Re: [PATCH] Fix user data corrupted by old value return of sysctl

[Coywolf Qi Hunt <coywolf@gmail.com>]

2005/12/30, Yi Yang <yang.y.yi@gmail.com>:
> If the user reads a sysctl entry which is of string type
>   by sysctl syscall, this call probably corrupts the user data
>   right after the old value buffer, the issue lies in sysctl_string
>   seting 0 to oldval[len], len is the available buffer size
>   specified by the user, obviously, this will write to the first
>   byte of the user memory place immediate after the old value buffer
> , the correct way is that sysctl_string doesn't set 0, the user
> should do it by self in the program.
>
> The following program verifies this point:
>
> #include <linux/unistd.h>
> #include <linux/types.h>
> #include <linux/sysctl.h>
> #include <errno.h>
>
> _syscall1(int, _sysctl, struct __sysctl_args *, args);
> int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
>             void *newval, size_t newlen)
> {
>          struct __sysctl_args args
>                 = {name,nlen,oldval,oldlenp,newval,newlen};
>
>          return _sysctl(&args);
> }
>
> #define SIZE(x) sizeof(x)/sizeof(x[0])
> #define OSNAMESZ 4
>
> struct mystruct {
>         char osname[OSNAMESZ];
>         int target;
>         int osnamelth;
> } myos;
>
> int name[] = { CTL_KERN, KERN_NODENAME };
>
> int main(int argc, char * argv[])
> {
>         myos.target = 1;
>         printf("target = %d\n", myos.target);
>         myos.osnamelth = SIZE(myos.osname);
>          if (sysctl(name, SIZE(name), myos.osname,
>                         &myos.osnamelth, 0, 0))
>                  perror("sysctl");
>         else {
>                  printf("Current host name: %s\n", myos.osname);
>         }
>         printf("target = %d\n", myos.target);
>          return 0;
> }
>
> Copy it to file sysctl-safe.c, then
> $ hostname
> mylocalmachine
> $ gcc sysctl-safe.c
> $ ./a.out
> target = 1
> Current host name: mylo
> target = 0
> $
>
> After apply this patch:
> $ hostname
> mylocalmachine
> $ gcc sysctl-safe.c
> $ ./a.out
> target = 1
> Current host name: mylo

You didn't set the trailing '\0', I wonder how your printf did work
properly ever. You've just been lucky or something.

-- Coywolf


> target = 1
>
> Signed-off-by: Yi Yang <yang.y.yi@gmail.com>
>
>
> --- a/kernel/sysctl.c.orig      2005-12-30 09:21:34.000000000 +0000
> +++ b/kernel/sysctl.c   2005-12-30 15:58:15.000000000 +0000
> @@ -2207,8 +2207,6 @@ int sysctl_string(ctl_table *table, int
>                                 len = table->maxlen;
>                         if(copy_to_user(oldval, table->data, len))
>                                 return -EFAULT;
> -                       if(put_user(0, ((char __user *) oldval) + len))
> -                               return -EFAULT;
>                         if(put_user(len, oldlenp))
>                                 return -EFAULT;
>                 }
>
>
>
--
Coywolf Qi Hunt