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

[Yi Yang <yang.y.yi@gmail.com>]

Coywolf Qi Hunt wrote:

>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
>  
>
The variable target does it, its value is 0x00000001, so you mustn't 
worry it.
osname only has 4-bytes space, so if you set '\0' to its tail, a byte 
information will be lost.

>
>  
>
>>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
>
>  
>