\_string,  
  

•           sizeof(user\_kdgkb->kb\_string));  
  

•   if (IS\_ERR(kbs))  
  

•       return PTR\_ERR(kbs);
  
    fnw = NULL;  
    fnw\_sz = 0;  
  

@@ -2084,7 +2082,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
else
fj = first_free;
/* buffer usage increase by new entry */
- delta = (q ? -strlen(q) : 1) + strlen(kbs->kb_string);

•   delta = (q ? -strlen(q) : 1) + strlen(kbs);
  
    if (delta <= funcbufleft) {     /\* it fits in current buf \*/  
        if (j < MAX\_NR\_FUNC) {  
  

@@ -2136,7 +2134,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
funcbufsize = sz;
}
/* finally insert item itself */
- strcpy(func_table[kb_func], kbs->kb_string);

•   strcpy(func\_table\[kb\_func\], kbs);  
    spin\_unlock\_irqrestore(&func\_buf\_lock, flags);  
    break;  
  

  }
  –
  2.29.1